White dwarfs

THE fate of a star after its nuclear fuel burns out depends on the mass it was born with. Stars with a lot of mass become neutron stars or black holes. A low- or medium-mass star (less than about five times the solar mass, Ms) will become a white dwarf. When such sun-like medium-mass stars are generating energy by nuclear fusion of hydrogen in their cores, there is a delicate balance between gravity that pulls the star matter inward and the pressure of hot gas in the core that pushes outward. When the star exhausts its fuel, gravity wins and the star begins to collapse, resulting in a dense object called white dwarf, which is about as massive as the sun but only slightly bigger then the earth. Its density is about 200,000 times that of the earth. White dwarfs are one of the densest forms of matter in the universe, next only to black holes.

As the large electron population in such collapsing stars will begin to be confined to smaller and smaller volumes, they will exert their own outward pressure (a quantum phenomenon arising from what is known as electron degeneracy). In a stable white dwarf, gravity’s inward pull is balanced by this outward electron-degeneracy pressure. Chandrasekhar had shown in his celebrated work that a white dwarf cannot be more massive than 1.44 Ms beyond which the gravitational force becomes stronger and as such further collapse cannot be prevented.

White dwarfs are composed of degenerate matter, mostly carbon and oxygen formed by the fusion of helium nuclei in a sea of degenerate electrons. If such a white dwarf in a binary star system gains more mass, typically through matter accretion from a companion star, its mass can exceed the Chandrasekhar limit and it will begin to collapse because of its own gravity. This contraction results in an increase in the core temperature that is enough to initiate nuclear fusion again among the carbon and oxygen nuclei to produce elements up to nickel (Ni-56). Within seconds, a substantial fraction of the white dwarf undergoes a runaway thermonuclear reaction releasing an enormous amount of energy. This causes the complete destruction of the star leaving no remnant.

R. Ramachandran