White Dwarf
Remnant of a starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. with mass <8 Msun. White dwarfs have masses <1.4 Msun (the Chandrasekhar mass) and are supported by electron degeneracy pressureThe force that supports white dwarf stars against gravity. Quantum mechanics restricts the number of electrons that can have low energy. When electrons are packed together, the number of available low energy states is too small and many electrons are forced into high energy states. When this happens the electrons. White dwarfs have radii ~Rearth (<0.02 Rsun) and densitiesMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much ~105-6 g/cm3. No
nuclear fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion or gravitational contraction occurs in white dwarfs, they shine by residual heat. The surface has a very high temperature and radiates mainly in
the ultraviolet. White dwarf varieties are defined by the elements that dominate their surfaces as revealed by their spectra. However, if material is accreting onto the surface of a white dwarf from a companion star, the spectral will reflect this fact. Nearly all white dwarfs have C-O cores. Three varieties (DA, DB and DO) have
nearly pure surfaces of either H or He; PG 1159 stars appear to have partially exposed cores. White dwarfs with mixtures of elements on their surfaces receive
compound classifications. For example, DAB stars contain H and He, whereas DAO stars have H and ionized He.
Image source: http://www.daviddarling.info/encyclopedia/W/whitedwarf.html.
