STAR

Large celestial body composed of gravitationally contained hot gases emitting electromagnetic radiation, especially light, as a result of nuclear reactions inside the star.

The sun is a star. With the sole exception of the sun, the stars appear to be fixed, maintaining the same pattern in the skies year after year. In fact the stars are in rapid motion, but their distances are so great that their relative changes in position become apparent only over the centuries.

The number of stars visible to the naked eye from earth has been estimated to total 8000, of which 4000 are visible from the northern hemisphere and 4000 from the southern hemisphere.

At any one time in either hemisphere, only about 2000 stars are visible. The other 2000 are located in the daytime sky and are obscured by the much brighter light of the sun. Astronomers have calculated that the stars in the Milky Way, the galaxy to which the sun belongs, number in the hundreds of billions. The Milky Way, in turn, is only one of several hundred million such galaxies within the viewing range of the larger modern telescopes. The individual stars visible in the sky are simply those that lie closest to the solar system in the Milky Way.

The star nearest to our solar system is the triple star Proxima Centauri, which is about 40 trillion km (about 25 trillion mi) from earth. In terms of the speed of light, the common standard used by astronomers for expressing distance, this triple-star system is about 4.29 light-years distant; light traveling at about 300,000 km per sec (about 186,000 mi per sec) takes more than four years and three months to travel from this star to earth.

Physical Description

The sun is a typical star, with a visible surface called a photosphere, an overlying atmosphere of hot gases, and above them a more diffuse corona and an outflowing stream of particles called the solar (stellar) wind. Cooler areas of the photosphere, such as the sunspots on the sun, are likely present on other typical stars; their existence on some large nearby stars has been inferred by a technique called speckle interferometry. The internal structure of the sun and other stars cannot be directly observed, but studies indicate convection currents and layers of increasing density and temperature until the core is reached where thermonuclear reactions take place. Stars consist mainly of hydrogen and helium, with varying amounts of heavier elements.

The largest stars known are supergiants with diameters that are more than 400 times that of the sun, whereas the small stars known as white dwarfs have diameters that may be only 0.01 times that of the sun. Giant stars are usually diffuse, however, and may be only 40 times more massive than the sun, whereas white dwarfs are extremely dense and may have masses about 0.1 times that of the sun despite their small size. Supermassive stars are suspected that could be 1000 times more massive than the sun, and, at the lower range, hot balls of gases may exist that are too small to initiate nuclear reactions. One possible such brown dwarf was first observed in 1987, and others have been detected since then.

Star brightness is described in terms of magnitude. The brightest stars may be as much as 1,000,000 times brighter than the sun; white dwarfs are about 1000 times less bright.

Star Catalogs

Except for the comparatively few stars visible to the naked eye, stars are named by numbers according to the various star atlases and catalogs issued by astronomical observatories. The first such star catalog was compiled by the Egyptian astronomer Ptolemy in the 2nd century AD.

Called the Almagest, it listed the names and locations of 1028 stars. In 1603 a star atlas was published in Augsburg by the German astronomer Johann Bayer. Bayer listed a much larger number of stars than did Ptolemy, and he designated stars by a Greek letter and the constellation, or the celestial configuration, in which the star appears.

In the 18th century the English astronomer John Flamsteed also published an atlas in which stars were named according to their constellation, but Flamsteed differentiated them with numbers rather than letters. This atlas contained the locations of approximately 3000 stars.

The first modern star catalog, that issued in 1862 by the observatory of Bonn, Germany, contains the locations of more than 300,000 stars.

In 1887 an international committee began work on an elaborate star catalog. The charts were to be compiled from photographs taken by about 20 collaborating observatories and comprising some 21,600 individual plates. From these photographs an exhaustive catalog is to list between 8 million and 10 million stars.

Modern catalogs of stars consist not of books, but of copies of glass photographic plates taken with large wide-field telescopes. The first such major survey was completed in the mid-1950s, using the 48-in. (1.22-m) Schmidt telescope on Mount Palomar. Each plate covers a region of the sky 6° by 6°, and 1035 charts cover all the sky visible from Mount Palomar. A corresponding set of charts of the southern sky is currently being made with the use of Schmidt telescopes in Australia and Chile.

Classification of Stellar Spectra

The photographic study of stellar spectra was initiated in 1885 by the American astronomer

Edward Charles Pickering at the Harvard College Observatory and carried out principally by the American astronomer Annie J. Cannon. This research led to the important discovery that stellar spectra can be arranged in a continuous sequence, based on the relative intensity of certain absorption lines occurring in the spectra. The observed variations within the sequence provide clues to the age of the different stars and their stages of development. See Spectrum.

The various stages in the spectrum sequence, which are designated by the letters O, B, A, F, G, K, and M, are characterized especially by variations in the intensity of the hydrogen lines that occur throughout the sequence. In addition, the lines of other elements become prominent at different stages. Subscripts from 0 to 9 are used to denote gradations in the pattern within each class.

Class O

This group is primarily characterized by the lines of helium, oxygen, and nitrogen, besides the hydrogen lines. The O group, which comprises extremely hot stars, includes those showing bright-line spectra of hydrogen and helium, as well as those exhibiting dark lines of the same elements.

