White dwarf

A white dwarf is a very dense type of star: in an Earth-sized volume, it packs a mass that is comparable to the Sun. A white dwarf radiates light from residual heat and eventual crystallization, not from nuclear fusion. Stars like the Sun, whose mass is not high enough to collapse into a neutron star or black hole, are expected to become white dwarf stars later in their evolution. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white. This would become known as the first white dwarf. The name white dwarf was coined by Willem Jacob Luyten in 1922. In 1931 Subrahmanyan Chandrasekhar developed a physical model of white dwarfs and he won the 1983 Nobel Prize in Physics for studies in the evolution of stars. These compact stars are composed mostly of a highly compressed form of matter. The composition of the white dwarf produced will depend on the initial mass of the star. Once formed, the material in a white dwarf no longer undergoes fusion reactions and thus lacks a heat source to support it against gravitational collapse. Instead, it is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit— approximately 1.44 times the mass of the Sun— beyond which electron degeneracy pressure cannot support it. Observed white dwarf masses are largely between 0.5 and 0.7 M☉. White dwarfs are very hot when they form, but cool gradually as they radiate their energy. This radiation, which initially has a high...