In the United States, the American Association for the Advancement of Science (AAAS) plays a key role in fostering the public understanding of science and in promoting scientific research. Founded in 1848, it has nearly 300 affiliated organizations, many of which originally developed from AAAS special-interest groups.
Since the late 19th century, communication among scientists has also been improved by international organizations, such as the International Bureau of Weights and Measures, founded in 1873, the International Council of Research, founded in 1919, and the World Health Organization, founded in 1948. Other organizations act as international forums for research in particular fields. For example, the Intergovernmental Panel on Climate Change (IPCC), established in 1988, as research on how climate change occurs, and what affects change is likely to have on humans and their environment.
Classifying sciences involves arbitrary decisions because the universe is not easily split into separate compartments. This article divides science into five major branches: mathematics, physical sciences, earth sciences, life sciences, and social sciences. A sixth branch, technology, draws on discoveries from all areas of science and puts them to practical use. Each of these branches itself consists of numerous subdivisions. Many of these subdivisions, such as astrophysics or biotechnology, combine overlapping disciplines, creating yet more areas of research.
The 20th century mathematics made rapid advances on all fronts. The foundations of mathematics became more solidly grounded in logic, while at the same time mathematics advanced the development of symbolic logic. Philosophy was not the only field to progress with the help of mathematics. Physics, too, benefited from the contributions of mathematicians to relativity theory and quantum theory. In fact, mathematics achieved broader applications than ever before, as new fields developed within mathematics (computational mathematics, game theory, and chaos theory) and other branches of knowledge, including economics and physics, achieved firmer grounding through the application of mathematics. Even the most abstract mathematics seemed to find application, and the boundaries between pure mathematics and applied mathematics grew ever fuzzier Mathematicians searched for unifying principles and general statements that applied to large categories of numbers and objects. In algebra, the study of structure continued with a focus on structural units called rings, fields, and groups, and at mid-century it extended to the relationships between these categories. Algebra became an important part of other areas of mathematics, including analysis, number theory, and topology, as the search for unifying theories moved ahead. Topology—the studies of the properties of objects that remain constant during transformation, or stretching-became a fertile research field, bringing together geometry, algebra, and analysis. Because of the abstract and complex nature of most 20th-century mathematics, most of the remaining sections of this article will discuss practical developments in mathematics with applications in more familiar fields.
Until the 20th century the centres of mathematics research in the West were all located in Europe. Although the University of Göttingen in Germany, the University of Cambridge in England, the French Academy of Sciences and the University of Paris, and the University of Moscow in Russia retained their importance, the United States rose in prominence and reputation for mathematical research, especially the departments of mathematics at Princeton University and the University of Chicago.
At the Second International Congress of Mathematicians held in Paris in 1900, German mathematician David Hilbert spoke to the assembly. Hilbert was a professor at the University of Göttingen, the former academic home of Gauss and Riemann. Hilbert’s speech at Paris was a survey of twenty-three mathematical problems that he felt would guide the work being done in mathematics during the coming century. These problems stimulated a great deal of the mathematical research of the 20th century, and many of the problems were solved. When news breaks that another ‘Hilbert problem’ has been solved, mathematicians worldwide impatiently await further details.
Hilbert contributed to most areas of mathematics, starting with his classic Grundlagen der Geometric (Foundations of Geometry), published in 1899. Hilbert’s work created the field of functional analysis (the analysis of functions as a group), a field that occupied many mathematicians during the 20th century. He also contributed to mathematical physics. From 1915 on he fought to have Emmy Noether, a noted German mathematician, hired at Göttingen. When the university refused to hire her because of objections to the presence of a woman in the faculty senate, Hilbert countered that the senate was not the changing room for a swimming pool. Noether later made major contributions to ring theory in algebra and wrote a standard text on abstract algebra.
In some ways pure mathematics became more abstract in the 20th century, as it joined forces with the field of symbolic logic in philosophy. The scholars who bridged the fields of mathematics and philosophy early in the century were Alfred North Whitehead and Bertrand Russell, who worked together at Cambridge University. They published their major work, Principia Mathematica (Principles of Mathematics), in three volumes from 1910 to 1913. In it they demonstrated the principles of mathematical logic and attempted to show that all of the mathematics could be deduced from a few premises and definitions by the rules of formal logic. In the late 19th century, German mathematician Gottlob Frége had provided the system of notation for mathematical logic and paved the way for the work of Russell and Whitehead. Mathematical logic influenced the direction of 20th-century mathematics, including the work of Hilbert.
Hilbert proposed that the underlying consistency of all mathematics could be demonstrated within mathematics. Nevertheless, logician Kurt Gödel in Austria proved that the goal of establishing the completeness and consistency of every mathematical theory is impossible. Despite its negative conclusion Gödel’s Theorem, published in 1931, opened new areas in mathematical logic. One area, known as recursion theory, played a major role in the development of computers.
Several revolutionary theories, including relativity and quantum theory, challenged existing assumptions in physics in the early 20th century. The work of a number of mathematicians contributed to these theories. Among them was Noether, whose gender had denied her a paid position at the University of Göttingen. Noether’s mathematical formulations on invariants (quantities that remain unchanged as other quantities change) contributed to Einstein’s theory of relativity. Russian mathematician Hermann Minkowski contributed to relativity the notion of the space-time continuum, with time as a fourth dimension. Hermann Weyl, a student of Hilbert’s, investigated the geometry of relativity and applied group theory to quantum mechanics. Weyl’s investigations helped advance the field of topology. Early in the century Hilbert quipped, “Physics is getting too difficult for physicists.”
Hungarian-born American mathematician John von Neumann built a solid mathematical basis for quantum theory with his text Mathematische Grundlagen der Quantenmechanik (1932, Mathematical Foundations of Quantum Mechanics). This investigation led him to explore algebraic operators and groups associated with them, opening a new area now known as Neumann algebra. Von Neumann, however, is probably best known for his work in game theory and computers.
During World War II (1939-1945) mathematicians and physicists worked together on developing radar, the atomic bomb, and other technology that helped defeat the Axis powers. Polish-born mathematician Stanislaw Ulam solved the problem of how to initiate fusion in the hydrogen bomb. Von Neumann participated in numerous US defence projects during the war.
Mathematics plays an important role today in cosmology and astrophysics, especially in research into big bang theory and the properties of black holes, antimatter, elementary particles, and other unobservable objects and events. Stephen Hawking, among the best-known cosmologists of the 20th century, in 1979 was appointed Lucasian Professor of Mathematics at Trinity College, Cambridge, a position once held by Newton.
Mathematics formed an alliance with economics in the 20th century as the tools of mathematical analysis, algebra, probability, and statistics illuminated economic theories. A specialty called econometrics links enormous numbers of equations to form mathematical models for use as forecasting tools.
Game theory began in mathematics but had immediate applications in economics and military strategy. This branch of mathematics deals with situations in which some sort of decision must be made to maximize a profit-that is, too win. Its theoretical foundations were supplied by von Neumann in a series of papers written during the 1930s and 1940s. Von Neumann and economist Oskar Morgenstern published results of their investigations in The Theory of Games and Economic Behaviour (1944). John Nash, the Princeton mathematician profiled in the motion picture A Beautiful Mind, shared the 1994 Nobel Prize in economics for his work in game theory.
Mathematicians, physicists, and engineers contributed to the development of computers and computer science. Nevertheless, the early, theoretical work came from mathematicians. English mathematician Alan Turing, working at Cambridge University, introduced the idea of a machine that could considerably equate of equal value the mathematical operations and solve equations. The Turing machine, as it became known, was a precursor of the modern computer. Through his work Turing brought together the elements that form the basis of computer science: symbolic logic, numerical analysis, electrical engineering, and a mechanical vision of human thought processes.
Computer theory is the third area with which von Neumann is associated, in addition to mathematical physics and game theory. He established the basic principles on which computers operate. Turing and von Neumann both recognized the usefulness of the binary arithmetic system for storing computer programs.
The first large-scale digital computers were pioneered in the 1940s. Von Neumann completed the EDVAC (Electronic Discrete Variable Automatic Computer) at the Institute of Advanced Study in Princeton in 1945. Engineers John Eckert and John Mauchly built ENIAC (Electronic Numerical Integrator and Calculator), which began operation at the University of Pennsylvania in 1946. As increasingly complex computers are built, the field of artificial intelligence has drawn attention. Researchers in this field attempt to develop computer systems that can mimic human thought processes.
Mathematician Norbert Wiener, working at the Massachusetts Institute of Technology (MIT), also became interested in automatic computing and developed the field known as cybernetics. Cybernetics grew out of Wiener’s work on increasing the accuracy of bombsights during World War II. From this came a broader investigation of how information can be translated into improved performance. Cybernetics is now applied to communication and control systems in living organisms.
Computers have exercised an enormous influence on mathematics and its applications. As ever more complex computers are developed, their applications proliferate. Computers have given great impetus to areas of mathematics such as numerical analysis and finite mathematics. Computer science has suggested new areas for mathematical investigation, such as the study of algorithms. Computers also have become powerful tools in areas as diverse as number theory, differential equations, and abstract algebra. In addition, the computer has made possible the solution of several long-standing problems in mathematics, such as the four-colours theorem first proposed in the mid-19th century.
The four-colour theorem stated that four colours are sufficient to colour any map, given that any two countries with a contiguous boundary require different colours. Mathematicians at the University of Illinois finally confirmed the theorem in 1976 by means of a large-scale computer that reduced the number of possible maps too less than 2,000. The program they wrote ran thousands of lines in length and took more than 1,200 hours to run. Many mathematicians, however, do not accept the result as a proof because it has not been checked. Verification by hand would require far too many human hours. Some mathematicians object to the solution’s lack of elegance. This complaint has been paraphrased, “a good mathematical proof is like a poem-this are a telephone directory."
Hilbert inaugurated the 20th century by proposing twenty-three problems that he expected to occupy mathematicians for the next 100 years. A number of these problems, such as the Riemann hypothesis about prime numbers, remain unsolved in the early 21st century. Hilbert claimed, “If I were to awaken after having slept for a thousand years, my first question would be: Has the Riemann hypothesis been proven?”
The existence of old problems, along with new problems that continually arise, ensures that mathematics research will remain challenging and vital through the 21st century. Influenced by Hilbert, the Clay Mathematics Institute at Harvard University announced the Millennium Prize in 2000 for solutions to mathematics problems that have long resisted solution. Among the seven problems is the Riemann hypothesis. An award of $1 million awaits the mathematician who solves any of these problems.
Minkowski, Hermann (1864-1909), Russian mathematician, who developed the concept of the space-time continuum. He was born in Russia and attended and then taught at German universities. To the three dimensions of space, Minkowski added the concept of a fourth dimension, time. This concept developed from Albert Einstein's 1905 relativity theory, and became, in turn, the framework for Einstein's 1916 general theory of relativity.
Gravitation is one of the four fundamental forces of nature, along with electromagnetism and the weak and strong nuclear forces, which hold together the particles that make up atoms. Gravitation is by far the weakest of these forces and, as a result, is not important in the interactions of atoms and nuclear particles or even of moderate-sized objects, such as people or cars. Gravitation is important only when very large objects, such as planets, are involved. This is true for several reasons. First, the force of gravitation reaches great distances, while nuclear forces operate only over extremely short distances and decrease in strength very rapidly as distance increases. Second, gravitation is always attractive. In contrast, electromagnetic forces between particles can be repulsive or attractive depending on whether the particles both have a positive or negative electrical charge, or they have opposite electrical charges. These attractive and repulsive forces tend to cancel each other out, leaving only a weak net force. Gravitation has no repulsive force and, therefore, no such cancellation or weakening.
After presenting his general theory of relativity in 1915, German-born American physicist Albert Einstein tried in vain to unify his theory of gravitation with one that would include all the fundamental forces in nature. Einstein discussed his special and general theories of relativity and his work toward a unified field theory in a 1950 Scientific American article. At the time, he was not convinced that he had discovered a valid solution capable of extending his general theory of relativity to other forces. He died in 1955, leaving this problem unsolved. open sidebar.
Gravitation plays a crucial role in most processes on the earth. Theocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives whether patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space.
The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size.
Falling objects accelerate in response to the force exerted on them by Earth’s gravity. Different objects accelerate at the same rate, regardless of their mass. This illustration shows the speed at which a ball and a cat would be moving and the distance each would have fallen at intervals of a tenth of a second during a short fall
If an object held near the surface of the earth is released, it will fall and accelerate, or pick up speed, as it descends. This acceleration is caused by gravity, the force of attraction between the object and the earth. The force of gravity on an object is also called the object's weight. This force depends on the object's mass, or the amount of matter in the object. The weight of an object is equal to the mass of the object multiplied by the acceleration due to gravity.
A bowling ball that weighs 16 lb. is being pulled toward the earth with a force of 16 lb? In the metric system, the bowling ball is pulled toward the earth with a force of seventy-one newtons (a metric unit of force abbreviated N). The bowling ball also pulls on the earth with a force of 16 lb. (71 N), but the earth is so massive that it does not move appreciably. In order to hold the bowling ball up and keep it from falling, a person must exert an upward force of 16 lb (71 N) on the ball. This upward force acts to oppose the 16 lb. (71 N) downward weight force, leaving a total force of zero. The total force on an object determines the object's acceleration.
If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb. (71 N) bowling ball and a 500 lb. (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m. sec.(thirty-two ft./sec.), resulting in an acceleration of 9.8 m/sec/sec (32 ft. sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.
The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the centre of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb. (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earth's poles than at the equator.
The special theory of relativity dealt only with constant, as opposed to accelerated, motion of the frames of reference, and the Lorentz transformations apply to frames moving with uniform motion with respect to each other. In 1915-1916, Einstein extended relativity to account for the more general case of accelerated frames of reference in his general theory of relativity, the central idea in general relativity theory, which accounts for accelerated motion, is that distinguishing between the effects of gravity is impossible and of nonuniform motion, if we did not know, for example, that we were on a spacecraft accelerating at a constant speed and dropped a cup of coffee, we cold not determined whether the mess on the floor was due to the effects of gravity or the accelerated motion, this inability to distinguish between a nonuniform motion, like an acceleration, and gravity is known as the ‘principle of equivalence’.