Class B

In this group the helium lines attain maximum intensity at the subdivision B2 and fade progressively in higher subdivisions. The intensity of the hydrogen lines steadily increases throughout the subdivisions. The group is typified by the star Epsilon (e) Orionis.

Class A

This group comprises the so-called hydrogen stars with spectra dominated by the absorption lines of hydrogen. a typical star of this group is Sirius, the Dog Star.

Class F

This group comprises stars in which the so-called H and K lines of calcium and the characteristic lines of hydrogen are strong. A notable star in this category is d Aquilae.

Class G

This group comprises stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of many metals, notably iron, are also present. The sun belongs to this group, and the G stars are therefore frequently called solar stars.

Class K

This group comprises stars having strong calcium lines and lines indicating the presence of other metals. The violet light of the spectrum is less intense, compared with the red light, than in the classes previously mentioned. The group is typified by Arcturus.

Class M

This group comprises stars with spectra dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectrum is less intense than that in the K stars. The star a Orionis is typical of this group.

All these characteristics are compatible with the conclusion that stars of these classes are all of similar chemical composition and are arranged in a temperature order from hottest to coolest. The absolute surface temperatures of the various star groups are approximately the following: O, 22,200° C (40,000° F); B, 13,900° C (25,000° F); A, 10,000° C (18,000° F); F, 6650° C (12,000° F); G, 5540° C (10,000° F); K, 3870° C (7000° F); and M, 1760° C (3200° F).

The interior temperature of the average star is about 20,000,000° C (36,000,000° F).

Double Stars

More than half of the stars in the sky are actually members of two-star (binary) systems or multiple-star systems. Some nearby double stars appear separate when viewed telescopically, but many more are detected as doubles only by spectroscopic means. A double-star system consists of two stars that are physically close to each other and that revolve in an orbit around their common center of mass. Such double stars were first recognized by the British astronomer Sir William Herschel in 1803.

Spectroscopic binaries, first identified in 1889, are not visually separable by the telescope but can nevertheless be recognized by means of doubling or broadening of the spectrum lines as the star pair revolves. When one component moves away from earth and the other approaches it as they revolve in their orbit, the spectrum lines from the receding star shift toward the red, while those from the advancing star shift toward the violet.

Another type of double star is the so-called eclipsing variable. Stars of this type are composed of a brighter and a darker component. As seen from earth, when the orbit is such that the darker star eclipses the brighter one, the intensity of the light coming from the star fluctuates regularly.

Investigation has shown that about one of every two or three stars visible with telescopes of moderate size is a double star of the physical-double type. Many thousands of visual binaries and many hundred spectroscope binaries have been studied intensively. Such stars are the main source of information about stellar masses.

Variable Stars

All stars probably vary slightly in their brightness on a more or less periodic basis, including the sun. Such variations may be scarcely measurable. Some stars, however, change greatly in brightness and are called variable stars. There are many types. Some repeat cycles with almost clocklike precision; others are highly irregular. Some may require only hours or days to return to a starting brightness; others may require years. The brightness of such stars may change almost imperceptibly or violently.

The most spectacular variable is the so-called temporary star, or nova. Novas may brighten up to as much as 200,000 times the sun's brightness by blowing off perhaps a hundredth or a thousandth of 1 percent of the sun's mass at speeds up to 960 km per sec (up to 600 mi per sec).

Some novas repeat this process, periodically until they lose too much mass to continue. Although supernovas are similarly named, they are a far more catastrophic phenomenon and not periodic at all. They represent the true explosion of a star, sometimes brightening for a few days to 10 billion times the sun's true brightness before fading away permanently. They leave behind expanding wreckage seen as bright gaseous clouds, or nebulas; the Crab nebula is an example, first observed from earth as a supernova in 1054. Sometimes a pulsar is also left as a remnant in the center of the wreckage. Novas occur fairly frequently in the Milky Way, perhaps one or two being observed each year, but supernovas are much rarer. The most recent supernova in the Milky Way appeared in 1604, although one in a nearby galaxy drew great attention in 1987.

Many variable stars change their brightness by pulsating, that is, by expanding and contracting somewhat like a balloon. One important type, named Cepheid variables after d Cephei, repeat their brightness cycles rather accurately. Their periods range from about a day to hundreds of days, and they are all hundreds of times more luminous than the sun. The longer the period of a Cepheid variable, the greater the average brightness of the star. This period-luminosity relation, discovered by Henrietta Leavitt of the Harvard College Observatory, has proved invaluable in measuring the stellar distances, particularly to nearby galaxies of stars.

Only the period and average apparent brightness of a Cepheid need be observed to provide a measure of its distance. Novas and especially supernovas are also important distance measures because their incredible brilliance at maximum light makes them observable at huge distances in the universe.

Variable stars are of unusual interest because their variation is usually caused by some peculiarity of their internal structure that develops with age. Variable stars thus can reveal information about stellar evolution. Supernovas, for example, have burned up their nuclear fuel and must blow off matter because they become unstable as they collapse gravitationally.

