White Dwarf Stars

The first white dwarf star to be discovered was the companion (component B) of Sirius, a main sequence star (component A). The spectrum and color, hence temperature, of the two stars are very similar, but the dwarf star is some 10,000 times fainter, hence much smaller by factor of 100 in radius, than the main sequence companion. With a mass of about one solar mass and a size about that of Earth, this star has an average mass density of 3,000,000 grams per cubic centimeter. Its surface gravity is such that a person who weighs 180 pounds on Earth would weight 50,000 tons on the surface of Sirius B.

Numerous white dwarfs are known (their number is second only to the main sequence stars) and as a class, they are hot stars with typical surface temperatures of 10,000 K but luminosities less than about 1 percent of the Sun. These stars have no internal thermonuclear energy source. The energy they radiate is the loss of their residual heat. This loss, however, is slow due to their small surface areas. Comparing luminosities with their (theoretical) store of internal thermal energy suggests that cooling times are long—from 10 to 100 billion years. Ultimately they will end up as black dwarfs, too cold to have any significant radiation.

White dwarfs are the final stage of the evolution of low‐mass stars, which (according to current theory) originally had no more than about 8 solar masses while on the main sequence. Mass loss during the red giant and planetary nebula stages can reduce this to no more than about 1.4 solar masses. This number is known as the Chandrasekhar limit, or the maximum mass for which the electron degeneracy pressure is able to balance the inward pull of gravity in a star of this size. More massive main sequence stars tend to produce more massive white dwarfs, with the most common mass being about 0.6 solar mass. This remaining mass has been converted basically to carbon and oxygen. Unlike a normal star in which gas pressure from the motion of all the particles—nuclei and electrons—provides the balance against gravity, in a white dwarf the nuclei of carbon and oxygen play little role. Ultimately, when they cool below 4,000 K, these nuclei will lock into a solid lattice structure, the electrons providing the pressure to balance gravity. There is a limit to which this works. Unlike main sequence stars that are larger for greater mass, increasing the mass of a white dwarf star produces a smaller star. If the mass is made larger, the star's larger gravity produces a smaller white dwarf. At a mass greater than the Chandrasekhar limit, the electrons are unable to provide sufficient pressure to balance gravity. Gravity would thus compact the object to a much smaller size; but this would no longer be a white dwarf. As the properties of white dwarfs are so different from those of normal stars they sometimes are referred to not as stars but as compact objects. See Figure 1




Figure 1
Mass‐radius relation for white dwarf stars.