In this context, Einstein posited the laws elating space and time measurements carried out by two observers moving uniformly, as of one observer in an accelerating spacecraft and another on Earth. Force fields, like gravity, cause space-like, Einstein concluded, to become warped or curved and hence non-Euclidean in form. In the general theory the motion of material points, including light, is not along straight lines, as in Euclidean space, but along geodesics was confirmed in an experiment performed during a total eclipse of the Sun by Arthur Eddington in 1919.
Here, as in the special theory, visualization may help to understand the situation but does not really describe it. This is nicely illustrated in the typical visual analogy used to illustrate what spatial geodesics base. In this tremendous sheet of paper, extends infinitely in all directions. The inhabitants of this flatland, the flatlanders, are not only aware of the third dimension. Since the world here is perfectly Euclidean, any measurement of the sum of lines, no mater how far expended, would never meet.
We are then asked to move our flatlanders to a new land on the surface of a large sphere. Initially, our relocated population would perceive their new world as identical to the old, or as Euclidean and flat. Next we suppose them to send a kind of laser light along the surface of their two world for thousands of mile s. the discovery is then made that if the two beams of light are sent in parallel directions, the come together after travelling a thousand miles.
After experiencing utter confusion in the face of these results, the flatlaners eventually realize that their world is non-Euclidean or curved and invert Riemannian geometry to describe the curved space. The analogy normally concludes with the suggestion that we are the flatlanders, with the difference being that our story takes place in three, rather than two, dimensions in space. Just as the shadow creatures could not visualize the curved two-dimensional surface of their world, so we cannot visualize a three-dimensional curved space.
Thus a visual analogy to illustrate the reality described by the general theory is useful only to the extent that it entices us into an acceptance of the proposition that the reality is unvisualizable. Yet here, as in the special theory, there is no ambiguity in the mathematical description of this reality. Although curved geodesics are not any more unphysical than straight lines, visualizing the three spatial dimensions as a ‘surface’ in’ in the higher four-dimensional space-time cannot be done. Visualization may help us better understand what is implied by the general theory, but it dies no t disclose what is really meant by the theory,
The ancient Greek philosophers developed several theories about the force that caused objects to fall toward the earth. In the 4th century Bc, the Greek philosopher Aristotle proposed that all things were made from some combination of the four elements, earth, air, fire, and water. Objects that were similar in nature attracted one another, and as a result, objects with more earth in them were attracted to the earth. Fire, by contrast, was dissimilar and therefore tended to rise from the earth. Aristotle also developed a cosmology, that is, a theory describing the universe, that was geocentric, or earth-entered, with the moon, sun, planets, and stars moving around the earth on spheres. The Greek philosophers, however, did not propose a connection between the force behind planetary motion and the force that made objects fall toward the earth.
At the beginning of the 17th century, the Italian physicist and astronomer Galileo discovered that all objects fall toward the earth with the same acceleration, regardless of their weight, size, or shape, when gravity is the only force acting on them. Galileo also had a theory about the universe, which he based on the ideas of the Polish astronomer Nicolaus Copernicus. In the mid-16th century, Copernicus had proposed a heliocentric, or sun-centred system, in which the planets moved in circles around the sun, and Galileo agreed with this cosmology. However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. Like the Greek philosophers, he saw no connection between the force behind planetary motion and gravitation on earth.
In the late 16th and early 17th centuries the heliocentric model of the universe gained support from observations by the Danish astronomer Tycho Brahe, and his student, the German astronomer Johannes Kepler. These observations, made without telescopes, were accurate enough to determine that the planets did not move in circles, as Copernicus had suggested. Kepler calculated that the orbits had to be ellipses (slightly elongated circles). The invention of the telescope made even more precise observations possible, and Galileo was one of the first to use a telescope to study astronomy. In 1609 Galileo observed that moons orbited the planet Jupiter, a fact that could not presumably fit into an earth-centred model of the heavens.
The new heliocentric theory changed scientists' views about the earth's place in the universe and opened the way for new ideas about the forces behind planetary motion. However, it was not until the late 17th century that Isaac Newton developed a theory of gravitation that encompassed both the attraction of objects on the earth and planetary motion.
Gravitational forces because the Moon has significantly less mass than Earth, the weight of an object on the Moon’s surface is only one-sixth the object’s weight on Earth’s surface. This graph shows how much and object that weigh on Earth would weigh at different points between the Earth and Moon. Since the Earth and Moon pull in opposite directions, there is a point, about 346,000 km (215,000 mi) from Earth, where the opposite gravitational forces would cancel, and the object's weight would be zero.
To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object be motion at a constant speed on a straight line, and that it takes a force too slow, speed, or change the path of an object. Newton also invented calculus, a new branch of mathematics that became an important tool in the calculations of his theory of gravitation.
Newton proposed his law of gravitation in 1687 and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles' masses divided by the square of the distance between them. The gravitational force between two objects can be expressed by the following equation: F= GMm/d2 where F is the gravitational force, G is a constant known as the universal constant of gravitation, M and m are the masses of each object, and d is the distance between them. Newton considered a particle to be an object with a mass that was concentrated in a small point. If the mass of one or both particles increases, then the attraction between the two particles increases. For instance, if the mass of one particle is doubled, the force of attraction between the two particles is doubled. If the distance between the particles increases, then the attraction decreases as the square of the distance between them. Doubling the distance between two particles, for instance, will make the force of attraction one quarter as great as it was.
According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts similar between their centres. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the centre of gravity, at which all the force of gravity can be considered to be acting.
Newton's law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. It is therefore known as the universal law of gravitation. In order to know the strength of gravitational forces overall, however, it became necessary to find the value of ‘G’, the universal constant of gravitation. Scientists needed to re-enact an experiment, but gravitational forces are very weak between objects found in a common laboratory and thus hard to observe. In 1798 the English chemist and physicist Henry Cavendish finally measured G with a very sensitive experiment in which he nearly eliminated the effects of friction and other forces. The value he found was 6.754 x 10-11 N-m2/kg2-close to the currently accepted value of 6.670 x 10-11 N-m2/kg2 (a decimal point followed by ten zeros and then the number 6670). This value is so small that the force of gravitation between two objects with a mass of 1 metric ton each, 1 metre from each other, is about sixty-seven millionths of a newton, or about fifteen millionths of a pound.
Gravitation may also be described in a completely different way. A massive object, such as the earth, may be thought of as producing a condition in space around it called a gravitational field. This field causes objects in space to experience a force. The gravitational field around the earth, for instance, produces a downward force on objects near the earth surface. The field viewpoint is an alternative to the viewpoint that objects can affect each other across distance. This way of thinking about interactions has proved to be very important in the development of modern physics.
Newton's law of gravitation was the first theory to describe the motion of objects on the earth accurately as well as the planetary motion that astronomers had long observed. According to Newton's theory, the gravitational attraction between the planets and the sun holds the planets in elliptical orbits around the sun. The earth's moon and moons of other planets are held in orbit by the attraction between the moons and the planets. Newton's law led to many new discoveries, the most important of which was the discovery of the planet Neptune. Scientists had noted unexplainable variations in the motion of the planet Uranus for many years. Using Newton's law of gravitation, the French astronomer Urbain Leverrier and the British astronomer John Couch each independently predicted the existence of a more distant planet that was perturbing the orbit of Uranus. Neptune was discovered in 1864, in an orbit close to its predicted position.
Frames of Reference, as only a situation can appear different when viewed from different frames of reference. Try to imagine how an observer's perceptions could change from frame to frame in this illustration.
Scientists used Newton's theory of gravitation successfully for many years. Several problems began to arise, however, involving motion that did not follow the law of gravitation or Newtonian mechanics. One problem was the observed and unexplainable deviations in the orbit of Mercury (which could not be caused by the gravitational pull of another orbiting body).
Another problem with Newton's theory involved reference frames, that is, the conditions under which an observer measures the motion of an object. According to Newtonian mechanics, two observers making measurements of the speed of an object will measure different speeds if the observers are moving relative to each other. A person on the ground observing a ball that is on a train passing by will measure the speed of the ball as the same as the speed of the train. A person on the train observing the ball, however, will measure the ball's speed as zero. According to the traditional ideas about space and time, then, there could not be a constant, fundamental speed in the physical world because all speed is relative. However, near the end of the 19th century the Scottish physicist James Clerk Maxwell proposed a complete theory of electric and magnetic forces that contained just such a constant, which he called c. This constant speed was 300,000 km/sec (186,000 mi/sec) and was the speed of electromagnetic waves, including light waves. This feature of Maxwell's theory caused a crisis in physics because it indicated that speed was not always relative.
Albert Einstein resolved this crisis in 1905 with his special theory of relativity. An important feature of Einstein's new theory was that no particle, and even no information, could travel faster than the fundamental speed c. In Newton's gravitation theory, however, information about gravitation moved at infinite speed. If a star exploded into two parts, for example, the change in gravitational pull would be felt immediately by a planet in a distant orbit around the exploded star. According to Einstein's theory, such forces were not possible.
Though Newton's theory contained several flaws, it is still very practical for use in everyday life. Even today, it is sufficiently accurate for dealing with earth-based gravitational effects such as in geology (the study of the formation of the earth and the processes acting on it), and for most scientific work in astronomy. Only when examining exotic phenomena such as black holes (points in space with a gravitational force so strong that not even light can escape them) or in explaining the big bang (the origin of the universe) is Newton's theory inaccurate or inapplicable.
The gravitational attraction of objects for one another is the easiest fundamental force to observe and was the first fundamental force to be described with a complete mathematical theory by the English physicist and mathematician Sir Isaac Newton. A more accurate theory called general relativity was formulated early in the 20th century by the German-born American physicist Albert Einstein. Scientists recognize that even this theory is not correct for describing how gravitation works in certain circumstances, and they continue to search for an improved theory.
Gravitation plays a crucial role in most processes on the earth. The ocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives weather patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space.
The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size.
If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.
The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the centre of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earth's poles than at the equator.
The ancient Greek philosophers developed several theories about the force that caused objects to fall toward the earth. In the 4th century Bc, the Greek philosopher Aristotle proposed that all things were made from some combination of the four elements, earth, air, fire, and water. Objects that were similar in nature attracted one another, and as a result, objects with more earth in them were attracted to the earth. Fire, by contrast, was dissimilar and therefore tended to rise from the earth. Aristotle also developed a cosmology, that is, a theory describing the universe, that was geocentric, or earth-entered, with the moon, sun, planets, and stars moving around the earth on spheres. The Greek philosophers, however, did not propose a connection between the force behind planetary motion and the force that made objects fall toward the earth.
At the beginning of the 17th century, the Italian physicist and astronomer Galileo discovered that all objects fall toward the earth with the same acceleration, regardless of their weight, size, or shape, when gravity is the only force acting on them. Galileo also had a theory about the universe, which he based on the ideas of the Polish astronomer Nicolaus Copernicus. In the mid-16th century, Copernicus had proposed a heliocentric, or sun-entered system, in which the planets moved in circles around the sun, and Galileo agreed with this cosmology. However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. Like the Greek philosophers, he saw no connection between the force behind planetary motion and gravitation on earth.
In the late 16th and early 17th centuries the heliocentric model of the universe gained support from observations by the Danish astronomer Tycho Brahe, and his student, the German astronomer Johannes Kepler. These observations, made without telescopes, were accurate enough to determine that the planets did not move in circles, as Copernicus had suggested. Kepler calculated that the orbits had to be ellipses (slightly elongated circles). The invention of the telescope made even more precise observations possible, and Galileo was one of the first to use a telescope to study astronomy. In 1609 Galileo observed that moons orbited the planet Jupiter, a fact that could not presumably fit into an earth-centred model of the heavens.
The new heliocentric theory changed scientists' views about the earth's place in the universe and opened the way for new ideas about the forces behind planetary motion. However, it was not until the late 17th century that Isaac Newton developed a theory of gravitation that encompassed both the attraction of objects on the earth and planetary motion.
Gravitational Forces Because the Moon has significantly less mass than Earth, the weight of an object on the Moon’s surface is only one-sixth the object’s weight on Earth’s surface. This graph shows how much an object that weighs ‘w’ on Earth would weigh at different points between the Earth and Moon. Since the Earth and Moon pull in opposite directions, there is a point, about 346,000 km (215,000 mi) from Earth, where the opposite gravitational forces would cancel, and the object's weight would be zero.
To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object be motion at a constant speed on a straight line, and that it takes a force too slow, speed, or change the path of an object. Newton also invented calculus, a new branch of mathematics that became an important tool in the calculations of his theory of gravitation.
Newton proposed his law of gravitation in 1687 and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles' masses divided by the square of the distance between them. The gravitational force between two objects can be expressed by the following equation: F= GMm/d2 where F is the gravitational force, ‘G’ is a constant known as the universal constant of gravitation, ‘M’ and ‘m’ are the masses of each object, and d is the distance between them. Newton considered a particle to be an object with a mass that was concentrated in a small point. If the mass of one or both particles increases, then the attraction between the two particles increases. For instance, if the mass of one particle is doubled, the force of attraction between the two particles is doubled. If the distance between the particles increases, then the attraction decreases as the square of the distance between them. Doubling the distance between two particles, for instance, will make the force of attraction one quarter as great as it was.
According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts similarly between their centres. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the centre of gravity, at which all the force of gravity can be considered to be acting.
Newton's law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. It is therefore known as the universal law of gravitation. In order to know the strength of gravitational forces overall, however, it became necessary to find the value of G, the universal constant of gravitation. Scientists needed to re-enact an experiment, but gravitational forces are very weak between objects found in a common laboratory and thus hard to observe. In 1798 the English chemist and physicist Henry Cavendish finally measured ‘G’ with a very sensitive experiment in which he nearly eliminated the effects of friction and other forces. The value he found was 6.754 x 10-11 N-m2/kg2-close to the currently accepted value of 6.670 x 10-11 N-m2/kg2 (a decimal point followed by ten zeros and then the number 6670). This value is so small that the force of gravitation between two objects with a mass of 1 metric ton each, 1 metre from each other, is about sixty-seven millionths of a newton, or about fifteen millionths of a pound.
