White dwarfs, compact stars with masses of the order of the mass of the Sun Mο and radii ≈1% of the radius of the Sun Rο; they make up 3—10% of the total number of stars in the Galaxy.
The equilibrium of white dwarfs is maintained, at a mean density of the substance of ~105—106 g/cm3, by the pressure of the degenerate electron gas. For physics white dwarfs are of interest above all as objects to which the theory of superdense plasma is applied. White dwarfs become stars at the end of their evolution (after the reserves of thermonuclear fuel in the stars have been exhausted). Normal stars with an initial mass M<5Mο turn into white dwarfs after shedding their outer layers. The exposed core has a very high surface temperature; cooling gradually, the stellar core passes into the state of a white dwarf. The hottest known white dwarf has a surface temperature of ~7·104 K, the coldest (“red” white dwarfs) — about 5·103K. The main source of the luminosity of a white dwarf - the energy of the thermal motion of the ions stored in the star.
White dwarfs exist thanks to a stable equilibrium between the forces of gravitation and the internal pressure of the degenerate electron gas. The concentration of the practically free electrons ne in the substance of a white dwarf is so large that their zero-point quantum-mechanical momentum pe≈hn1/3e creates a pressure sufficient for the existence of a white dwarf with the observed values of the radii. The mass — radius relation for a white dwarf at M<0.5 Mο has the form R≈M-1/3, that is, the more massive white dwarfs have the smaller radius. The theory predicts an upper limit to the mass of a white dwarf (the so-called Chandrasekhar limit MCh≈ 1.4 Mο); exceeding this limit leads to the gravitational collapse of the star. The existence of the Chandrasekhar limit is due to the fact that, as the density grows, the electron gas becomes relativistic, so that its pressure can no longer withstand the forces of gravitation.
The theoretical dependence of the luminosity of a white dwarf on its age is broadly confirmed by observations (a white dwarf luminosity of ~10-3 Lο corresponds to an age of ~109 years). If a white dwarf belongs to a close binary system, a significant contribution to its luminosity may come from the thermonuclear burning of hydrogen flowing onto the white dwarf from the second star of the system. This burning, however, is usually non-stationary in character (outbursts of novae and nova-like stars). In the spectra obtained for white dwarfs (in about 10 out of 500) a strong polarisation of the radiation or a Zeeman splitting of the spectral lines is observed, which points to the existence in some white dwarfs of magnetic fields of ~106 — 108 G. In about 10 white dwarfs optical pulsations with periods of ~102—103 s have been detected, which have so far found no final explanation.