Star

I

Introduction

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 8,000, of which 4,000 are in the northern hemisphere of the sky and 4,000 in the southern hemisphere. At any one time during the night in either hemisphere, only about 2,000 stars are visible. The others are obscured by atmospheric haze, especially near the horizon, and by faint sky light. 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 visible through large 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 Proxima Centauri, one component of the triple star Alpha Centauri, which is about 40 trillion km (25 trillion mi) from the Earth. In terms of the speed of light, the common standard used by astronomers for expressing distance, Proxima Centauri is about 4.22 light years distant; light travelling at about 300,000 km/s (186,000 mi/s) takes more than four years and two months to travel from this star to the Earth.

II

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, which on the Sun are called sunspots, are probably present on other typical stars; their existence on some large nearby stars has been inferred by a technique called speckle interferometry (seeInterferometer). The internal structure of the Sun and other stars cannot be directly observed, but studies indicate convection currents and density and temperature that increase 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 1,000 times as massive as the Sun may exist, and, at the lower range, hot balls of gases may exist that are too small to initiate nuclear reactions. An object that may be such a 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 1,000 times less bright.

III

Star Catalogues

Except for the comparatively few stars visible to the naked eye, stars are named by numbers according to the various star atlases and catalogues issued by astronomical observatories. The first such star catalogue was compiled by the Egyptian astronomer Ptolemy in the 2nd century ad. Called the Almagest, it listed the names and locations of 1,028 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 3,000 stars. The first modern star catalogue, that issued in 1862 by the observatory of Bonn, in Germany, contains the locations of more than 300,000 stars.

In 1887 an international committee began work on an elaborate star catalogue. It was compiled from photographs taken by about 20 collaborating observatories, comprising some 21,600 individual plates, showing some 8 to 10 million stars.

Modern catalogues 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 (seeTelescope: History) on Mount Palomar. Each plate covers a region of the sky 6° by 6°, and 1,035 charts cover all the sky visible from Mount Palomar. Corresponding set of charts of the southern sky have been made by the use of Schmidt telescopes in Australia and Chile.

IV

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 on the basis of the relative intensity of certain absorption lines. The observed variations within the sequence provide clues to the ages of the different stars and their stages of development.

The various stages in the sequence of spectra, 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.