ETYM Old Eng. sterre, as. steorra.
Luminous globe of gas, mainly hydrogen and helium, which produces its own heat and light by nuclear reactions. Although stars shine for a very long time—many billions of years —they are not eternal, and have been found to change in appearance at different stages in their lives.
Stars are born when nebulae (giant clouds of dust and gas) contract under the influence of gravity. As each new star contracts, the temperature and pressure in its core rises. At about 10 millionşC the temperature is hot enough for a nuclear reaction to begin (the fusion of hydrogen nuclei to form helium nuclei); vast amounts of energy are released, contraction stops, and the star begins to shine. Stars at this stage are called main-sequence stars; the Sun is such a star and is expected to remain at this stage for the next 5 billion years. Their surface temperatures range from 2,000şC/3,600şF to above 30,000şC/54,000şF and the corresponding colors range from red to blue-white.
The smallest mass possible for a star is about 8% that of the Sun (80 times the mass of the planet Jupiter), otherwise nuclear reactions do not occur. Objects with less than this critical mass shine only dimly, and are termed brown dwarfs.
When all the hydrogen at the core of a main-sequence star has been converted into helium, the star swells to become a red giant, about 100 times its previous size and with a cooler, redder surface. When, after this brief stage, the star can produce no more nuclear energy, its outer layers drift off into space to form a planetary nebula, and its core collapses in on itself to form a small and very dense body called a white dwarf.
Eventually the white dwarf fades away, leaving a non-luminous dark body.
Some very large main-sequence stars do not end their lives as white dwarfs—they pass through their life cycle quickly, becoming red supergiants that eventually explode into brilliant supernovae. Part of the core remaining after such an explosion may collapse to form a small superdense star, which consists almost entirely of neutrons and is therefore called a neutron star. Neutron stars, called pulsars, spin very quickly, giving off pulses of radio waves (rather as a lighthouse gives off flashes of light). If the collapsing core of the supernova is very massive it does not form a neutron star; instead it forms a black hole, a region so dense that its gravity not only draws in all nearby matter but also all radiation, including its own light.
See also binary star, Hertzsprung–Russell diagram, and variable star.
1. (Astronomy) A celestial body of hot gases that radiates energy derived from thermonuclear reactions in the interior.
2. A plane figure with 5 or more points; often used as an emblem.
3. Any celestial body visible (as a point of light) from the Earth at night.