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People of the ancient world thought that stars were tiny lights on the inner side of a great, hollow globe. They made up stories about them and gave names to the patterns that they saw in the sky night after night and year after year (see Astrology; Constellation). Only with the birth of the modern science of astronomy did the true nature of the universe begin to reveal itself (see Astronomy). Scientists still cannot say exactly what a star is. They do, however, know many facts about these myriad companions to the sun, which lights and warms the Earth. The star that we know best is our own sun. It is the center of our solar system, and the Earth revolves around it. The sun is only one among billions of stars. Likewise, our solar system is only a small segment of the great galaxy we call the Milky Way. Many other galaxies are visible through telescopes. (See also Sun; Planets.)

Nature of the Stars

Astronomers generally agree that most stars have approximately the same diameter as our sun. Some, however, are only one tenth its size; while others may be more than 100 times as large. Stars are actually great globes of incandescent gases--their brightness depending upon their size and temperature. These glowing spheres are enormous powerhouses of atomic energy, and it is now believed that this energy is released by a process similar to the thermonuclear reaction that takes place in a hydrogen bomb explosion (see Nuclear Energy). The chemical content of a star is determined through the science that is known as astrophysics. In many stars the gases may be unbelievably thin, with the particles or atoms of matter in the gas far enough apart to make it a thousand times less dense than the air we breathe. Yet, for all its thinness, matter is there, perhaps a million times as much as we have in the Earth. Hydrogen, oxygen, and nitrogen are there, and perhaps iron, calcium, and other elements too. In cooler stars the matter may be more nearly liquid, more like the boiling iron in a blast furnace. In some old and comparatively cold stars, the matter may be packed so densely that a cubic inch of it would weigh a ton. Such stars are called dead or dark. Astrophysicists determine these facts with spectroscopes. With these instruments they can tell from the light a star gives what kinds of matter it contains and how hot it is (see Spectrum and Spectroscope). How do astronomers locate dead stars that give no light? Some are detected because they are near bright stars, and gravitation keeps the two swinging around each other. From the motion of the bright star, the nature of the dark star can be determined. In some double stars, or binaries, the dark one swings regularly in front of the bright one and cuts down the light. Such a pair is called a variable star, or eclipsing binary. Some dark stars give off infrared radiation that can be photographed.

The Number of Stars

Astronomers can only estimate the total number of stars in the universe. One way this is done is to measure the amount of light and other effects given by a known number of stars and compare these with the effect from the entire sky. Some astronomers say that the Milky Way alone has more than 100 billion stars and that the Milky Way consists merely of the stars nearest us. These are gathered into an enormous cluster called a galaxy. Farther out there are estimated to be billions of other galaxies (sometimes called extragalactic nebulae). If the estimate for the Milky Way alone is near the truth, the total number of stars must be inconceivably large. (For more information about galaxies, see Astronomy; Solar System.)

Distances to the Stars

Astronomers measure the tremendous distances to the stars in light-years. (One light-year is the distance light travels in a year, at the rate of 186,000 miles a second.) The nearest visible star is Alpha Centauri, seen in the Southern Hemisphere. It is 41/3 light-years away. In the same constellation is a smaller and perhaps nearer star, Proxima Centauri, which can be seen only with a telescope. Countless other galaxies with stars are thought to be spaced about 1 million light-years apart. Most stars are so far away that they do not seem to change position even when seen from points on opposite sides of the Earth's orbit, 186 million miles apart. In a telescope, some nearer stars shift slightly among their neighbors when seen from these points. This shift is called the annual stellar parallax. From it, distances to the nearer stars can be learned. The first correct parallax determination of distance was made by the German astronomer Friedrich W. Bessel in 1838.

Movements of the Stars

If we look at the stars and then look at them again about an hour or so later, we see that all but the pole star, or North Star, have changed positions in the heavens. This change is caused by the rotation of the Earth on its axis. The stars seem to wheel across the sky from east to west, because the Earth is turning beneath them from west to east. Otherwise the stars seem always to hold the same positions relative to each other (except for parallactic motion). For this reason, the ancients believed that most of the stars were fixed. They could see only a few bright ones, called planets, that moved. If Christopher Columbus were alive today, he would see scarcely any change in the position of the stars since the day he arrived in the New World. However, delicate tests show that the stars are moving at tremendous speeds. Our sun and solar system are moving through space toward the constellation of Hercules at about 12 miles per second. A star's motion is measured against the general background of stars, or the celestial sphere, by correcting the apparent motion for the effect of the solar system's motion. The result is the star's proper motion.

Some Important Stars

The brightest of all the stars is Sirius, the Dog Star. It is more than three times the size of our sun; and it is about 9 light-years away. It is seen best early in March, in the southern sky. In 1844 Bessel announced that Sirius has an unseen companion star about half as large. The pair is rushing toward the Earth at about 360 miles a minute. Bessel's telescope was not powerful enough to separate them; but an American, Alvan G. Clark, found the companion star with a telescope he had built. The companion of Sirius does not block the light which Sirius sends to the Earth. The bright star Algol, in the constellation Perseus, is an eclipsing binary. For about two and a half days, Algol is almost as bright as the polestar. Suddenly the companion star comes between Algol and the Earth, and the light is reduced by about two thirds for a few hours. Today astronomers know more than 50 such eclipsing variables, or binaries. More than 13,000 double stars have been observed. The change of light due to eclipse is not the same as twinkling. This occurs to all stars, because changes in atmospheric conditions alter the refraction of starlight and change its brightness. The most important star for navigators is Polaris, the polestar, or North Star (see Directions). It is not among the brightest stars, being at an immense distance (about 680 light-years). Powerful telescopes show that it is really three stars in a group.