Gravitation may also be described in a completely different way. A massive object, such as the earth, may be thought of as producing a condition in space around it called a gravitational field. This field causes objects in space to experience a force. The gravitational field around the earth, for instance, produces a downward force on objects near the earth surface. The field viewpoint is an alternative to the viewpoint that objects can affect each other across distance. This way of thinking about interactions has proved to be very important in the development of modern physics.
Newton's law of gravitation was the first theory to describe the motion of objects on the earth accurately as well as the planetary motion that astronomers had long observed. According to Newton's theory, the gravitational attraction between the planets and the sun holds the planets in elliptical orbits around the sun. The earth's moon and moons of other planets are held in orbit by the attraction between the moons and the planets. Newton's law led to many new discoveries, the most important of which was the discovery of the planet Neptune. Scientists had noted unexplainable variations in the motion of the planet Uranus for many years. Using Newton's law of gravitation, the French astronomer Urbain Leverrier and the British astronomer John Couch each independently predicted the existence of a more distant planet that was perturbing the orbit of Uranus. Neptune was discovered in 1864, in an orbit close to its predicted position.
Einstein's general relativity theory predicts special gravitational conditions. The Big Bang theory, which describes the origin and early expansion of the universe, is one conclusion based on Einstein's theory that has been verified in several independent ways.
Another conclusion suggested by general relativity, as well as other relativistic theories of gravitation, is that gravitational effects move in waves. Astronomers have observed a loss of energy in a pair of neutron stars (stars composed of densely packed neutrons) that are orbiting each other. The astronomers theorize that energy-carrying gravitational waves are radiating from the pair, depleting the stars of their energy. Very violent astrophysical events, such as the explosion of stars or the collision of neutron stars, can produce gravitational waves strong enough that they may eventually be directly detectable with extremely precise instruments. Astrophysicists are designing such instruments with the hope that they will be able to detect gravitational waves by the beginning of the 21st century.
Another gravitational effect predicted by general relativity is the existence of black holes. The idea of a star with a gravitational force so strong that light cannot escape from its surface can be traced to Newtonian theory. Einstein modified this idea in his general theory of relativity. Because light cannot escape from a black hole, for any object-a particle, spacecraft, or wave-to escape, it would have to move past light. Nevertheless, light moves outward at the speed c. According to relativity 'c', is the highest attainable speed, so nothing can pass it. The black holes that Einstein envisioned, then, allow no escape whatsoever. An extension of this argument shows that when gravitation is this strong, nothing can even stay in the same place, but must move inward. Even the surface of a star must move inward, and must continue the collapse that created the strong gravitational force. What remains then is not a star, but a region of space from which emerges a tremendous gravitational force.
Einstein's theory of gravitation revolutionized 20th-century physics. Another important revolution that took place was quantum theory. Quantum theory states that physical interactions, or the exchange of energy, cannot be made arbitrarily small. There is a minimal interaction that comes in a packet called the quantum of an interaction. For electromagnetism the quantum is called the photon. Like the other interactions, gravitation also must be quantized. Physicists call a quantum of gravitational energy a graviton. In principle, gravitational waves arriving at the earth would consist of gravitons. In practice, gravitational waves would consist of apparently continuous streams of gravitons, and individual gravitons could not be detected.
Einstein's theory did not include quantum effects. For most of the 20th century, theoretical physicists have been unsuccessful in their attempts to formulate a theory that resembles Einstein's theory but also includes gravitons. Despite the lack of a complete quantum theory, making some partial predictions about quantized gravitation is possible. In the 1970s, British physicist Stephen Hawking showed that quantum mechanical processes in the strong gravitational pull just outside of black holes would create particles and quanta that move away from the black hole, thereby robbing it of energy.
Astronomy, is the study of the universe and the celestial bodies, gas, and dust within it. Astronomy includes observations and theories about the solar system, the stars, the galaxies, and the general structure of space. Astronomy also includes cosmology, the study of the universe and its past and future. People whom analysis astronomy is called astronomers, and they use a wide variety of methods to achieve of what in finality is obtainably resolved through their research. These methods usually involve ideas of physics, so most astronomers are also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas of astronomy also use techniques of chemistry, geology, and biology.
Astronomy is the oldest science, dating back thousands of years to when primitive people noticed objects in the sky overhead and watched the way the objects moved. In ancient Egypt, he first appearance of certain stars each year marked the onset of the seasonal flood, an important event for agriculture. In 17th-century England, astronomy provided methods of keeping track of time that were especially useful for accurate navigation. Astronomy has a long tradition of practical results, such as our current understanding of the stars, day and night, the seasons, and the phases of the Moon. Much of today's research in astronomy does not address immediate practical problems. Instead, it involves basic research to satisfy our curiosity about the universe and the objects in it. One day such knowledge may be of practical use to humans.
Astronomers use tools such as telescopes, cameras, spectrographs, and computers to analyse the light that astronomical objects emit. Amateur astronomers observe the sky as a hobby, while professional astronomers are paid for their research and usually work for large institutions such as colleges, universities, observatories, and government research institutes. Amateur astronomers make valuable observations, but are often limited by lack of access to the powerful and expensive equipment of professional astronomers.
A wide range of astronomical objects is accessible to amateur astronomers. Many solar system objects-such as planets, moons, and comets-are bright enough to be visible through binoculars and small telescopes. Small telescopes are also sufficient to reveal some of the beautiful detail in nebulas-clouds of gas and dust in our galaxy. Many amateur astronomers observe and photograph these objects. The increasing availability of sophisticated electronic instruments and computers over the past few decades has made powerful equipment more affordable and allowed amateur astronomers to expand their observations too much fainter objects. Amateur astronomers sometimes share their observations by posting their photographs on the World Wide Web, a network of information based on connections between computers.
Amateurs often undertake projects that require numerous observations over days, weeks, months, or even years. By searching the sky over a long period of time, amateur astronomers may observe things in the sky that represent sudden change, such as new comets or novas (stars that brightens suddenly). This type of consistent observation is also useful for studying objects that change slowly over time, such as variable stars and double stars. Amateur astronomers observe meteor showers, sunspots, and groupings of planets and the Moon in the sky. They also participate in expeditions to places in which special astronomical events-such as solar eclipses and meteor showers-are most visible. Several organizations, such as the Astronomical League and the American Association of Variable Star Observers, provide meetings and publications through which amateur astronomers can communicate and share their observations.
Professional astronomers usually have access to powerful telescopes, detectors, and computers. Most work in astronomy includes three parts, or phases. Astronomers first observe astronomical objects by guiding telescopes and instruments to collect the appropriate information. Astronomers then analyse the images and data. After the analysis, they compare their results with existing theories to determine whether their observations match with what theories predict, or whether the theories can be improved. Some astronomers work solely on observation and analysis, and some work solely on developing new theories.
Astronomy is such a broad topic that astronomers specialize in one or more parts of the field. For example, the study of the solar system is a different area of specialization than the study of stars. Astronomers who study our galaxy, the Milky Way, often use techniques different from those used by astronomers who study distant galaxies. Many planetary astronomers, such as scientists who study Mars, may have geology backgrounds and not consider they astronomers at all. Solar astronomers use different telescopes than nighttime astronomers use, because the Sun is so bright. Theoretical astronomers may never use telescopes at all. Instead, these astronomers use existing data or sometimes only previous theoretical results to develop and test theories. An increasing field of astronomy is computational astronomy, in which astronomers use computers to simulate astronomical events. Examples of events for which simulations are useful include the formation of the earliest galaxies of the universe or the explosion of a star to make a supernova.
Astronomers learn about astronomical objects by observing the energy they emit. These objects emit energy in the form of electromagnetic radiation. This radiation travels throughout the universe in the form of waves and can range from gamma rays, which have extremely short wavelengths, to visible light, to radio waves, which are very long. The entire range of these different wavelengths makes up the electromagnetic spectrum.
Astronomers gather different wavelengths of electromagnetic radiation depending on the objects that are being studied. The techniques of astronomy are often very different for studying different wavelengths. Conventional telescopes work only for visible light and the parts of the spectrum near visible light, such as the shortest infrared wavelengths and the longest ultraviolet wavelengths. Earth’s atmosphere complicates studies by absorbing many wavelengths of the electromagnetic spectrum. Gamma-ray astronomy, X-ray astronomy, infrared astronomy, ultraviolet astronomy, radio astronomy, visible-light astronomy, cosmic-ray astronomy, gravitational-wave astronomy, and neutrino astronomy all use different instruments and techniques.
Observational astronomers use telescopes or other instruments to observe the heavens. The astronomers who do the most observing, however, probably spend more time using computers than they do using telescopes. A few nights of observing with a telescope often provide enough data to keep astronomers busy for months analysing the data.
Until the 20th century, all observational astronomers studied the visible light that astronomical objects emit. Such astronomers are called optical astronomers, because they observe the same part of the electromagnetic spectrum that the human eye sees. Optical astronomers use telescopes and imaging equipment to study light from objects. Professional astronomers today hardly ever look through telescopes. Instead, a telescope sends an object’s light to a photographic plate or to an electronic light-sensitive computer chip called a charge-coupled device, or CCD. CCDs are about fifty times more sensitive than film, so today's astronomers can record in a minute an image that would have taken about an hour to record on film.
Telescopes may use either lenses or mirrors to gather visible light, permitting direct observation or photographic recording of distant objects. Those that use lenses are called refracting telescopes, since they use the property of refraction, or bending, of light. The largest refracting telescope is the 40-in (1-m) telescope at the Yerkes Observatory in Williams Bay, Wisconsin, founded in the late 19th century. Lenses bend different colours of light by different amounts, so different colours focus differently. Images produced by large lenses can be tinged with colour, often limiting the observations to those made through filters. Filters limit the image to one colour of light, so the lens bends all of the light in the image the same amount and makes the image more accurate than an image that includes all colours of light. Also, because light must pass through lenses, lenses can only be supported at the very edges. Large, heavy lenses are so thick that all the large telescopes in current use are made with other techniques.
Reflecting telescopes, which use mirrors, are easier to make than refracting telescopes and reflect all colours of light equally. All the largest telescopes today are reflecting telescopes. The largest single telescopes are the Keck telescopes at Mauna Kea Observatory in Hawaii. The Keck telescope mirrors are 394 in (10.0 m) in diameter. Mauna Kea Observatory, at an altitude of 4,205 m (13,796 ft), is especially high. The air at the observatory is clear, so many major telescope projects are located there.
The Hubble Space Telescope (HST), a reflecting telescope that orbits Earth, has returned the clearest images of any optical telescope. The main mirror of the HST is only ninety-four in. (2.4 m.) across, far smaller than that of the largest ground-based reflecting telescopes. Turbulence in the atmosphere makes observing objects as clearly as the HST can see impossible for ground-based telescopes. HST images of visible light are about five times finer than any produced by ground-based telescopes. Giant telescopes on Earth, however, collect much more light than the HST can. Examples of such giant telescopes include the twin 32-ft (10-m) Keck telescopes in Hawaii and the four 26-ft (8-m) telescopes in the Very Large Telescope array in the Atacama Desert in northern Chile (the nearest city is Antofagasta, Chile). Often astronomers use space and ground-based telescopes in conjunction.
Astronomers usually share telescopes. Many institutions with large telescopes accept applications from any astronomer who wishes to use the instruments, though others have limited sets of eligible applicants. The institution then divides the available time between successful applicants and assigns each astronomer an observing period. Astronomers can collect data from telescopes remotely. Data from Earth-based telescopes can be sent electronically over computer networks. Data from space-based telescopes reach Earth through radio waves collected by antennas on the ground.
Gamma rays have the shortest wavelengths. Special telescopes in orbit around Earth, such as the National Aeronautics and Space Administration’s (NASA’s) Compton Gamma-Ray Observatory, gather gamma rays before Earth’s atmosphere absorbs them. X rays, the next shortest wavelengths, also must be observed from space. NASA’s Chandra x-ray Observatory (CXO) is a school-bus-sized spacecraft scheduled to begin studying X-rays from orbit in 1999. It is designed to make high-resolution images.
Ultraviolet light has wavelengths longer than X rays, but shorter than visible light. Ultraviolet telescopes are similar to visible-light telescopes in the way they gather light, but the atmosphere blocks most ultraviolet radiation. Most ultraviolet observations, therefore, must also take place in space. Most of the instruments on the Hubble Space Telescope (HST) are sensitive to ultraviolet radiation. Humans cannot see ultraviolet radiation, but astronomers can create visual images from ultraviolet light by assigning particular colours or shades to different intensities of radiation.
Infrared astronomers study parts of the infrared spectrum, which consists of electromagnetic waves with wavelengths ranging from just longer than visible light to 1,000 times longer than visible light. Earth’s atmosphere absorbs infrared radiation, so astronomers must collect infrared radiation from places where the atmosphere is very thin, or from above the atmosphere. Observatories for these wavelengths are located on certain high mountaintops or in space. Most infrared wavelengths can be observed only from space. Every warm object emits some infrared radiation. Infrared astronomy is useful because objects that are not hot enough to emit visible or ultraviolet radiation may still emit infrared radiation. Infrared radiation also passes through interstellar and intergalactic gas and dusts more easily than radiation with shorter wavelengths. Further, the brightest part of the spectrum from the farthest galaxies in the universe is shifted into the infrared. The Next Generation Space Telescope, which NASA plans to launch in 2006, will operate especially in the infrared.
Radio waves have the longest wavelengths. Radio astronomers use giant dish antennas to collect and focus signals in the radio part of the spectrum. These celestial radio signals, often from hot bodies in space or from objects with strong magnetic fields, come through Earth's atmosphere to the ground. Radio waves penetrate dust clouds, allowing astronomers to see into the centre of our galaxy and into the cocoons of dust that surround forming stars.
Sometimes astronomers study emissions from space that are not electromagnetic radiation. Some of the particles of interest to astronomers are neutrinos, cosmic rays, and gravitational waves. Neutrinos are tiny particles with no electric charge and very little or no mass. The Sun and supernovas emit neutrinos. Most neutrino telescopes consist of huge underground tanks of liquid. These tanks capture a few of the many neutrinos that strike them, while the vast majority of neutrinos pass right through the tanks.
