A white dwarf is not a member of the characteristically named group of seven “hi-ho-ing” Disney entourage but a scientific term applied to a star which is nearing the end of its evolutionary life-cycle. Through modern advances and understanding. it is known that stars are not celestial nymphs with giant torches but burning balls of gaseous matter.
From a biological point of view, stars can be categorized by the stage of their development according to the Hertzsprung-Russell diagram. They begin their existence due to the pull of gravity on a giant molecular cloud which is basically floating errant matter, detrius of the universe and often of other stars. This proto-star becomes a fulled fledged star when its heat energy (from the conversation of potential gravitional energy) reaches 10 million Kelvins. (A scale of temperature measurement.)
When this occurs the very light atoms of hydrogen fuse together to form helium in chemical reactions which dispel energy. Eventually the star stops contracting as the gravitional pull finds equilibrium with the electron degeneracy pressure. (Basically the released electrons from fusion build to such a density that they themselves are continuously excited into higher energy states so that there is room for them to co-exist without occupying identical positions in time and space, as theorised in Quantum Physics – this is known as the Pauli Exclusion Principal.) Thus the star is now temporarily stable.
Even stars, however, do not live forever and when they have consumed their available hydrogen fuel, they die. How this occurs depends upon the mass of the star but a medium sized star, such as the sun, will again begin to collapse in on itself, trapping the remaining layers of hydrogen tight to its dense core. This speeds the rate of fusion and forces these molecules further from the core. Even if the star has begun to fuse helium and carbon fusion this only results in forming heavier matter that shrinks the core to an even denser state allowing these escaped molecules to breach further from its gravitional pull. This is known as a Red Giant and scientists can tell that this has occurred due to the luminousity and isotopes observed. This layer begins to cool and eventually will dissapate all together forming a planetary nebula.
All that now remains is a White Dwarf (its name first applied by Willem Luyten in 1922) which is a very dense, thermal omiting star at the end of its life. White dwarfs are stable (again due to degeneracy pressure) and can radiate with residual heat for billions of years. When they are first born they are brimming with heat energy and shine brightly in the sky but as they cool they become dimmer and their omissions morph to redder shades in the light spectrum.
A white dwarf, like the one the earth’s sun will become, will be composed mainly of carbon and oxgyen. Although larger stars with more complex chemical elements may have been able to parcipitate carbon fusion, the core temperature would not fuse neon or magnesium which will remain and add to the oxygen/carbon composition. Of course if the reactions occuring are violent enough the star may blow to a supernova.
A White dwarf may gain mass from the planetary nebula of other stars and grow in size to beyond the 1.4 solar masses of the Chandrasekhar limit at which point electron degeneracy pressure is not enough to protect stability of the star and the gravitional pull causes a carbon detonation that results in supernova.
With no fuel, no more fusion occurs at the heart of this ancient creation and as even its thermal energy is depleted, the White Dwarf no longer shines, becoming a cold black dwarf. However, considering the length of time this whole process takes and the comparatively infantile age of the universe, this has not been witnessed.