White Dwarf Stars
After the fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple - alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen - neon - magnesium white dwarf may be formed. The dwarf may have an initial temperature of up to 25000K, and be roughly the size of planet Earth. The density of the star will be comparable to the density of the nucleus – about 10 9 Kg/m 3 . If it can be seen – which may be a problem, given it's small size, hence small luminosity – it will have a bluish white colour hence the name white dwarf..
Most of the stars in the Universe – more than 90% - are low mass stars which will end up as white dwarves. Because low mass stars are also much longer lived than high mass stars, there will probably be a tendency in the long term for the proportion of low mas stars to increase – heavy stars burn bright for so very short a time, but low mass stars burn for billions of years.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a white dwarf, the Chandrasekhar limit — approximately 1.4 solar masses — beyond which it cannot be supported by degeneracy pressure.
When the white dwarf has been formed – with an initial temperature of up to may 25000K, which means it is small – it starts to cool. Over billions of years it will, cool and dim, eventually becoming a cold, “Black Dwarf”. Truly the end.