The eclipsing variable, mentioned in the previous section, varies because of external rather than internal causes. The star Algol in the constellation Perseus is typical.

Algol is a double star composed of one bright and one comparatively faint component with an orbit in a plane almost exactly in the line of sight from earth. As the darker component eclipses the brighter, the apparent brightness of the pair falls off sharply, and a similar but less intense diminution occurs when the brighter component eclipses the darker. Astronomers have observed many thousands of eclipsing variable stars, which are valuable in measuring stellar masses.

Pulsars and Neutron Stars

A number of distinct sources of radio pulses, referred to as pulsars, have been discovered with radio telescopes. Typical pulsation periods of the pulsars are near 1 sec. The periods range from several seconds to a tiny fraction of a second, as confirmed by optical and X-ray observations. The pulsation periods are so constant that only the most precise clocks can detect a slight increase in the average pulse interval for several pulsars; this increase indicates that it would take approximately 1 million years for typical periods to double.

The evidence strongly suggests that pulsars are rotating neutron stars with diameters of perhaps only about 16 km (about 10 mi). Probably they rotate once per pulsation period.

Their density is so enormous that if the volume of the ball on a ballpoint pen were packed with neutrons, as in a pulsar, it would contain more than 91,000 metric tons of mass.

Evolution of Stars

The formation and development of stars have been the subject of many hypotheses and conjectures by scientists. Theories of stellar evolution are based primarily on clues obtained from studies of the stellar spectra related to luminosity. Observation has shown that many known stars can be systematized in a regular sequence in which the brightest stars are the hottest and the smallest stars are the coolest and faintest. This series of stars is known as the main sequence on the temperature-luminosity diagram developed from the work of the Dutch astronomer Ejnar



Hertzsprung and the American astronomer Henry Norris Russell and known as the Hertzsprung-Russell diagram. Two exceptions to this grouping are the so-called red giants and white dwarfs. The red giants are bright stars of comparatively large dimensions; white dwarfs are low in brightness, small, and extremely dense.

A star begins its life as a large and comparatively cool mass of gas. The contraction of this gas and the subsequent rise of temperature continue until the interior temperature of the star reaches a value of about 1,000,000° C (about 1,800,000° F). At this point a nuclear reaction takes place in which the nuclei of hydrogen atoms combine with heavy hydrogen deuterons (nuclei of so-called heavy hydrogen atoms) to form the nucleus of the inert gas helium. The latter reaction liberates large amounts of nuclear energy, and the further contraction of the star probably is halted.

When the release of energy from the deuteron-hydrogen nucleus reaction ends, contraction begins anew, and the temperature of the star increases again until it reaches a point at which a nuclear reaction can occur between hydrogen and lithium and other light metals present in the body of the star. Again energy is released and contraction stops. When the lithium and other light materials are consumed, contraction resumes, and the star enters the final stage of development in which hydrogen is transformed into helium at extremely high temperatures through the catalytic action of carbon and nitrogen. This thermonuclear reaction is characteristic of the main sequence of stars mentioned above and continues until all the available hydrogen is consumed. The star gradually swells and becomes a red giant. It attains its greatest size when all its central hydrogen has been converted into helium. If it is to continue shining, its temperature at the center must rise high enough to cause fusion of the helium nuclei. During this process the star probably becomes much smaller and denser. When it has exhausted all possible sources of nuclear energy, it may contract further and become a white dwarf. This final stage may be marked by the stellar explosions known as novas. When a star sheds its outer envelope explosively as a nova or supernova, it returns to the interstellar medium elements heavier than hydrogen that it has synthesized in its interior. Future generations of stars formed from this material will therefore start life with a richer supply of heavier elements than the earlier generations of stars. Stars that shed their outer layers in a nonexplosive fashion become planetary nebulas, or old stars surrounded by spheres of radiating gases.

Massive stars, many times the mass of the sun, run through their cycle of evolution rapidly in astronomical time, perhaps only a few million years from birth to a supernova-type disruption. The remainder may then become a neutron star. A limit exists for the size of neutron stars, however, beyond which such stars are gravitationally bound to keep contracting until they become a black hole, from which light radiation cannot escape. Typical stars such as the sun may persist for many billion of years. The final fate of low-mass dwarfs is unknown, except that they cease to radiate appreciably. Most likely they become burned-out cinders, or black dwarfs. For discussion of the nuclear processes of stellar evolution.

The birth of stars is intimately connected with the presence of dust grains and molecules, as in the Orion nebula region of earth's galaxy. Here, molecular hydrogen (H2) is compressed to high densities and temperatures, dissociating the molecules. The atomic hydrogen then recollapses and forms a dense stellar core that gravitationally attracts surrounding material. The hot core dispels the cocoon of the overlying molecules, and the new star emerges.

Further gravitational heating raises the temperature until nuclear processes can occur.

Stars are generally born in small groups at one edge of a large molecular cloud.

Successive generations of stars eat into the edge of the cloud more and more, leaving a trail of stars of increasing ages.

The birth of stars has been observed in photographs taken of a sky region over a period of years. Modern techniques of space-based ultraviolet, infrared, and radio astronomy have further pinpointed sites of star formation and actual processes taking place.