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Most small and medium-size stars will end up as white dwarfs, after all the hydrogen they contain is fused into helium. 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.
A typical white dwarf is half as massive as the Sun, yet only slightly bigger than Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars and quark stars. The higher the mass of the white dwarf, the smaller the size. There is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun), after which the pressure of the electrons is no longer able to balance gravity, and the star continues to contract, eventually forming a neutron star.
Despite this limit, most stars end their lives as white dwarves, since they tend to eject most of their mass into space before the final collapse (often with spectacular results, see planetary nebula). It is thought that even stars 8 times as massive as the Sun will in the end die as white dwarfs.
Many white dwarfs are approximately the size of the Earth, typically 100 times smaller than the Sun. They may have the same mass as the Sun and so are very compact. 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. Such matter is called degenerate. 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.
If, for all observed stars, one makes a diagram of (absolute) brightness versus color ( Hertzsprung-Russell diagramIn stellar astronomy, the Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram shows the mathematical relationship between absolute magnitude, luminosity, stellar classification, and surface temperature. This was devised, c.), not all combinations of brightness and color occur. Few stars are in the low-brightness-hot-color region (the white dwarfs), but most stars follow a strip, called the main sequenceThe main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. Stars located on this band are known as main-sequence stars or dwarf stars . The coolest dwarfs are the red dwarfs. This line is so. Low mass main sequence stars are small and cool. They look red and are called red dwarfThis article is about red dwarfs, the type of star. Red Dwarf is also the name of a British television series. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spes or (even cooler) brown dwarfBrown dwarfs are sub- stellar objects (~13 to 75 Jupiter masses) that never fuse hydrogen into helium in their cores, as do stars on the main sequence. They do fuse deuterium. A useful criterion for telling brown dwarfs from low mass stars is obtained thrs. These form an entirely different class of heavenly bodies than white dwarfs. In red dwarfs, as in all main-sequence stars, the pressure counterbalancing the weight is caused by the thermal motion of the hot gas. The pressure obeys the ideal gas law. Another class of stars is called giants: stars in the high-brightness part of the brightness-color diagram. These are stars blown up by radiation pressure and are very large.
White dwarf stars are extremely hot; hence the bright white light they emit. This heat is a remnant of that generated from the star's collapse, and is not being replenished (unless they accrete matter from other close by stars), but since white dwarfs have an extremely small surface area from which to radiate this heat energy they remain hot for a long period of time.
Eventually, a white dwarf will cool into a black dwarfA black dwarf is the remains of a Sun-sized star which has evolved to a white dwarf and subsequently cooled down such that it no longer gives out radiation. None exist in our universe, as the time taken for a white dwarf to cool to such a degree is longer. Black dwarfs are ambient temperature entities and radiate weakly in the radio spectrum, according to theory. However, the universe has not existed long enough for any white dwarfs to have cooled down this far yet, and so no black dwarfs are thought to exist.
Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultraviolet observations enable astronomers to study the composition and structure of the thin atmospheres of these stars.
White dwarfs cannot be over 1.4 solar masses, the Chandrasekhar limit, but there is a working method to get them over this limit. Like a nova, a white dwarf can accrete material from a companion. Unlike a nova, the material accretes slowly and remains stable. The mass of the white dwarf increases until it hits the 1.4 solar mass limit, at which degeneracy pressure cannot support the star. This creates a type Ia supernova and is the most powerful of all the supernovae.