Cosmic rays are electrically charged particles that come to Earth from outer space at almost the speed of light. They are made up of negatively charged particles called electrons and positively charged nuclei of atoms. Astronomers do not know where most cosmic rays come from, but they use cosmic-ray detectors to study the particles. Cosmic-ray detectors are usually grids of wires that produce an electrical signal when a cosmic ray passes close to them.
Gravitational waves are a predicted consequence of the general theory of relativity developed by German-born American physicist Albert Einstein. Set off up in the 1960s astronomers have been building detectors for gravitational waves. Older gravitational-wave detectors were huge instruments that surrounded a carefully measured and positioned massive object suspended from the top of the instrument. Lasers trained on the object were designed to measure the object’s movement, which theoretically would occur when a gravitational wave hit the object. At the end of the 20th century, these instruments had picked up no gravitational waves. Gravitational waves should be very weak, and the instruments were probably not yet sensitive enough to register them. In the 1970s and 1980s American physicists Joseph Taylor and Russell Hulse observed indirect evidence of gravitational waves by studying systems of double pulsars. A new generation of gravitational-wave detectors, developed in the 1990s, used interferometers to measure distortions of space that would be caused by passing gravitational waves.
Some objects emit radiation more strongly in one wavelength than in another, but a set of data across the entire spectrum of electromagnetic radiation is much more useful than observations in anyone wavelength. For example, the supernova remnant known as the Crab Nebula has been observed in every part of the spectrum, and astronomers have used all the discoveries together to make a complete picture of how the Crab Nebula is evolving.
Whether astronomers take data from a ground-based telescope or have data radioed to them from space, they must then analyse the data. Usually the data are handled with the aid of a computer, which can carry out various manipulations the astronomer requests. For example, some of the individual picture elements, or pixels, of a CCD may be more sensitive than others. Consequently, astronomers sometimes take images of blank sky to measure which pixels appear brighter. They can then take these variations into account when interpreting the actual celestial images. Astronomers may write their own computer programs to analyse data or, as is increasingly the case, use certain standard computer programs developed at national observatories or elsewhere.
Often an astronomer uses observations to test a specific theory. Sometimes, a new experimental capability allows astronomers to study a new part of the electromagnetic spectrum or to see objects in greater detail or through special filters. If the observations do not verify the predictions of a theory, the theory must be discarded or, if possible, modified.
Up to about 3,000 stars are visible at a time from Earth with the unaided eye, far away from city lights, on a clear night. A view at night may also show several planets and perhaps a comet or a meteor shower. Increasingly, human-made light pollution is making the sky less dark, limiting the number of visible astronomical objects. During the daytime the Sun shines brightly. The Moon and bright planets are sometimes visible early or late in the day but are rarely seen at midday.
Earth moves in two basic ways: It turns in place, and it revolves around the Sun. Earth turns around its axis, an imaginary line that runs down its centre through its North and South poles. The Moon also revolves around Earth. All of these motions produce day and night, the seasons, the phases of the Moon, and solar and lunar eclipses.
Earth is about 12,000 km. (about 7,000 mi.) in diameter. As it revolves, or moves in a circle, around the Sun, Earth spins on its axis. This spinning movement is called rotation. Earth’s axis is tilted 23.5° with respect to the plane of its orbit. Each time Earth rotates on its axis, its corrective velocity to enable it of travelling, or free falling through into a new day, in other words, its rotational inertia or axial momentum carries it through one day, a cycle of light and dark. Humans artificially divide the day into 24 hours and then divide the hours into 60 minutes and the minutes into 60 seconds.
Earth revolves around the Sun once every year, or 365.25 days (most people use a 365-day calendar and take care of the extra 0.25 day by adding a day to the calendar every four years, creating a leap year). The orbit of Earth is almost, but not quite, a circle, so Earth is sometimes a little closer to the Sun than at other times. If Earth were upright as it revolved around the Sun, each point on Earth would have exactly twelve hours of light and twelve hours of dark each day. Because Earth is tilted, however, the northern hemisphere sometimes points toward the Sun and sometimes points away from the Sun. This tilt is responsible for the seasons. When the northern hemisphere points toward the Sun, the northernmost regions of Earth see the Sun 24 hours a day. The whole northern hemisphere gets more sunlight and gets it at a more direct angle than the southern hemisphere does during this period, which lasts for half of the year. The second half of this period, when the northern hemisphere points most directly at the Sun, is the northern hemisphere's summer, which corresponds to winter in the southern hemisphere. During the other half of the year, the southern hemisphere points more directly toward the Sun, so it is spring and summer in the southern hemisphere and fall and winters in the northern hemisphere.
One revolution of the Moon around Earth takes a little more than twenty-seven days seven hours. The Moon rotates on its axis in this same period of time, so the same face of the Moon is always presented to Earth. Over a period a little longer than twenty-nine days twelve hours, the Moon goes through a series of phases, in which the amount of the lighted half of the Moon we see from Earth changes. These phases are caused by the changing angle of sunlight hitting the Moon. (The period of phases is longer than the period of revolution of the Moon, because the motion of Earth around the Sun changes the angle at which the Sun’s light hits the Moon from night to night.)
The Moon’s orbit around Earth is tilted five from the plane of Earth’s orbit. Because of this tilt, when the Moon is at the point in its orbit when it is between Earth and the Sun, the Moon is usually a little above or below the Sun. At that time, the Sun lights the side of the Moon facing away from Earth, and the side of the Moon facing toward Earth is dark. This point in the Moon’s orbit corresponds to a phase of the Moon called the new moon. A quarter moon occurs when the Moon is at right angles to the line formed by the Sun and Earth. The Sun lights the side of the Moon closest to it, and half of that side is visible from Earth, forming a bright half-circle. When the Moon is on the opposite side of Earth from the Sun, the face of the Moon visible from Earth is lit, showing the full moon in the sky
Because of the tilt of the Moon's orbit, the Moon usually passes above or below the Sun at new moon and above or below Earth's shadow at full moon. Sometimes, though, the full moon or new moon crosses the plane of Earth's orbit. By a coincidence of nature, even though the Moon is about 400 times smaller than the Sun, it is also about 400 times closer to Earth than the Sun is, so the Moon and Sun look almost the same size from Earth. If the Moon lines up with the Sun and Earth at new moon (when the Moon is between Earth and the Sun), it blocks the Sun’s light from Earth, creating a solar eclipse. If the Moon lines up with Earth and the Sun at the full moon (when Earth is between the Moon and the Sun), Earth’s shadow covers the Moon, making a lunar eclipse.
A total solar eclipse is visible from only a small region of Earth. During a solar eclipse, the complete shadow of the Moon that falls on Earth is only about 160 km. (about 100 mi.) wide. As Earth, the Sun, and the Moon move, however, the Moon’s shadow sweeps out a path up to 16,000 km. (10,000 mi.) long. The total eclipse can only be seen from within this path. A total solar eclipse occurs about every eighteen months. Off to the sides of the path of a total eclipse, a partial eclipse, in which the Sun is only partly covered, is visible. Partial eclipses are much less dramatic than total eclipses. The Moon’s orbit around Earth is elliptical, or egg-shaped. The distance between Earth and the Moon varies slightly as the Moon orbits Earth. When the Moon is farther from Earth than usual, it appears smaller and may not cover the entire Sun during an eclipse. A ring, or annulus, of sunlight remains assimilated through visibility. Making an annular eclipse. An annular solar eclipse also occurs about every eighteen months. Additional partial solar eclipses are also visible from Earth in between.
At a lunar eclipse, the Moon is existent in Earth's shadow. When the Moon is completely in the shadow, the total lunar eclipse is visible from everywhere on the half of Earth from which the Moon is visible at that time. As a result, more people see total lunar eclipses than see total solar eclipses.
In an open place on a clear dark night, streaks of light may appear in a random part of the sky about once every ten minutes. These streaks are meteors-bits of rock-turning up in Earth's atmosphere. The bits of rock are called meteoroids, and when these bits survive Earth’s atmosphere intact and land on Earth, they are known as meteorites.
Every month or so, Earth passes through the orbit of a comet. Dust from the comet remains in the comet's orbit. When Earth passes through the band of dust, the dust and bits of rock burn up in the atmosphere, creating a meteor shower. Many more meteors are visible during a meteor shower than on an ordinary night. The most observed meteor shower is the Perseid shower, which occurs each year on August 11th or 12th.
Humans have picked out landmarks in the sky and mapped the heavens for thousands of years. Maps of the sky helped to potentially lost craft in as much as sailors have navigated using the celestially fixed stars to find refuge away from being lost. Now astronomers methodically map the sky to produce a universal format for the addresses of stars, galaxies, and other objects of interest.
Some of the stars in the sky are brighter and more noticeable than others are, and some of these bright stars appear to the eye to be grouped together. Ancient civilizations imagined that groups of stars represented figures in the sky. The oldest known representations of these groups of stars, called constellations, are from ancient Sumer (now Iraq) from about 4000 Bc. The constellations recorded by ancient Greeks and Chinese resemble the Sumerian constellations. The northern hemisphere constellations that astronomers recognize today are based on the Greek constellations. Explorers and astronomers developed and recorded the official constellations of the southern hemisphere in the 16th and 17th centuries. The International Astronomical Union (IAU) officially recognizes eighty-eight constellations. The IAU defined the boundaries of each constellation, so the eighty-eight constellations divide the sky without overlapping.
A familiar group of stars in the northern hemisphere is called the Big Dipper. The Big Dipper is part of an official constellation-Ursa Major, or the Great Bear. Groups of stars that are not official constellations, such as the Big Dipper, are called asterisms. While the stars in the Big Dipper appear in approximately the same part of the sky, they vary greatly in their distance from Earth. This is true for the stars in all constellations or asterisms-the stars accumulating of the group do not really occur close to each other in space, they merely appear together as seen from Earth. The patterns of the constellations are figments of humans’ imagination, and different artists may connect the stars of a constellation in different ways, even when illustrating the same myth.
Astronomers use coordinate systems to label the positions of objects in the sky, just as geographers use longitude and latitude to label the positions of objects on Earth. Astronomers use several different coordinate systems. The two most widely used are the altazimuth system and the equatorial system. The altazimuth system gives an object’s coordinates with respect to the sky visible above the observer. The equatorial coordinate system designates an object’s location with respect to Earth’s entire night sky, or the celestial sphere.
One of the ways astronomers give the position of a celestial object is by specifying its altitude and its azimuth. This coordinate system is called the altazimuth system. The altitude of an object is equal to its angle, in degrees, above the horizon. An object at the horizon would have an altitude of zero, and an object directly overhead would have an altitude of ninety. The azimuth of an object is equal to its angle in the horizontal direction, with north at zero, east at ninety, south at 180°, and west at 270°. For example, if an astronomer were looking for an object at twenty-three altitude and eighty-seven azimuth, the astronomer would know to look low in the sky and almost directly east.
As Earth rotates, astronomical objects appear to rise and set, so their altitudes and azimuths are constantly changing. An object’s altitude and azimuth also vary according to an observer’s location on Earth. Therefore, astronomers almost never use altazimuth coordinates to record an object’s position. Instead, astronomers with altazimuth telescopes translate coordinates from equatorial coordinates to find an object. Telescopes that use an altazimuth mounting system may be simple to set up, but they require many calculated movements to keep them pointed at an object as it moves across the sky. These telescopes fell out of use with the development of the equatorial coordinate and mounting system in the early 1800s. However, computers have made the return to popularity possible for altazimuth systems. Altazimuth mounting systems are simple and inexpensive, and-with computers to do the required calculations and control the motor that moves the telescope-they are practical.
The equatorial coordinate system is a coordinate system fixed on the sky. In this system, a star keeps the same coordinates no matter what the time is or where the observer is located. The equatorial coordinate system is based on the celestial sphere. The celestial sphere is a giant imaginary globe surrounding Earth. This sphere has north and south celestial pole directly above Earth’s North and South poles. It has a celestial equator, directly above Earth’s equator. Another important part of the celestial sphere is the line that marks the movement of the Sun with respect to the stars throughout the year. This path is called the ecliptic. Because Earth is tilted with respect to its orbit around the Sun, the ecliptic is not the same as the celestial equator. The ecliptic is tilted 23.5° to the celestial equator and crosses the celestial equator at two points on opposite sides of the celestial sphere. The crossing points are called the vernal (or spring) equinox and the autumnal equinox. The vernal equinox and autumnal equinox mark the beginning of spring and fall, respectively. The points at which the ecliptic and celestial equator are farthest apart are called the summer solstice and the winter solstice, which mark the beginning of summer and winter, respectively.
As Earth rotates on its axis each day, the stars and other distant astronomical objects appear to rise in the eastern part of the sky and set in the west. They seem to travel in circles around Earth’s North or South poles. In the equatorial coordinate system, the celestial sphere turns with the stars (but this movement is really caused by the rotation of Earth). The celestial sphere makes one complete rotation every twenty-three hours fifty-six minutes, which is four unexpected moments than a day measured by the movement of the Sun. A complete rotation of the celestial sphere is called a sidereal day. Because the sidereal day is shorter than a solar day, the stars that an observer sees from any location on Earth change slightly from night to night. The difference between a sidereal day and a solar day occurs because of Earth’s motion around the Sun.
The equivalent of longitude on the celestial sphere is called right ascension and the equivalent of latitude is declination. Specifying the right ascension of a star is equivalent to measuring the east-west distance from a line called the prime meridian that runs through Greenwich, England, for a place on Earth. Right ascension starts at the vernal equinox. Longitude on Earth is given in degrees, but right ascension is given in units of time-hours, minutes, and seconds. This is because the celestial equator is divided into 24 equal parts-each called an hour of right ascension instead of fifteen. Each hour is made up of 60 minutes, each of which is equal to 60 seconds. Measuring right ascension in units of time makes determine when will be the best time for observing an object easier for astronomers. A particular line of right ascension will be at its highest point in the sky above a particular place on Earth four minutes earlier each day, so keeping track of the movement of the celestial sphere with an ordinary clock would be complicated. Astronomers have special clocks that keep sidereal time (24 sidereal hours are equal to twenty-three hours fifty-six minutes of familiar solar time). Astronomers compare the current sidereal time with the right ascension of the object they wish to view. The object will be highest in the sky when the sidereal time equals the right ascension of the object.
