White dwarf stars are extremely hot hence the bright white light they emit. These are stars blown up by radiation pressure and are very large. Another class of stars is called giants: stars in the high-brightness part of the brightness-color diagram. In red dwarfs, as in all main-sequence stars, the pressure counterbalancing the weight is caused by the thermal motion of the hot gas. These form an entirely different class of heavenly bodies than white dwarfs. They look red and are called red dwarfs or (even cooler) brown dwarfs. Low mass main sequence stars are small and cool. Few stars are in the low-brightness-hot-color region (the white dwarfs), but most stars follow a strip, called the main sequence. If, for all observed stars, one makes a diagram of (absolute) brightness versus color ( Hertzsprung-Russell diagram), not all combinations of brightness and color occur. In the 1930's the explanation is given as a quantum mechanical effect: the weight of the white dwarf is supported by the pressure of electrons ( electron degeneracy), which only depends on density and not on temperature. A radius which is 100 times smaller, implies that the same amount of matter is packed in a volume that is typically 100³=1,000,000 smaller than the Sun and so the average density of matter in white dwarfs is 1,000,000 times denser than the average density of the Sun. They may have the same mass as the Sun and so are very compact. Many white dwarfs are approximately the size of the Earth, typically 100 times smaller than the Sun. It is thought that even stars eight times as massive as the Sun will in the end die as white dwarfs, cooling gradually to become black dwarfs. When this limit is exceeded, the pressure exerted by electrons is no longer able to balance the force of gravity, and the star continues to contract, eventually forming a neutron star.ĭespite this limit, most stars end their lives as white dwarfs since they tend to eject most of their mass into space before the final collapse (often with spectacular results - see planetary nebula). There is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun). The higher the mass of the white dwarf, the smaller the size. Near the end of its nuclear burning stage, such a star goes through a red giant phase and then expels most of its outer material (creating a planetary nebula) until only the hot ( T > 100,000 K) core remains, which then settles down to become a young white dwarf which shines from residual heat.Ī typical white dwarf has half the mass of the Sun yet is only slightly bigger than Earth this makes white dwarfs one of the densest forms of matter (10 9kg.m -3), surpassed only by neutron stars and hypothetical quark stars. Most small and medium-size stars will end up as white dwarfs, after all the hydrogen they contain is fused into helium. However, over the universe's lifetime to the present (about 13.7 billion years) even the oldest white dwarfs still radiate temperatures of a few thousand kelvins. A white dwarf which exceeds this limit (known as the Chandrasekhar limit), typically by mass transfer from a companion star, may explode as a supernova.Įventually, over hundreds of billions of years, white dwarfs cool to temperatures at which they are no longer visible. The maximum mass of a white dwarf, beyond which degeneracy pressure can no longer support it, is about 1.4 solar masses. The white dwarf is supported only by electron degeneracy pressure. The core, no longer supported against gravitational collapse by fusion reactions, becomes extremely dense, with a typical mass of about half that of the sun contained in a volume about equal to that of the Earth. This core has no further source of energy, and so will gradually radiate away its energy and cool down. These stars are not heavy enough to generate the core temperatures required to fuse carbon in nucleosynthesis reactions, and after they have become a red giant during their helium-burning phase, they will shed their outer layers to form a planetary nebula, leaving behind an inert core consisting mostly of carbon and oxygen. A white dwarf is an astronomical object which is produced when a low to medium mass star dies.
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