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 change in appearance at different stages in their lives (they are said to have a ‘life cycle’). Stars seen at night belong to our galaxy, the Milky Way. The Sun is the nearest star to Earth; other stars in the Milky Way are large distances away (to get to the nearest would take about 4 years travelling at the speed of light).
The smallest mass possible for a star is about 8% that of the Sun (80 times that of Jupiter), otherwise nuclear reactions do not occur. Objects with less than this critical mass shine only dimly, and are termed brown dwarfs.
The most massive stars known are Eta Carinae and the Pistol Star in Sagittarius, each with a mass approximately 150 times that of the Sun - at the limit of what astronomers believe on theoretical grounds to be possible.
Stars are born when nebulae (giant clouds of dust and gas) contract under the influence of gravity. These clouds consist mainly of hydrogen and helium, with traces of other elements and dust grains. The temperature and pressure in the cloud's core rise as the star grows smaller and denser. As the star is forming, it is surrounded by evaporating gaseous globules (EGGs), the oldest of which was photographed in the Eta Carina Nebula in 1996 by the Hubble Space Telescope.
At first the temperature of the star scarcely rises, as dust grains radiate away much of the heat, but as it grows denser less of the heat generated can escape, and it gradually warms up. At about 10 million°C/18 million°F the temperature is hot enough for a nuclear reaction to begin, and hydrogen nuclei fuse 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. Enough energy is produced in the nuclear reaction to replace that being lost at the surface, so the star does not contract further until its nuclear energy sources are exhausted. Until this happens the star remains practically unaltered. Where the star is on the main sequence, how long it takes to contract before it gets there, and how long it remains there, are all determined by the mass of the star; the larger the mass, the shorter the period and the brighter the star. A star with the mass of the Sun takes a few million years to reach the main sequence, and then remains on it for about 10 billion years, a little more than twice the present age of the Sun. The Sun is thus expected to remain at this stage for another 5 billion years. Surface temperatures of main-sequence stars range from 2,000°C/3,600°F to above 30,000°C/54,000°F. The corresponding colours range from red to blue-white. The nuclear reactions take place near the centre, so the star gradually acquires an inert helium core, surrounded by a thin shell of burning hydrogen. 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 a hundred times its previous size and with a cooler, redder surface.
What happens next depends on the mass of the star. If this is less than 1.2 times that of the Sun, the star's 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 nonluminous dark body.
If the mass is greater than 1.2 times that of the Sun, the star does not end as a white dwarf, but passes through its life cycle quickly, becoming a red supergiant. As the star's core grows hotter, further nuclear transformations take place, resulting in the helium being converted first into carbon and oxygen, then into heavier elements, and finally into iron. The star eventually explodes into a brilliant supernova. Part of the core remaining after the explosion may collapse to form a small superdense star, consisting almost entirely of neutrons and therefore called a neutron star. The possibility of such stars was first pointed out by Lev Landau in 1932. Neutron stars spin very quickly, emitting beams of radio waves from their poles. In the small proportion of cases where such a beam sweeps across the Earth, we observe the neutron star as a pulsating radio source, or pulsar.
If the collapsing core of the supernova has a mass more than three times that of the Sun 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, as the velocity of escape from its surface exceeds that of light. As French astronomer and mathematician Pierre Laplace pointed out in 1798, such a mass would not be visible from the outside. Black holes are responsible for various high-energy processes, including quasars and X-ray sources such as Cygnus X-1.
In 2005 the three largest stars ever found were reported by astronomers. Three red ‘supergiant’ stars were imaged, each more than 1.5 billion km/.93 million mi in diameter. They were designated KY Cygni, V354 Cephei, and KW Sagitarii, situated at distances of 5,200, 9,000 and 9,800 light years respectively from Earth.
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