The direction perpendicular to right ascension-and the equivalent to latitude on Earth-is declination. Declination is measured in degrees. These degrees are divided into arcminutes and arcseconds. One arcminute is equal to 1/60 of a degree, and one arcsecond is equal to 1/60 of an arcminute, or 1/360 of a degree. The celestial equator is at declination zero, the north celestial pole is at declination ninety, and the south celestial pole has a declination of -90°. Each star has a right ascension and a declination that mark its position in the sky. The brightest star, Sirius, for example, has right ascension six hours forty-five minutes (abbreviated as 6h. 45m.) and declination-16 degrees forty-three arcminutes
Stars are so far away from Earth that the main star motion we see results from Earth’s rotation. Stars do move in space, however, and these proper motions slightly change the coordinates of the nearest stars over time. The effects of the Sun and the Moon on Earth also cause slight changes in Earth’s axis of rotation. These changes, called precession, cause a slow drift in right ascension and declination. To account for precession, astronomers redefine the celestial coordinates every fifty years or so.
Solar systems, both our own and those located around other stars, are a major area of research for astronomers. A solar system consists of a central star orbited by planets or smaller rocky bodies. The gravitational force of the star holds the system together. In our solar system, the central star is the Sun. It holds all the planets, including Earth, in their orbits and provides light and energy necessary for life. Our solar system is just one of many. Astronomers are just beginning to be able to study other solar systems.
Our solar system contains the Sun, nine planets (of which Earth is third from the Sun), and the planets’ satellites. It also contains asteroids, comets, and interplanetary dust and gases.
Until the end of the 18th century, humans knew of five planets-Mercury, Venus, Mars, Jupiter, and Saturn-in addition to Earth. When viewed without a telescope, planets appear to be dots of light in the sky. They shine steadily, while stars seem to twinkle. Twinkling results from turbulence in Earth's atmosphere. Stars are so far away that they appear as tiny points of light. A moment of turbulence can change that light for a fraction of a second. Even though they look the same size as stars to unaided human eyes, planets are close enough that they take up more space in the sky than stars do. The disks of planets are big enough to average out variations in light caused by turbulence and therefore do not twinkle.
Between 1781 and 1930, astronomers found three more planets-Uranus, Neptune, and Pluto. This brought the total number of planets in our solar system to nine. In order of increasing distance from the Sun, the planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
Astronomers call the inner planets-Mercury, Venus, Earth, and Mars-the terrestrial planets. Terrestrial (from the Latin word terra, meaning ‘Earth’) planets are Earthlike in that they have solid, rocky surfaces. The next group of planets-Jupiter, Saturn, Uranus, and Neptune-is called the Jovian planets, or the giant planets. The word Jovian has the same Latin root as the word Jupiter. Astronomers call these planets the Jovian planets because they resemble Jupiter in that they are giant, massive planets made almost entirely of gas. The mass of Jupiter, for example, is 318 times the mass of Earth. The Jovian planets have no solid surfaces, although they probably have rocky cores several times more massive than Earth. Rings of chunks of ice and rock surround each of the Jovian planets. The rings around Saturn are the most familiar.
Pluto, the outermost planet, is tiny, with a mass about one five-hundredth the mass of Earth. Pluto seems out of place, with its tiny, solid body out beyond the giant planets. Many astronomers believe that Pluto is really just the largest, or one of the largest, of a group of icy objects in the outer solar system. These objects orbit in a part of the solar system called the Kuiper Belt. Even if astronomers decide that Pluto belongs to the Kuiper Belt objects, it will probably still be called a planet for historical reasons.
Most of the planets have moons, or satellites. Earth's Moon has a diameter about one-fourth the diameter of Earth. Mars has two tiny chunks of rock, Phobos and Deimos, each only about 10 km (about 6 mi) across. Jupiter has at least seventeen satellites. The largest four, known as the Galilean satellites, are Io, Europa, Ganymede, and Callisto. Ganymede is even larger than the planet Mercury. Saturn has at least eighteen satellites. Saturn’s largest moon, Titan, is also larger than the planet Mercury and is enshrouded by a thick, opaque, smoggy atmosphere. Uranus has at least seventeen moons, and Neptune has at least eight moons. Pluto had one moon, called Charon. Charon is more than half as big as Pluto.
Comets and asteroids are rocky and icy bodies that are smaller than planets. The distinction between comets, asteroids, and other small bodies in the solar system is a little fuzzy, but generally a comet is icier than an asteroid and has a more elongated orbit. The orbit of a comet takes it close to the Sun, then back into the outer solar system. When comets near the Sun, some of their ice turns from solid material into gas, releasing some of their dust. Comets have long tails of glowing gas and dust when they are near the Sun. Asteroids are rockier bodies and usually have orbits that keep them at always about the same distance from the Sun.
Both comets and asteroids have their origins in the early solar system. While the solar system was forming, many small, rocky objects called planetesimals condensed from the gas and dust of the early solar system. Millions of planetesimals remain in orbit around the Sun. A large spherical cloud of such objects out beyond Pluto forms the Oort cloud. The objects in the Oort cloud are considered comets. When our solar system passes close to another star or drifts closer than usual to the centre of our galaxy, the change in gravitational pull may disturb the orbit of one of the icy comets in the Oort cloud. As this comet falls toward the Sun, the ice turns into vapour, freeing dust from the object. The gas and dust form the tail or tails of the comet. The gravitational pull of large planets such as Jupiter or Saturn may swerve the comet into an orbit closer to the Sun. The time needed for a comet to make a complete orbit around the Sun is called the comet’s period. Astronomers believe that comets with periods longer than about 200 years come from the Oort Cloud. Short-period comets, those with periods less than about 200 years, probably come from the Kuiper Belt, a ring of planetesimals beyond Neptune. The material in comets is probably from the very early solar system, so astronomers study comets to find out more about our solar system’s formation.
When the solar system was forming, some of the planetesimals came together more toward the centre of the solar system. Gravitational forces from the giant planet Jupiter prevented these planetesimals from forming full-fledged planets. Instead, the planetesimals broke up to create thousands of minor planets, or asteroids, that orbit the Sun. Most of them are in the asteroid belt, between the orbits of Mars and Jupiter, but thousands are in orbits that come closer to Earth or even cross Earth's orbit. Scientists are increasingly aware of potential catastrophes if any of the largest of these asteroids hits Earth. Perhaps 2,000 asteroids larger than 1 km. (0.6 mi.) in diameter are potential hazards.
The Sun is the nearest star to Earth and is the centre of the solar system. It is only eight light-minutes away from Earth, meaning light takes only eight minutes to travel from the Sun to Earth. The next nearest star is four light-years away, so light from this star, Proxima Centauri (part of the triple star Alpha Centauri), takes four years to reach Earth. The Sun's closeness means that the light and other energy we get from the Sun dominate Earth’s environment and life. The Sun also provides a way for astronomers to study stars. They can see details and layers of the Sun that are impossible to see on more distant stars. In addition, the Sun provides a laboratory for studying hot gases held in place by magnetic fields. Scientists would like to create similar conditions (hot gases contained by magnetic fields) on Earth. Creating such environments could be useful for studying basic physics.
The Sun produces its energy by fusing hydrogen into helium in a process called nuclear fusion. In nuclear fusion, two atoms merge to form a heavier atom and release energy. The Sun and stars of similar mass start off with enough hydrogen to shine for about ten billion years. The Sun is less than halfway through its lifetime.
Although most telescopes are used mainly to collect the light of faint objects so that they can be studied, telescopes for planetary and other solar system studies are also used to magnify images. Astronomers use some of the observing time of several important telescopes for planetary studies. Overall, planetary astronomers must apply and compete for observing time on telescopes with astronomers seeking to study other objects. Some planetary objects can be studied as they pass in front of, or occult, distant stars. The atmosphere of Neptune's moon Triton and the shapes of asteroids can be investigated in this way, for example. The fields of radio and infrared astronomy are useful for measuring the temperatures of planets and satellites. Ultraviolet astronomy can help astronomers study the magnetic fields of planets.
During the space age, scientists have developed telescopes and other devices, such as instruments to measure magnetic fields or space dust, that can leave Earth's surface and travel close to other objects in the solar system. Robotic spacecraft have visited all of the planets in the solar system except Pluto. Some missions have targeted specific planets and spent much time studying a single planet, and some spacecraft have flown past a number of planets.
Astronomers use different telescopes to study the Sun than they use for nighttime studies because of the extreme brightness of the Sun. Telescopes in space, such as the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE), are able to study the Sun in regions of the spectrum other than visible light. X-rays, ultraviolet, and radio waves from the Sun are especially interesting to astronomers. Studies in various parts of the spectrum give insight into giant flows of gas in the Sun, into how the Sun's energy leaves the Sun to travel to Earth, and into what the interior of the Sun is like. Astronomers also study solar-terrestrial relations-the relation of activity on the Sun with magnetic storms and other effects on Earth. Some of these storms and effects can affect radio reception, cause electrical blackouts, or damage satellites in orbit.
Our solar system began forming about five billion years ago, when a cloud of gas and dust between the stars in our Milky Way Galaxy began contracting. A nearby supernova-an exploding star-may have started the contraction, but most astronomers believe a random change in density in the cloud caused the contraction. Once the cloud-known as the solar nebula-began to contract, the contraction occurred faster and faster. The gravitational energy caused by this contraction heated the solar nebula. As the cloud became smaller, it began to spin faster, much as a spinning skater will spin faster by pulling in his or her arms. This spin kept the nebula from forming a sphere; instead, it settled into a disk of gas and dust.
In this disk, small regions of gas and dust began to draw closer and stick together. The objects that resulted, which were usually less than 500 km (300 mi) across, are the planetesimals. Eventually, some planetesimals stuck together and grew to form the planets. Scientists have made computer models of how they believe the early solar system behaved. The models show that for a solar system to produce one or two huge planets like Jupiter and several other, much smaller planets is usual.
The largest region of gas and dust wound up in the centre of the nebula and formed the protosun (proto is Greek for ‘before’ and is used to distinguish between an object and its forerunner). The increasing temperature and pressure in the middle of the protosun vaporized the dust and eventually allowed nuclear fusion to begin, marking the formation of the Sun. The young Sun gave off a strong solar wind that drove off most of the lighter elements, such as hydrogen and helium, from the inner planets. The inner planets then solidified and formed rocky surfaces. The solar wind lost strength. Jupiter’s gravitational pull was strong enough to keep its shroud of hydrogen and helium gas. Saturn, Uranus, and Neptune also kept their layers of light gases.
The theory of solar system formation described above accounts for the appearance of the solar system as we know it. Examples of this appearance include the fact that the planets all orbit the Sun in the same direction and that almost all the planets rotate on their axes in the same direction. The recent discoveries of distant solar systems with different properties could lead to modifications in the theory, however
Studies in the visible, the infrared, and the shortest radio wavelengths have revealed disks around several young stars in our galaxy. One such object, Beta Pictoris (about sixty-two light-years from Earth), has revealed a warp in the disk that could be a sign of planets in orbit. Astronomers are hopeful that, in the cases of these young stars, they are studying the early stages of solar system formation.
Although astronomers have long assumed that many other stars have planets, they have been unable to detect these other solar systems until recently. Planets orbiting around stars other than the Sun are called extra solar planets. Planets are small and dim compared with stars, so they are lost in the glare of their parent stars and are invisible to direct observation with telescopes.
Astronomers have tried to detect other solar systems by searching for the way a planet affects the movement of its parent star. The gravitational attraction between a planet and its star pulls the star slightly toward the planet, so the star wobbles slightly as the planet orbits it. Throughout the mind and late 1900s, several observatories tried to detect wobbles in the nearest stars by watching the stars’ movement across the sky. Wobbles were reported in several stars, but later observations showed that the results were false.
In the early 1990s, studies of a pulsar revealed at least two planets orbiting it. Pulsars are compact stars that give off pulses of radio waves at very regular intervals. The pulsar, designated PSR 1257+12, is about 1,000 light-years from Earth. This pulsar's pulses sometimes came a little early and sometimes a little late in a periodic pattern, revealing that an unseen object was pulling the pulsar toward and away from Earth. The environment of a pulsar, which emits X rays and other strong radiation that would be harmful to life on Earth, is so extreme that these objects would have little resemblance to planets in our solar system.
The wobbling of a star changes the star’s light that reaches Earth. When the star moves away from Earth, even slightly, each wave of light must travel farther to Earth than the wave before it. This increases the distance between waves (called the wavelength) as the waves reach Earth. When a star’s planet pulls the star closer to Earth, each successive wavefront has less distance to travel to reach Earth. This shortens the wavelength of the light that reaches Earth. This effect is called the Doppler effect. No star moves fast enough for the change in wavelength to result in a noticeable change in colour, which depends on wavelength, but the changes in wavelength can be measured with precise instruments. Because the planet’s effect on the star is very small, astronomers must analyse the starlight carefully to detect a shift in wavelength. They do this by first using a technique called spectroscopy to separate the white starlight into its component colours, as water vapour does to sunlight in a rainbow. Stars emit light in a continuous range. The range of wavelengths a star emits is called the star’s spectrum. This spectrum had dark lines, called absorption lines, at wavelengths at which atoms in the outermost layers of the star absorb light.
Astronomers know what the exact wavelength of each absorption line is for a star that is not moving. By seeing how far the movement of a star shifts the absorption lines in its spectrum, astronomers can calculate how fast the star is moving. If the motion fits the model of the effect of a planet, astronomers can calculate the mass of the planet and how close it is to the star. These calculations can only provide the lower limit to the planet’s mass, because telling at what angle the planet orbits. The star is impossible for astronomers. Astronomers need to know the angle at which the planet orbits the star to calculate the planet’s mass accurately. Because of this uncertainty, some of the giant extra solar planets may be a type of failed star called a brown dwarf instead of planets. Most astronomers believe that many of the suspected planets are true planets.