Magnitudes and Sizes of Stars

Stars are ordinarily classified by magnitudes, in the order of their brightness. In the first magnitude are placed the 20 brightest stars--Sirius, Canopus, Alpha Centauri, Vega, Capella, Arcturus, Rigel, Procyon, Achernar, Beta Centauri, Betelgeuse, Altair, Acrux Aldebaran, Pollux, Spica, Antares, Fomalhaut, Deneb, and Regulus. (Those listed in italic type cannot be seen in northern latitudes.) The second group contains 50 stars, including the polestar and the two pointers. In the third there are 160; in the fourth, 500; in the fifth, 1,500; and in the sixth, 4,000. Most human eyes cannot see stars fainter than the sixth magnitude. The number of stars increases enormously in higher magnitudes. They can be seen and photographed in telescopes. Distance cannot be learned from a star's magnitude alone, because its magnitude depends upon its size and brightness as well. In addition all stars are so far away that they appear as point sources in the telescope. Measuring the image would tell nothing about its size. Astronomers can, however, measure the diameters of the closer bright stars. If a plate containing two parallel slits is placed over the objective of a telescope, the image of the star obtained through the slits is crossed by bars of light and darkness because of interference (see Light). If the slits are moved apart, the bars disappear. The amount of separation required to cause this disappearance depends upon the distance and diameter of the star. Betelgeuse is about 520 light-years away. It was the first star to be measured with the interferometer (see Michelson). In 1920, astronomers at the Mount Wilson Observatory determined that the diameter of Betelgeuse was then about 260 million miles, or 300 times as great as the sun's. The sizes of more distant stars are estimated by their light.

Classes of Stars by Age

Astronomers have found that the size of stars depends to a considerable extent upon the fact that stars seem to be born, mature, grow old, and die. While they do so, chemical elements and heat interact in them. This produces a succession of changes in color and luminosity, or brightness. Thus observations of these characteristics reveal the stage reached by each star in its life cycle. (In astronomy the

brightness of a star is compared with how bright the sun would be if both bodies were seen at the same distance.) The observable facts are arranged in a classification called the Draper Catalogue. Temperatures are measured on the Kelvin scale (centigrade scale, using absolute zero as the base temperature). The letters for the types of stars are rearranged from an earlier scale to suit later knowledge. Stars of the O class, such as Betelgeuse, Rigel, Deneb, and Antares, are called supergiants. They are enormous clouds of gas. Force of gravity is contracting them, and this provides radiant energy enough to make these stars the brightest of all. B stars such as Arcturus are giants. They too are still contracting. Most stars are in the main sequence from A to K. They consist largely of hydrogen and helium, with a scattering of heavier elements. They are dense enough to have extremely high interior temperatures; and this transmutes hydrogen to helium. The change yields enough energy to maintain heat and brightness, as it does in our sun, a star of G type (see Sun). Toward the end of the sequence, stars are cool enough to permit formation of molecules. These contain carbon in types R and N and zirconium oxide in type S. When nuclear fuels are exhausted, stars fall into an end dwarf type. In these the electrons of atoms are stripped from the nuclei, and all the particles are stripped from the nuclei, and all the particles are packed tightly. Matter in this degenerate state is so dense, a one-inch cube of it may weigh hundreds of tons. Dwarfs radiate enough light to be seen a short astronomical distance. Some show white light, like the companion of Sirius; others give red light.

Stars Grouped in Galaxies

On clear nights the Milky Way, a wispy band of faint white light, stretches roughly north and south across the sky. Actually, the band is a cluster of myriads of stars called a galaxy. Our sun is only one of them. The entire galaxy is lens shaped, with its center toward the constellation Sagittarius (the Archer), almost 33,000 light-years away. The galaxy is about 15,000 light-years thick at the center and about 100,000 light-years across. It contains more than 100 billion stars. As far as powerful telescopes can see, there are some tens of billions of other galaxies, spaced on the average about one million light-years apart. One of the nearest is the great galaxy in Andromeda. It is shaped like a giant pinwheel; and it rotates about its axis. Our galaxy is similar, and the sun makes one trip round approximately every 200 million years.

Quasars--Starlike Objects

In the early 1960s celestial bodies that resemble stars were discovered and identified by astronomers. The objects were named quasars, or quasi-stellar radio sources. Quasars emit intense light and radio waves and have masses millions of times greater than that of the sun. They are believed to be several billion light-years distant, traveling away from the Earth at tremendous speeds. One theory suggests that a quasar contains a core which produces thermonuclear energy. This stimulates two surrounding cloudlike layers. The visible inner layer is a luminous gas, or plasma. The outer layer contains electrons which spiral through the quasar's magnetic field and emit radio signals. Objects similar to quasars but which do not emit radio waves were designated quasi-stellar blue galaxies, or blue stellar objects (BSO). They are about 500 times more numerous than quasars and up to about 100 times brighter than an ordinary galaxy. Astronomers theorize that BSOs may be aging quasars that have stopped emitting radio signals and are galaxies in an early stage of evolution.

The Birth of a Star

Star formation begins when a dense, intersteller cloud of hydrogen and dust particles collapses inward under the influence of its own gravity. This gravitational contraction causes an increase in the cloud's density and internal temperature. The heat vaporizes 1

1From Compton's Interactive Encyclopedia Deluxe © 1998 The Learning Company, Inc.

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