white dwarf

white dwarf

one of a large class of small faint stars of enormous density (on average 10⁸ kg/m³) with diameters only about 1 per cent that of the sun, and masses less than the Chandrasekhar limit (about 1.4 solar masses). It is thought to mark the final stage in the evolution of a sun-like star

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

white dwarf

An extremely dense compact star that has a mass below the Chandrasekhar limit (about 1.4 solar masses) and has undergone gravitational collapse. Having diameters only 1% that of the Sun these stars are consequently very faint, with absolute magnitudes ranging from +10 to +15. The description ‘white’ is misleading because they display a range in color as they cool down, through white, yellow, and red, until finally ending up as cold black globes – called black dwarfs. However, the white ones are the brightest and were the first to be discovered.

White dwarfs are the final phase in the evolution of a low-mass star. Their progenitors are stars of up to 8 solar masses, which lose up to 90% of their matter in the form of planetary nebulae. The core shrinks to become a white dwarf following the exhaustion of its nuclear fuel. Because most of the matter in the core of the collapsing star is in a degenerate state – with the electrons stripped from their nuclei and packed tightly together – the star contracts until its gravity is balanced by the degeneracy pressure of the electrons, and the density rises to 10⁷–10¹¹ kg m^–3. Because of the peculiar behavior of degenerate material (which is subject to the quantum mechanical uncertainty principle), the most massive white dwarfs collapse to the smallest diameters and highest densities. Stars above the Chandrasekhar limit are too massive to be supported in this way and must collapse further to become neutron stars or black holes. In practice, the addition of extra matter to a carbon-oxygen white dwarf at the Chandrasekhar limit may alternatively produce a runaway explosion as a type Ia supernova.

The light from white dwarfs does not arise from internal nuclear reactions but in a thin gaseous atmosphere that slowly leaks away the star's heat into space. The spectral lines arising in this atmosphere are broadened by the extremely high surface gravity, and in extreme cases the light loses enough energy to suffer a measurable gravitational redshift. Over 75% of white dwarfs have hydrogen-rich atmospheres, and are designated DA (see below). Some white dwarfs show no hydrogen at all in their spectra, whereas others are enriched in helium, carbon, and calcium. A few have strong magnetic fields (10⁵ tesla) and several have high rotational velocities. Some are pulsating variables (see ZZ Ceti stars). Novae, recurrent novae, and dwarf novae are close binary stars in which one component is a white dwarf (see cataclysmic variable).

Spectroscopic classification of white dwarfs by spectral type is inadequate because of the variety of surface composition. A classification scheme, introduced in 1983 by E. Sion and others, is based on the following spectral characteristics and temperature information:

DA only lines of neutral hydrogen (H I)

DB neutral helium (He I); no H or metals

DC continuous spectrum

DO ionized helium (He II) strong; no H or He

DZ metal lines only; no H or He

DQ carbon features

White dwarfs are further designated with a temperature index from 0–9, and with appropriate letters for magnetic fields/polarization (H,P), variability (V), and peculiar or unclassified spectra (X).

It has been estimated that there may be 10¹⁰ white dwarfs in our Galaxy, many having by now cooled to become black dwarfs. Best known of all white dwarfs is Sirius B, companion to Sirius (αCMa), which was discovered in 1862 by Alvan Clark after F.W. Bessel had predicted its existence (in 1844) from the unusual motion of Sirius. Sirius B has a radius of only 10⁴ km, about twice that of the Earth, but a mass similar to that of the Sun.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006