Between 1995 and 1999 astronomers discovered more than a dozen extra solar planets. Astronomers now know of far more planets outside our solar system than inside our solar system. Most of these planets, surprisingly, are more massive than Jupiter and are orbiting so close to their parent stars that some of them have ‘years’ (the time it takes to orbit the parent star once) as long as only a few days on Earth. These solar systems are so different from our solar system that astronomers are still trying to reconcile them with the current theory of solar system formation. Some astronomers suggest that the giant extra solar planets formed much farther away from their stars and were later thrown into the inner solar systems by some gravitational interaction.
Stars are an important topic of astronomical research. Stars are balls of gas that shine or used to shine because of nuclear fusion in their cores. The most familiar star is the Sun. The nuclear fusion in stars produces a force that pushes the material in a star outward. However, the gravitational attraction of the star’s material for itself pulls the material inward. A star can remain stable as long as the outward pressure and gravitational force balance. The properties of a star depend on its mass, its temperature, and its stage in evolution.
Astronomers study stars by measuring their brightness or, with more difficulty, their distances from Earth. They measure the ‘colour’ of a star-the differences in the star’s brightness from one part of the spectrum to another-to determine its temperature. They also study the spectrum of a star’s light to determine not only the temperature, but also the chemical makeup of the star’s outer layers.
Many different types of stars exist. Some types of stars are really just different stages of a star’s evolution. Some types are different because the stars formed with much more or much less mass than other stars, or because they formed close to other stars. The Sun is a type of star known as a main-sequence star. Eventually, main-sequence stars such as the Sun swell into giant stars and then evolve into tiny, dense, white dwarf stars. Main-sequence stars and giants have a role in the behaviour of most variable stars and novas. A star much more massive than the Sun will become a supergiant star, then explode as a supernova. A supernova may leave behind a neutron star or a black hole.
In about 1910 Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently worked out a way to graph basic properties of stars. On the horizontal axis of their graphs, they plotted the temperatures of stars. On the vertical axis, they plotted the brightness of stars in a way that allowed the stars to be compared. (One plotted the absolute brightness, or absolute magnitude, of a star, a measurement of brightness that takes into account the distance of the star from Earth. The other plotted stars in a nearby galaxy, all about the same distance from Earth.)
On an H-R diagram, the brightest stars are at the top and the hottest stars are at the left. Hertzsprung and Russell found that most stars fell on a diagonal line across the H-R diagram from upper left lower to right. This line is called the main sequence. The diagonal line of main-sequence stars indicates that temperature and brightness of these stars are directly related. The hotter a main-sequence stars is, the brighter it is. The Sun is a main-sequence star, located in about the middle of the graph. More faint, cool stars exist than hot, bright ones, so the Sun is brighter and hotter than most of the stars in the universe.
At the upper right of the H-R diagram, above the main sequence, stars are brighter than main-sequence stars of the same colour. The only way stars of a certain colour can be brighter than other stars of the same colour is if the brighter stars are also bigger. Bigger stars are not necessarily more massive, but they do have larger diameters. Stars that fall in the upper right of the H-R diagram are known as giant stars or, for even brighter stars, supergiant stars. Supergiant stars have both larger diameters and larger masses than giant stars.
Giant and supergiant stars represent stages in the lives of stars after they have burned most of their internal hydrogen fuel. Stars swell as they move off the main sequence, becoming giants and—for more massive stars-supergiants.
A few stars fall in the lower left portion of the H-R diagram, below the main sequence. Just as giant stars are larger and brighter than main-sequences stars, these stars are smaller and dimmer. These smaller, dimmer stars are hot enough to be white or blue-white in colour and are known as white dwarfs.
White dwarf stars are only about the size of Earth. They represent stars with about the mass of the Sun that have burned as much hydrogen as they can. The gravitational force of a white dwarf’s mass is pulling the star inward, but electrons in the star resist being pushed together. The gravitational force is able to pull the star into a much denser form than it was in when the star was burning hydrogen. The final stage of life for all stars like the Sun is the white dwarf stage.
Many stars vary in brightness over time. These variable stars come in a variety of types. One important type is called a Cepheid variable, named after the star delta Cepheid, which is a prime example of a Cepheid variable. These stars vary in brightness as they swell and contract over a period of weeks or months. Their average brightness depends on how long the period of variation takes. Thus astronomers can determine how bright the star is merely by measuring the length of the period. By comparing how intrinsically bright these variable stars are with how bright they look from Earth, astronomers can calculate how far away these stars are from Earth. Since they are giant stars and are very bright, Cepheid variables in other galaxies are visible from Earth. Studies of Cepheid variables tell astronomers how far away these galaxies are and are very useful for determining the distance scale of the universe. The Hubble Space Telescope (HST) can determine the periods of Cepheid stars in galaxies farther away than ground-based telescopes can see. Astronomers are developing a more accurate idea of the distance scale of the universe with HST data.
Cepheid variables are only one type of variable star. Stars called long-period variables vary in brightness as they contract and expand, but these stars are not as regular as Cepheid variables. Mira, a star in the constellation Cetus (the whale), is a prime example of a long-period variable star. Variable stars called eclipsing binary stars are really pairs of stars. Their brightness varies because one member of the pair appears to pass in front of the other, as seen from Earth. A type of variable star called R Coronae Borealis stars varies because they occasionally give off clouds of carbon dust that dim these stars.
Sometimes stars brighten drastically, becoming as much as 100 times brighter than they were. These stars are called novas (Latin for ‘new stars’). They are not really new, just much brighter than they were earlier. A nova is a binary, or double, star in which one member is a white dwarf and the other is a giant or supergiant. Matter from the large star falls onto the small star. After a thick layer of the large star’s atmosphere has collected on the white dwarf, the layer burns off in a nuclear fusion reaction. The fusion produces a huge amount of energy, which, from Earth, appears as the brightening of the nova. The nova gradually returns to its original state, and material from the large star again begins to collect on the white dwarf.
Sometimes stars brighten many times more drastically than novas do. A star that had been too dim to see can become one of the brightest stars in the sky. These stars are called supernovas. Sometimes supernovas that occur in other galaxies are so bright that, from Earth, they appear as bright as their host galaxy.
There are two types of supernovas. One type is an extreme case of a nova, in which matter falls from a giant or supergiant companion onto a white dwarf. In the case of a supernova, the white dwarf gains so much fuel from its companion that the star increases in mass until strong gravitational forces cause it to become unstable. The star collapses and the core explodes, vaporizing a lot of the white dwarves and producing an immense amount of light. Only bits of the white dwarf remain after this type of supernova occurs.
The other type of supernova occurs when a supergiant star uses up all its nuclear fuel in nuclear fusion reactions. The star uses up its hydrogen fuel, but the core is hot enough that it provides the initial energy necessary for the star to begin ‘burning’ helium, then carbon, and then heavier elements through nuclear fusion. The process stops when the core is mostly iron, which is too heavy for the star to ‘burn’ in a way that gives off energy. With no such fuel left, the inward gravitational attraction of the star’s material for itself has no outward balancing force, and the core collapses. As it collapses, the core releases a shock wave that tears apart the star’s atmosphere. The core continues collapsing until it forms either a neutron star or a black hole, depending on its mass
Only a handfuls of supernovas are known in our galaxy. The last Milky Way supernova seen from Earth was observed in 1604. In 1987 astronomers observed a supernova in the Large Magellanic Cloud, one of the Milky Way’s satellite galaxies. This supernova became bright enough to be visible to the unaided eye and is still under careful study from telescopes on Earth and from the Hubble Space Telescope. A supernova in the process of exploding emits radiation in the X-ray range and ultraviolet and radio radiation studies in this part of the spectrum are especially useful for astronomers studying supernova remnants.
Neutron stars are the collapsed cores sometimes left behind by supernova explosions. Pulsars are a special type of neutron star. Pulsars and neutron stars form when the remnant of a star left after a supernova explosion collapses until it is about 10 km. (about 6 mi.) in radius. At that point, the neutrons-electrically neutral atomic particles-of the star resists being pressed together further. When the force produced by the neutrons, balances, the gravitational force, the core stops collapsing. At that point, the star is so dense that a teaspoonful has the mass of a billion metric tons.
Neutron stars become pulsars when the magnetic field of a neutron star directs a beam of radio waves out into space. The star is so small that it rotates from one to a few hundred times per second. As the star rotates, the beam of radio waves sweeps out a path in space. If Earth is in the path of the beam, radio astronomers see the rotating beam as periodic pulses of radio waves. This pulsing is the reason these stars are called pulsars.
Some neutron stars are in binary systems with an ordinary star neighbour. The gravitational pull of a neutron star pulls material off its neighbour. The rotation of the neutron star heats the material, causing it to emit X-rays. The neutron star’s magnetic field directs the X-rays into a beam that sweeps into space and may be detected from Earth. Astronomers call these stars X-ray pulsars.
Gamma-ray spacecraft detect bursts of gamma rays about once a day. The bursts come from sources in distant galaxies, so they must be extremely powerful for us to be able to detect them. A leading model used to explain the bursts are the merger of two neutron stars in a distant galaxy with a resulting hot fireball. A few such explosions have been seen and studied with the Hubble and Keck telescopes.
Black holes are objects that are so massive and dense that their immense gravitational pull does not even let light escape. If the core left over after a supernova explosion has a mass of more than about fives times that of the Sun, the force holding up the neutrons in the core is not large enough to balance the inward gravitational force. No outward force is large enough to resist the gravitational force. The core of the star continues to collapse. When the core's mass is sufficiently concentrated, the gravitational force of the core is so strong that nothing, not even light, can escape it. The gravitational force is so strong that classical physics no longer applies, and astronomers use Einstein’s general theory of relativity to explain the behaviour of light and matter under such strong gravitational forces. According to general relativity, space around the core becomes so warped that nothing can escape, creating a black hole. A star with a mass ten times the mass of the Sun would become a black hole if it were compressed to 90 km. (60 mi.) or less in diameter.
Astronomers have various ways of detecting black holes. When a black hole is in a binary system, matter from the companion star spirals into the black hole, forming a disk of gas around it. The disk becomes so hot that it gives off X rays that astronomers can detect from Earth. Astronomers use X-ray telescopes in space to find X-ray sources, and then they look for signs that an unseen object of more than about five times the mass of the Sun is causing gravitational tugs on a visible object. By 1999 astronomers had found about a dozen potential black holes.
The basic method that astronomers use to find the distance of a star from Earth uses parallax. Parallax is the change in apparent position of a distant object when viewed from different places. For example, imagine a tree standing in the centre of a field, with a row of buildings at the edge of the field behind the tree. If two observers stand at the two front corners of the field, the tree will appear in front of a different building for each observer. Similarly, a nearby star's position appears different when seen from different angles.
Parallax also allows human eyes to judge distance. Each eye sees an object from a different angle. The brain compares the two pictures to judge the distance to the object. Astronomers use the same idea to calculate the distance to a star. Stars are very far away, so astronomers must look at a star from two locations as far apart as possible to get a measurement. The movement of Earth around the Sun makes this possible. By taking measurements six months apart from the same place on Earth, astronomers take measurements from locations separated by the diameter of Earth’s orbit. That is a separation of about 300 million km (186 million mi). The nearest stars will appear to shift slightly with respect to the background of more distant stars. Even so, the greatest stellar parallax is only about 0.77 seconds of arc, an amount 4,600 times smaller than a single degree. Astronomers calculate a star’s distance by dividing one by the parallax. Distances of stars are usually measured in parsecs. A parsec is 3.26 light-years, and a light-year is the distance that light travels in a year, or about 9.5 trillion km (5.9 trillion mi). Proxima Centauri, the Sun’s nearest neighbour, has a parallax of 0.77 seconds of arc. This measurement indicates that Proxima Centauri’s distance from Earth is about 1.3 parsecs, or 4.2 light -years. Because Proxima Centauri is the Sun’s nearest neighbours, it has a larger parallax than any other star.
Astronomers can measure stellar parallaxes for stars up to about 500 light-years away, which is only about 2 percent of the distance to the centre of our galaxy. Beyond that distance, the parallax angle is too small to measure.
A European Space Agency spacecraft named Hipparcos (an acronym for High Precision Parallax Collecting Satellite), launched in 1989, gave a set of accurate parallaxes across the sky that was released in 1997. This set of measurements has provided a uniform database of stellar distances for more than 100,000 stars and to some degree less accurate database of more than one million stars. These parallax measurements provide the base for measurements of the distance scale of the universe. Hipparcos data are leading to more accurate age calculations for the universe and for objects in it, especially globular clusters of stars.
Astronomers use a star’s light to determine the star’s temperature, composition, and motion. Astronomers analyse a star’s light by looking at its intensity at different wavelengths. Blue light has the shortest visible wavelengths, at about 400 nanometres. (A nanometre, abbreviated ‘nm’, is one billionth of a metre, or about one forty-thousandth of an inch.) Red light has the longest visible wavelengths, at about 650 nm. A law of radiation known as Wien's displacement law (developed by German physicist Wilhelm Wien) links the wavelength at which the most energy is given out by an object and its temperature. A star like the Sun, whose surface temperature is about 6000 K (about 5730°C or about 10,350°F), gives off the most radiation in yellow-green wavelengths, with decreasing amounts in shorter and longer wavelengths. Astronomers put filters of different standard colours on telescopes to allow only light of a particular colour from a star to pass. In this way, astronomers determine the brightness of a star at particular wavelengths. From this information, astronomers can use Wien’s law to determine the star’s surface temperature.
Astronomers can see the different wavelengths of light of a star in more detail by looking at its spectrum. The continuous rainbow of colour of a star's spectrum is crossed by dark lines, or spectral lines. In the early 19th century, German physicist Josef Fraunhofer identified such lines in the Sun's spectrum, and they are still known as Fraunhofer lines. American astronomer Annie Jump Cannon divided stars into several categories by the appearance of their spectra. She labelled them with capital letters according to how dark their hydrogen spectral lines were. Later astronomers reordered these categories according to decreasing temperature. The categories are O, B, A, F, G, K, and M, where O stars are the hottest and M stars are the coolest. The Sun is a G star. An additional spectral type, L stars, was suggested in 1998 to accommodate some cool stars studied using new infrared observational capabilities. Detailed study of spectral lines shows the physical conditions in the atmospheres of stars. Careful study of spectral lines shows that some stars have broader lines than others of the same spectral type. The broad lines indicate that the outer layers of these stars are more diffuse, meaning that these layers are larger, but spread more thinly, than the outer layers of other stars. Stars with large diffuse atmospheres are called giants. Giant stars are not necessarily more massive than other stars-the outer layers of giant stars are just more spread out.
