A White Dwarf is a star with low absolute brightness and 'normal' color. Such stars are discovered in the 19th century and the first ones are white. The color of a star is a measure of the surface temperature: white stars are like the Sun, blue stars hotter and red stars are cooler. White dwarfs are so dim because they are small and not because they are cool. Color and size explain the name white dwarf. A more appropriate name for white dwarfs is degenerate dwarf (see below), an example of a degenerate star. Some white dwarfs are blue, rather than white. White dwarfs may have in principle any color and a 'blue degenerate dwarf' sounds better than a 'blue white dwarf'.
Many white dwarfs have 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 diagram), 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 sequence. Low mass main sequence stars are small and cool. They look red and are called red dwarfs or (even cooler) brown dwarfs. 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.
A star like our Sun will become a white dwarf when it has exhausted its nuclear fuel. 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. 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 life as white dwarf, 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.
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 dwarf. 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 I supernova and is the most powerful of all the supernovae.
1862. Alvan Graham Clark in 1862 discovered a dark companion of the brightest star Sirius (Alpha Canis Majoris). The companion, called Sirius B or the Pup, had a surface temperature of about 25,000 kelvins, so it was classified as a hot star. However, Sirius B was about 10,000 times fainter than the primary, Sirius A. Since it was very bright per unit of surface area, the Pup had to be much smaller than Sirius A, with roughly the diameter of the Earth.
Analysis of the orbit of the Sirius star system showed that the mass of the Pup was almost the same as that of our own Sun. This implied that Sirius B was thousands of times more dense than lead. As more white dwarfs were found, astronomers began to discover that white dwarfs are common in our Galaxy.
1926. R.H. Fowler explained the high densities in an article "Dense matter" (Monthly Notices R. Astron. Soc. 87, 114-122) using the
electron degenerate pressure a few months after the formulation of the Fermi-Dirac statistics for an electron, on which the electron pressure is based.
1930. S. Chandrasekhar discovered (Astroph. J. 74, 81-82) in an article called "The maximum mass of ideal white dwarfs" that no white dwarf can be more massive than about 1.2 solar masses". This is now called the Chandrasekhar limit. Fowler and Chandrasekhar both received the Nobel prize in 1983.
The hot white dwarf Sirius B, at 8 light years the closest known white dwarf, seen in X-rays as an intense x-ray source (lines radiating from Sirius B are image artifacts). The white dwarf is part of a binary system. Its companion, in this image the fainter source, is at the position of Sirius A (Alpha Canis Majoris). In visible light, Sirius A is the brightest star in the night sky. Sirius B is much dimmer and appears so close to the brilliant Sirius A that it was not actually sighted until 1862, during Alvan Clark's testing of a telescope. Both stars are at the same distance, the white dwarf being so much dimmer in optical light because of its dwarf size. It has a radius just less than the Earth's. In x-rays Sirius B is much brighter. In the high gravity of this star, Hydrogen (transparent for x-ray but not in optical light) floats on top. In x-rays one sees the deeper, hotter layers at around 200,000 kelvins, in stead of the actual surface temperature in optical light of 25,000 kelvins. NASA/CXS
History of discoveries
See also
brown dwarf,
timeline of white dwarfs, neutron stars, and supernovae
White Dwarf is also the name of a magazine.