white dwarf

A star in an advanced state of stellar evolution, composed of degenerate matter. A white dwarf is created when a star finally exhausts its possible sources of fuel for thermonuclear fusion. The star collapses under its own gravity, compressing the matter to a degenerate state in which atomic nuclei and electrons that have been completely stripped from atoms are all packed together. The process stops when a quantum mechanical effect, the exclusion principle, comes into play; the electrons cannot be compacted further and so exert a resistance called degeneracy pressure. S Chandrasekhar demonstrated theoretically that the upper mass limit for white dwarfs is 1.4 times the mass of the Sun - larger collapsing masses must become neutron stars or black holes. The first white dwarf to be recognized as such was 40 Eridani B, observed in 1910. It was shown to have a surface temperature of 17,000 K but a total luminosity so low that it could be explained only if the star were smaller than the Earth. Other well-known white dwarfs include van Maanen's star and Sirius B. Sirius B, first seen in 1862, has a mass about the same as the Sun's in a diameter only five times the Earth's, but is 10,000 times fainter than Sirius A, which is a normal A star. A few hundred white dwarfs are known but they may represent as much as 10 per cent of the stellar population. Their low intrinsic luminosity makes them difficult to detect. Though called "white" dwarfs as a group, these degenerate stars actually cover a range of temperatures and colours, from the hottest, which are white and have surface temperatures as high as 100,000 K, to cool red objects at only 4,000 K. Since they have no internal source of energy, white dwarfs are in a long process of gradually cooling off, during which the temperature declines. Their ultimate fate is to become a black dwarf - a non-luminous dead star. The spectra of white dwarfs are bewilderingly complex, reflecting a range of temperature and composition. Typically, the spectrum contains very broad absorption lines, though some show no lines at all. The line-forming region is only a few hundred metres thick. Some white dwarfs show only hydrogen lines, presumably because the helium and heavier elements have sunk to the bottom of the "atmosphere" under the strong gravitational force. In other stars, helium or metals are seen and no hydrogen remains. A new classification scheme for white dwarfs was proposed in 1983 by E. M. Sion and collaborators. The designations consist of three capital letters, the first being D for degenerate. The second letter indicates the primary spectrum: A, H only; B, neutral He with no H or metals; C, continuous; O, ionized He with neutral He or H; Z, metal lines only with no H or He; Q, carbon present. The third letter is for secondary spectral characteristics: P, magnetic with polarized light; H, magnetic without polarized light; X, peculiar or unclassifiable; V, variable. The old system was based on the usual spectral class sequence (O, B, A, F, G, K, M) prefixed by D.