Many stars have thousands of spectral lines from iron and other elements near iron in the periodic table. Other stars of the same temperature have very few spectral lines from such elements. Astronomers interpret these findings to mean that two different populations of stars exist. Some formed long ago, before supernovas produced the heavy elements, and others formed more recently and incorporated some heavy elements. The Sun is one of the more recent stars.
Spectral lines can also be studied to see if they change in wavelength or are different in wavelength from sources of the same lines on Earth. These studies tell us, according to the Doppler effect, how much the star is moving toward or away from us. Such studies of starlight can tell us about the orbits of stars in binary systems or about the pulsations of variable stars, for example.
Astronomers study galaxies to learn about the structure of the universe. Galaxies are huge collections of billions of stars. Our Sun is part of the Milky Way Galaxy. Galaxies also contain dark strips of dust and may contain huge black holes at their centres. Galaxies exist in different shapes and sizes. Some galaxies are spirals, some are oval, or elliptical, and some are irregular. The Milky Way is a spiral galaxy. Galaxies tend to group together in clusters.
Our Sun is only one of about 400 billion stars in our home galaxy, the Milky Way. On a dark night, far from outdoor lighting, a faint, hazy, whitish band spans the sky. This band is the Milky Way Galaxy as it appears from Earth. The Milky Way looks splotchy, with darker regions interspersed with lighter ones.
The Milky Way Galaxy is a pinwheel-shaped flattened disk about 75,000 light-years in diameter. The Sun is located on a spiral arm about two-thirds of the way out from the centre. The galaxy spins, but the centre spins faster than the arms. At Earth’s position, the galaxy makes a complete rotation about every 200 million years.
When observers on Earth look toward the brightest part of the Milky Way, which is in the constellation Sagittarius, they look through the galaxy’s disk toward its centre. This disk is composed of the stars, gas, and dust between Earth and the galactic centre. When observers look in the sky in other directions, they do not see as much of the galaxy’s gas and dust, and so can see objects beyond the galaxy more clearly.
The Milky Way Galaxy has a core surrounded by its spiral arms. A spherical cloud containing about 100 examples of a type of star cluster known as a globular cluster surrounds the galaxy. Still, farther out is a galactic corona. Astronomers are not sure what types of particles or objects occupy the corona, but these objects do exert a measurable gravitational force on the rest of the galaxy. Galaxies contain billions of stars, but the space between stars is not empty. Astronomers believe that almost every galaxy probably has a huge black hole at its centre.
The space between stars in a galaxy consists of low-density gas and dust. The dust is largely carbon given off by red-giant stars. The gas is largely hydrogen, which accounts for 90 percent of the atoms in the universe. Hydrogen exists in two main forms in the universe. Astronomers give complete hydrogen atoms, with a nucleus and an electron, a designation of the Roman numeral I, or HI. Ionized hydrogen, hydrogen made up of atoms missing their electrons, is given the designation II, or HII. Clouds, or regions, of both types of hydrogen exist between the stars. HI regions are too cold to produce visible radiation, but they do emit radio waves that are useful in measuring the movement of gas in our own galaxy and in distant galaxies. The HII regions form around hot stars. These regions emit diffuse radiation in the visual range, as well as in the radio, infrared, and ultraviolet ranges. The cloudy light from such regions forms beautiful nebulas such as the Great Orion Nebula.
Astronomers have located more than 100 types of molecules in interstellar space. These molecules occur only in trace amounts among the hydrogens. Still, astronomers can use these molecules to map galaxies. By measuring the density of the molecules throughout a galaxy, astronomers can get an idea of the galaxy’s structure. interstellar dust sometimes gathers to form dark nebulae, which appear in silhouette against background gas or stars from Earth. The Horsehead Nebula, for example, is the silhouette of interstellar dust against a background HI region.
The first known black holes were the collapsed cores of supernova stars, but astronomers have since discovered signs of much larger black holes at the centres of galaxies. These galactic black holes contain millions of times as much mass as the Sun. Astronomers believe that huge black holes such as these provide the energy of mysterious objects called quasars. Quasars are very distant objects that are moving away from Earth at high speed. The first ones discovered were very powerful radio sources, but scientists have since discovered quasars that don’t strongly emit radio waves. Astronomers believe that almost every galaxy, whether spiral or elliptical, has a huge black hole at its centre.
Astronomers look for galactic black holes by studying the movement of galaxies. By studying the spectrum of a galaxy, astronomers can tell if gas near the centre of the galaxy is rotating rapidly. By measuring the speed of rotation and the distance from various points in the galaxy to the centre of the galaxy, astronomers can determine the amount of mass in the centre of the galaxy. Measurements of many galaxies show that gas near the centre is moving so quickly that only a black hole could be dense enough to concentrate so much mass in such a small space. Astronomers suspect that a significant black hole occupies even the centre of the Milky Way. The clear images from the Hubble Space Telescope have allowed measurements of motions closer to the centres of galaxies than previously possible, and have led to the confirmation in several cases that giant black holes are present.
Galaxies are classified by shape. The three types are spiral, elliptical, and irregular. Spiral galaxies consist of a central mass with one, two, or three arms that spiral around the centre. An elliptical galaxy is oval, with a bright centre that gradually, evenly dims to the edges. Irregular galaxies are not symmetrical and do not look like spiral or elliptical galaxies. Irregular galaxies vary widely in appearance. A galaxy that has a regular spiral or elliptical shape but has, some special oddity is known as a peculiar galaxy. For example, some peculiar galaxies are stretched and distorted from the gravitational pull of a nearby galaxy.
Spiral galaxies are flattened pinwheels in shape. They can have from one to three spiral arms coming from a central core. The Great Andromeda Spiral Galaxy is a good example of a spiral galaxy. The shape of the Milky Way is not visible from Earth, but astronomers have measured that the Milky Way is also a spiral galaxy. American astronomer Edwin Hubble further classified spirals galaxies by the tightness of their spirals. In order of increasingly open arms, Hubble’s types are Sa, Sb., and Sc. Some galaxies have a straight, bright, bar-shaped feature across their centre, with the spiral arms coming off the bar or off a ring around the bar. With a capital B for the bar, the Hubble types of these galaxies are SBa, SBb, and Sbc.
Many clusters of galaxies have giant elliptical galaxies at their centres. Smaller elliptical galaxies, called dwarf elliptical galaxies, are much more common than giant ones. Most of the two dozen galaxies in the Milky Way’s Local Group of galaxies are dwarf elliptical galaxies.
Astronomers classify elliptical galaxies by how oval they look, ranging from E0 for very round to E3 for intermediately oval to E7 for extremely elongated. The galaxy class E7 is also called S0, which is also known as a lenticular galaxy, a shape with an elongated disk but no spiral arms. Because astronomers can see other galaxies only from the perspective of Earth, the shape astronomers see is not necessarily the exact shape of a galaxy. For instance, they may be viewing it from an end, and not from above or below.
Some galaxies have no structure, while others have some trace of structure but do not fit the spiral or elliptical classes. All of these galaxies are called irregular galaxies. The two small galaxies that are satellites to the Milky Way Galaxy are both irregular. They are known as the Magellanic Clouds. The Large Magellanic Cloud shows signs of having a bar in its centre. The Small Magellanic Cloud is more formless. Studies of stars in the Large and Small Magellanic Clouds have been fundamental for astronomers’ understanding of the universe. Each of these galaxies provides groups of stars that are all at the same distance from Earth, allowing astronomers to compare the absolute brightness of these stars.
In the late 1920s American astronomer Edwin Hubble discovered that all but the nearest galaxies to us are receding, or moving away from us. Further, he found that the farther away from Earth a galaxy is, the faster it is receding. He made his discovery by taking spectra of galaxies and measuring the amount by which the wavelengths of spectral lines were shifted. He measured distance in a separate way, usually from studies of Cepheid variable stars. Hubble discovered that essentially all the spectra of all the galaxies were shifted toward the red, or had red-shifts. The red-shifts of galaxies increased with increasing distance from Earth. After Hubble’s work, other astronomers made the connection between red-shift and velocity, showing that the farther a galaxy is from Earth, the faster it moves away from Earth. This idea is called Hubble’s law and is the basis for the belief that the universe is uniformly expanding. Other uniformly expanding three-dimensional objects, such as a rising cake with raisins in the batter, also demonstrate the consequence that the more distant objects (such as the other raisins with respect to any given raisin) appear to recede more rapidly than nearer ones. This consequence is the result of the increased amount of material expanding between these more distant objects.
Hubble's law state that there is a straight-line, or linear, relationship between the speed at which an object is moving away from Earth and the distance between the object and Earth. The speed at which an object is moving away from Earth is called the object’s velocity of recession. Hubble’s law indicates that as velocity of recession increases, distance increases by the same proportion. Using this law, astronomers can calculate the distance to the most-distant galaxies, given only measurements of their velocities calculated by observing how much their light is shifted. Astronomers can accurately measure the red-shifts of objects so distant that the distance between Earth and the objects cannot be measured by other means.
The constant of proportionality that relates velocity to distance in Hubble's law is called Hubble's constant, or H. Hubble's law is often written v Hd, or velocity equals Hubble's constant multiplied by distance. Thus determining Hubble's constant will give the speed of the universe's expansion. The inverse of Hubble’s constant, or 1/H, theoretically provides an estimate of the age of the universe. Astronomers now believe that Hubble’s constant has changed over the lifetime of the universe, however, so estimates of expansion and age must be adjusted accordingly.
The value of Hubble’s constant probably falls between sixty-four and 78 kilometres per second per mega-parsec (between forty and 48 miles per second per mega-parsec). A mega-parsec is one million parsecs and a parsec is 3.26 light-years. The Hubble Space Telescope studied Cepheid variables in distant galaxies to get an accurate measurement of the distance between the stars and Earth to refine the value of Hubble’s constant. The value they found is 72 kilometres per second per mega-parsec (45 miles per second per mega-parsec), with an uncertainty of only 10 percent
The actual age of the universe depends not only on Hubble's constant but also on how much the gravitational pull of the mass in the universe slows the universe’s expansion. Some data from studies that use the brightness of distant supernovas to assess distance indicate that the universe's expansion is speeding up instead of slowing. Astronomers invented the term ‘dark energy’ for the unknown cause of this accelerating expansion and are actively investigating these topics. The ultimate goal of astronomers is to understand the structure, behaviour, and evolution of all of the matter and energy that exist. Astronomers call the set of all matter and energy the universe. The universe is infinite in space, but astronomers believe it does have a finite age. Astronomers accept the theory that about fourteen billion years ago the universe began as an explosive event resulting in a hot, dense, expanding sea of matter and energy. This event is known as the big bang Astronomers cannot observe that far back in time. Many astronomers believe, however, the theory that within the first fraction of a second after the big bang, the universe went through a tremendous inflation, expanding many times in size, before it resumed a slower expansion.
As the universe expanded and cooled, various forms of elementary particles of matter formed. By the time the universe was one second old, protons had formed. For approximately the next 1,000 seconds, in the era of nucleosynthesis, all the nuclei of deuterium (hydrogen with both a proton and neutron in the nucleus) that are present in the universe today formed. During this brief period, some nuclei of lithium, beryllium, and helium formed as well.
When the universe was about one million years old, it had cooled to about 3000 K (about 3300°C or about 5900°F). At that temperature, the protons and heavier nuclei formed during nucleosynthesis could combine with electrons to form atoms. Before electrons combined with nuclei, the travel of radiation through space was very difficult. Radiation in the form of photons (packets of light energy) could not travel very far without colliding with electrons. Once protons and electrons combined to form hydrogen, photons became able to travel through space. The radiation carried by the photons had the characteristic spectrum of a hot gas. Since the time this radiation was first released, it has cooled and is now 3 K (-270°C or-450°F). It is called the primeval background radiation and has been definitively detected and studied, first by radio telescopes and then by the Cosmic Background Explorer (COBE) and Wilkinson Microwave Anisotropy Probe (WMAP) spacecrafts. COBE, WMAP, and ground-based radio telescopes detected tiny deviations from uniformity in the primeval background radiation; these deviations may be the seeds from which clusters of galaxies grew.
The gravitational force from invisible matter, known as dark matter, may have helped speed the formation of structure in the universe. Observations from the Hubble Space Telescope have revealed older galaxies than astronomers expected, reducing the interval between the big bang and the formation of galaxies or clusters of galaxies.
From about two billion years after the big bang for another two billion years, quasars formed as active giant black holes in the cores of galaxies. These quasars gave off radiation as they consumed matter from nearby galaxies. Few quasars appear close to Earth, so quasars must be a feature of the earlier universe.
A population of stars formed out of the interstellar gas and dust that contracted to form galaxies. This first population, known as Population II, was made up almost entirely of hydrogen and helium. The stars that formed evolved and gave out heavier elements that were made through fusion in the stars’ cores or that was formed as the stars exploded as supernovas. The later generation of stars, to which the Sun belongs, is known as Population I and contains heavy elements formed by the earlier population. The Sun formed about five billion years ago and is almost halfway through its 11-billion-year lifetime
About 4.6 billion years ago, our solar system formed. The oldest fossils of a living organism date from about 3.5 billion years ago and represent Cyanobacteria. Life evolved, and sixty-five million years ago, the dinosaurs and many other species were extinguished, probably from a catastrophic meteor impact. Modern humans evolved no earlier than a few hundred thousand years ago, a blink of an eye on the cosmic timescale.
Will the universe expand forever or eventually stop expanding and collapse in on itself? Jay M. Pasachoff, professor of astronomy at Williams College in Williamstown, Massachusetts, confronts this question in this discussion of cosmology. Whether the universe will go on expanding forever, depends on whether there is enough critical density to halt or reverse the expansion, and the answer to that question may, in turn, depend on the existence of something the German-born American physicist Albert Einstein once labelled the cosmological constant.
