star

star hot incandescent sphere of gas, held together by its own gravitation , and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy .

Properties of Stars

Stars differ widely in mass , size, temperature , and total energy output, or luminosity . The sun , a typical star, has a mass of about 2 × 10 ³³ grams, a radius of about 7 × 10 ¹⁰ cm, a surface temperature of about 6,000°C, and a luminosity of about 4 × 10 ³³ erg/sec. More than 90% of all stars have masses between one tenth and 50 times that of the sun. Other stellar quantities vary over a much larger range. The most luminous stars (excluding supernovas ) are about ten million times more powerful than the sun, while the least luminous are only one hundredth as powerful. Red giants , the largest stars, are fifteen-hundred times greater in size than the sun; if one were placed at the sun's position, it would stretch to halfway between Jupiter and Saturn. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius.

The visible stars are divided into six classes according to apparent brightness; the brightest are first magnitude and the faintest are sixth magnitude. The stars differ in apparent brightness both because they lie at different distances from us and because they vary in actual or intrinsic brightness. Variable stars do not shine steadily but fluctuate in either a regular or irregular fashion. The supernova, or exploding star, is the most spectacular variable star; the eclipsing binary, where the two stars alternately hide and then reinforce each other's light, is not a true variable.

Light received from a star consists of a spectrum of wavelengths; the hotter the star, the shorter the wavelength at which the light is most intense. The color of a star is closely related to its surface temperature. Red stars have surface temperatures around 3,000°C and blue-white stars have surface temperatures above 20,000°C (see spectral class ).

Stellar Structure and Stellar Evolution

The theory of stellar structure applies the laws of physics to calculation of the equilibrium configurations of stars. According to this theory, the mass and chemical composition of a star determine all its other characteristics. Because most stars are more than 90% hydrogen, variations in chemical composition are small and have a small effect. Variation in mass is the main factor; a doubling in mass increases the luminosity more than 10 times. For a star to be stable, the compressive force of gravitation must be exactly balanced by the tendency of the gas to expand. Thus, the size and temperature of a star are important, interrelated factors.

Despite the tremendous pressure generated by the massive layers above it, the central region, or core, of a star remains gaseous. This is possible because the core has a temperature of millions of degrees. At this temperature, nuclear energy is released by the fusion of hydrogen to form helium; the principle is the same as that of the hydrogen bomb. By the time nuclear energy reaches the surface of the star, it has been largely converted into visible light with a spectrum characteristic of a very hot body (see black body ). The theory of stellar evolution states that a star must change as it consumes its hydrogen in the nuclear reactions that power it. Ultimately each star must die, rarely in a supernova explosion, when its capability for nuclear reactions is exhausted. The heavy atoms created in supernovas (see nucleosynthesis ) are spewed out to become part of the interstellar matter from which new stars are continuously formed.

Location and Motion of Stars

The universe contains billions of galaxies, and each galaxy contains billions of stars. The stars visible to the unaided eye are all in our own galaxy, the Milky Way . Stars are not spread uniformly through a galaxy. They are frequently bunched together in star clusters of as many as 100,000 stars. Many stars that appear as single points of light in even the most powerful telescopes are actually systems of two or more stars orbiting one another, bound together by their mutual gravitational attraction; the binary stars are most common among these multiple star systems.

In ancient times, the stars were believed to be motionless; their fixed patterns in the sky were designated as the constellations . It is now known that the stars move through space, although their motion is too small to be detected during a human lifetime without exacting measurements. From the observed proper motion (change in apparent position on the celestial sphere ), distance of the star from the earth, and radial velocity (motion along the line of sight), the true velocity of a star through space can be determined. See also brown dwarf .

Bibliography

See C. de Jager, The Brightest Stars (1980); G. O. Abell, Exploration of the Universe (5th ed. 1987); R. J. Taylor, The Stars: Their Structure and Evolution (1994); A. C. Phillips, The Physics of Stars (1994).

stars Stars and Bars the flag of the Confederate States, which had two horizontal red bars separated by a narrow white bar, and in the top left-hand corner, a circle of eleven white stars on a blue background for the eleven states of the Confederacy.
Stars and Stripes the national flag of the US. When first adopted by Congress (14th June 1777) it contained 13 stripes and 13 stars, representing the 13 states of the Union; it now has 13 stripes and 50 stars. An informal name for the flag is Old Glory.

See also seven stars, star.