New technology allows astronomers to peer further into the universe than ever before. The science of cosmology, the study of the universe as a whole, has become an observational science. Scientists may now verify, modify, or disprove theories that were partially based on guesswork.
In the 1920s, the early days of modern cosmology, it took an astronomer all night at a telescope to observe a single galaxy. Current surveys of the sky will likely compile data for a million different galaxies within a few years. Building upon advances in cosmology over the past century, our understanding of the universe should continue to accelerate
Modern cosmology began with the studies of Edwin Hubble, who measured the speeds that galaxies move toward or away from us in the mid-1920s. By observing red-shift-the change in wavelength of the light that galaxies give off as they move away from us-Hubble realized that though the nearest galaxies are approaching us, all distant galaxies are receding. The most-distant galaxies are receding most rapidly. This observation is consistent with the characteristics of an expanding universe. Since 1929 an expanding universe has been the first and most basic pillar of cosmology.
In 1990 the National Aeronautics and Space Administration (NASA) launched the Hubble Space Telescope (HST), named to honour the pioneer of cosmology. Appropriately, determining the rate at which the universe expands was one of the telescope’s major tasks.
One of the HST’s key projects was to study Cepheid variables (stars that varies greatly in brightness) and to measure distances in space. Another set of Hubble’s observations focuses on supernovae, exploding stars that can be seen at very great distances because they are so bright. Studies of supernovae in other galaxies reveal the distances to those galaxies.
The term big bang refers to the idea that the expanding universe can be traced back in time to an initial explosion. In the mid-1960s, physicists found important evidence of the big bang when they detected faint microwave radiation coming from every part of the sky. Astronomers think this radiation originated about 300,000 years after the big bang, when the universe thinned enough to become transparent. The existence of cosmic microwave background radiation, and its interpretation, is the second pillar of modern cosmology.
Also in the 1960s, astronomers realized that the lightest of the elements, including hydrogen, helium, lithium, and boron, were formed mainly at the time of the big bang. What is most important, deuterium (the form of hydrogen with an extra neutron added to normal hydrogen's single proton) was formed only in the era of nucleosynthesis? This era started about one second after the universe was formed and made up the first three minutes or so after the big bang. No sources of deuterium are known since that early epoch. The current ratio of deuterium to regular hydrogen depends on how dense the universe was at that early time, so studies of the deuterium that can now be detected indicate how much matter the universe contains. These studies of the origin of the light elements are the third pillar of modern cosmology.
Until recently many astronomers disagreed on whether the universe was expected to expand forever or eventually stop expanding and collapse in on itself in a ‘big crunch.’
At the General Assembly of the International Astronomical Union (IAU) held in August 2000, a consistent picture of cosmology emerged. This picture depends on the current measured value for the expansion rate of the universe and on the density of the universe as calculated from the abundances of the light elements. The most recent studies of distant supernovae seem to show that the universe's expansion is accelerating, not slowing. Astronomers have recently proposed a theoretical type of negative energy-which would provide a force that opposes the attraction of gravity-to explain the accelerating universe.
For decades scientists have debated the rate at which the universe is expanding. We know that the further away a galaxy is, the faster it moves away from us. The question is: How fast are galaxies receding for each unit of distance they are away from us? The current value, as announced at the IAU meeting, is 75 km/s/Mpc, that is, for each mega-parsec of distance from us (where each mega-parsec is 3.26 million light-years), the speed of expansion increases by 75 kilometres per second.
What’s out there, exactly?
In the picture of expansion held until recently, astronomers thought the universe contained just enough matter and energy so that it would expand forever but expand at a slower and slower rate as time went on. The density of matter and energy necessary for this to happen is known as the critical density.
Astronomers now think that only 5 percent or so of the critical density of the universe is made of ordinary matter. Another 25 percent or so of the critical density is made of dark matter, a type of matter that has gravity but that has not been otherwise detected. The accelerating universe, further, shows that the remaining 70 percent of the critical density is made of a strange kind of energy, perhaps that known as the cosmological constant, an idea tentatively invoked and then abandoned by Albert Einstein in equations for his general theory of relativity.
Some may be puzzled: Didn't we learn all about the foundations of physics when we were still at school? The answer is ‘yes’ or ‘no’, depending on the interpretation. We have become acquainted with concepts and general relations that enable us to comprehend an immense range of experiences and make them accessible to mathematical treatment. In a certain sense these concepts and relations are probably even final. This is true, for example, of the laws of light refraction, of the relations of classical thermodynamics as far as it is based on the concepts of pressure, volume, temperature, heat and work, and of the hypothesis of the nonexistence of a perpetual motion machine.
What, then, impels us to devise theory after theory? Why do we devise theories at all? The answer to the latter question is simple: Because we enjoy ‘comprehending’, i.e., reducing phenomena by the process of logic to something already known or (apparently) evident. New theories are first of all necessary when we encounter new facts that cannot be ‘explained’ by existing theories. Nevertheless, this motivation for setting up new theories is, so to speak, trivial, imposed from without. There is another, more subtle motive of no less importance. This is the striving toward unification and simplification of the premises of the theory as a whole (i.e., Mach's principle of economy, interpreted as a logical principle).
There exists a passion for comprehension, just as there exists a passion for music. That passion is altogether common in children, but gets lost in most people later on. Without this passion, there would be neither mathematics nor natural science. Time and again the passion for understanding has led to the illusion that man is able to comprehend the objective world rationally, by pure thought, without any empirical foundations-in short, by metaphysics. I believe that every true theorist is a kind of tamed metaphysicist, no matter how pure ‘positivist’, he may fancy himself. The metaphysicist believes that the logically simple are also the real. The tamed metaphysicist believes that not all that is logically simple is embodied in experienced reality, but that the totality of all sensory experience can be ‘comprehended’ on the basis of a conceptual system built on premises of great simplicity. The skeptic will say that this is a ‘miracle creed’. Admittedly so, but it is a miracle creed that has been borne out to an amazing extent by the development of science.
The rise of atomism is a good example. How may Leucippus have conceived this bold idea? When water freezes and becomes ice-apparently something entirely different from water-why is it that the thawing of the ice forms something that seems indistinguishable from the original water? Leucippus is puzzled and looks for an ‘explanation’. He is driven to the conclusion that in these transitions the ‘essence’, of the thing has not changed at all. Maybe the thing consists of immutable particles and the change is only a change in their spatial arrangement. Could it not be that the same is true of all material objects that emerge again and again with nearly identical qualities?
This idea is not entirely lost during the long hibernation of occidental thought. Two thousand years after Leucippus, Bernoulli wonders why gas exerts pressure on the walls of a container. Should this be ‘explained’ by mutual repulsion of the parts of the gas, in the sense of Newtonian mechanics? This hypothesis appears absurd, for the gas pressure depends on the temperature, all other things being equal. To assume that the Newtonian forces of interaction depend on temperature is contrary to the spirit of Newtonian mechanics. Since Bernoulli is aware of the concept of atomism, he is bound to conclude that the atoms (or molecules) collide with the walls of the container and in doing so exert pressure. After all, one has to assume that atoms are in motion; how else can one account for the varying temperature of gases?
A simple mechanical consideration shows that this pressure depends only on the kinetic energy of the particles and on their density in space. This should have led the physicists of that age to the conclusion that heat consists in random motion of the atoms. Had they taken this consideration as seriously as it deserved to be taken, the development of the theory of heat-in particular the discovery of the equivalence of heat and mechanical energy-would have been considerably facilitated.
This example is meant to illustrate two things. The theoretical idea (atomism in this case) does not arise apart and independent of experience; nor can it be derived from experience by a purely logical procedure. It is produced by a creative act. Once a theoretical idea has been acquired, one does well to hold fast to it until it leads to an untenable conclusion.
In Newtonian physics the elementary theoretical concept on which the theoretical description of material bodies is based is the material point, or particle. Thus, matter is considered theoretically to be discontinuous. This makes it necessary to consider the action of material points on one another as ‘action at a distance’. Since the latter concept seems quite contrary to everyday experience, it is only natural that the contemporaries of Newton-and in fact, Newton himself found it difficult to accept. Owing to the almost miraculous success of the Newtonian system, however, the succeeding generations of physicists became used to the idea of action at a distance. Any doubt was buried for a long time to come.
All the same, when, in the second half of the 19th century, the laws of electrodynamics became known, it turned out that these laws could not be satisfactorily incorporated into the Newtonian system. It is fascinating to muse: Would Faraday have discovered the law of electromagnetic induction if he had received a regular college education? Unencumbered by the traditional way of thinking, he felt that the introduction of the ‘field’ as an independent element of reality helped him to coordinate the experimental facts. It was Maxwell who fully comprehended the significance of the field concept; he made the fundamental discovery that the laws of electrodynamics found their natural expression in the differential equations for the electric and magnetic fields. These equations implied the existence of waves, whose properties corresponded to those of light as far as they were known at that time.
This incorporation of optics into the theory of electromagnetism represents one of the greatest triumphs in the striving toward unification of the foundations of physics; Maxwell achieved this unification by purely theoretical arguments, long before it was corroborated by Hertz' experimental work. The new insight made it possible to dispense with the hypothesis of action at a distance, at least in the realm of electromagnetic phenomena; the intermediary field now appeared as the only carrier of electromagnetic interaction between bodies, and the field's behaviour was completely determined by contiguous processes, expressed by differential equations.
Now a question arose: Since the field exists even in a vacuum, should one conceive of the field as a state of a ‘carrier’, or should it be endowed with an independent existence not reducible to anything else? In other words, is there an ‘ether’ which carries the field; the ether being considered in the undulatory state, for example, when it carries light waves?
The question has a natural answer: Because one cannot dispense with the field concept, not introducing in addition a carrier with hypothetical properties is preferable. However, the pathfinder who first recognized the indispensability of the field concept were still too strongly imbued with the mechanistic tradition of thought to accept unhesitatingly this simple point of view. Nevertheless, in the course of the following decades this view imperceptibly took hold.
The introduction of the field as an elementary concept gave rise to an inconsistency of the theory as a whole. Maxwell's theory, although adequately describing the behaviour of electrically charged particles in their interaction with one another, does not explain the behaviours of electrical densities, i.e., it does not provide a theory of the particles themselves. They must therefore be treated as mass points on the basis of the old theory. The combination of the idea of a continuous field with that of material points discontinuous in space appears inconsistent. A consistent field theory requires continuity of all elements of the theory, not only in time but also in space, and in all points of space. Hence the material particle has no place as a fundamental concept in a field theory. Thus, even apart from the fact that gravitation is not included. Maxwell’s electrodynamics cannot be considered a complete theory.
Maxwell's equations for empty space remain unchanged if the spatial coordinates and the time are subjected to a particular linear transformations-the Lorentz transformations (‘covariance’ with respect to Lorentz transformations). Covariance also holds, of course, for a transformation that is composed of two or more such transformations; this is called the ‘group’ property of Lorentz transformations.
Maxwell's equations imply the ‘Lorentz group’, but the Lorentz group does not imply Maxwell's equations. The Lorentz group may effectively be defined independently of Maxwell's equations as a group of linear transformations that leave a particular value of the velocity-the velocity of light-invariant. These transformations hold for the transition from one ‘inertial system to another that is in uniform motion relative to the first. The most conspicuous novel property of this transformation group is that it does away with the absolute character of the concept of simultaneity of events distant from each other in space. On this account it is to be expected that all equations of physics are covariant with respect to Lorentz transformations (special theory of relativity). Thus it came about that Maxwell's equations led to a heuristic principle valid far beyond the range of the applicability or even validity of the equations themselves.
Special relativity has this in common with Newtonian mechanics: The laws of both theories are supposed to hold only with respect to certain coordinate systems: those known as ‘inertial systems’. An inertial system is a system in a state of motion such that ‘force-free’ material points within it are not accelerated with respect to the coordinate system. However, this definition is empty if there is no independent means for recognizing the absence of forces. Nonetheless, such a means of recognition does not exist if gravitation is considered as a ‘field’.
Let ‘A’ be a system uniformly accelerated with respect to an ‘inertial system’ I. Material points, not accelerated with respect to me, are accelerated with respect to ‘A’, the acceleration of all the points being equal in magnitude and direction. They behave as if a gravitational field exists with respect to ‘A’, for it is a characteristic property of the gravitational field that the acceleration is independent of the particular nature of the body. There is no reason to exclude the possibility of interpreting this behaviour as the effect of a ‘true’ gravitational field (principle of equivalence). This interpretation implies that ‘A’ is an ‘inertial system,’ even though it is accelerated with respect to another inertial system. (It is essential for this argument that the introduction of independent gravitational fields is considered justified even though no masses generating the field are defined. Therefore, to Newton such an argument would not have appeared convincing.) Thus the concepts of inertial system, the law of inertia and the law of motion are deprived of their concrete meaning-not only in classical mechanics but also in special relativity. Moreover, following up this train of thought, it turns out that with respect to A time cannot be measured by identical clocks; effectively, even the immediate physical significance of coordinate differences is generally lost. In view of all these difficulties, should one not try, after all, to hold on to the concept of the inertial system, relinquishing the attempt to explain the fundamental character of the gravitational phenomena that manifest themselves in the Newtonian system as the equivalence of inert and gravitational mass? Those who trust in the comprehensibility of nature must answer: No.
This is the gist of the principle of equivalence: In order to account for the equality of inert and gravitational mass within the theory admitting nonlinear transformations of the four coordinates is necessary. That is, the group of Lorentz transformations and hence the set of the "permissible" coordinate systems has to be extended.
What group of coordinate transformations can then be substituted for the group of Lorentz transformations? Mathematics suggests an answer that is based on the fundamental investigations of Gauss and Riemann: namely, that the appropriate substitute is the group of all continuous (analytical) transformations of the coordinates. Under these transformations the only thing that remains invariant is the fact that neighbouring points have nearly the same coordinates; the coordinate system expresses only the topological order of the points in space (including its four-dimensional character). The equations expressing the laws of nature must be covariant with respect to all continuous transformations of the coordinates. This is the principle of general relativity.
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