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it's something for you to google or look up on Wikipedia

 

roughly the story is that whitedwarf is the quiet END OF LIFE stage of a low to medium-mass star

 

an important mass threshhold is called the Chandrasekhar limit.

cooling masses ABOVE the chandra limit have enough gravity to crush themselves down to neutron stars

 

cooling masses below the chandra limit (which is around 1.4 solar masses, so our sun is below the limit)

just remain dead stars and gradually contract

the contraction heats them white hot, so they glow,

but they have no more available fusion reactions so all their energy comes from the gravitational energy released by their slow contraction

 

there are more details to the story which you need to find out from Wiki or some encyclo like that

 

I got a link for you:

http://en.wikipedia.org/wiki/White_Dwarf

 

======sample quote===

A white dwarf is an astronomical object which is produced when a low or medium mass star dies. These stars are not heavy enough to generate the core temperatures required to fuse carbon in nucleosynthesis reactions. After such a star has become a red giant during its helium-burning phase, it will shed its outer layers to form a planetary nebula, leaving behind an inert core consisting mostly of carbon and oxygen.

This core has no further source of energy, and so will gradually radiate away its energy and cool down. The core, no longer supported against gravitational collapse by fusion reactions, becomes extremely dense, with a typical mass of that of the sun contained in a volume about equal to that of the Earth. The white dwarf is supported only by electron degeneracy pressure. The maximum mass of a white dwarf, beyond which degeneracy pressure can no longer support it, is about 1.4 solar masses. A white dwarf which approaches this limit (known as the Chandrasekhar limit), typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation.

Eventually, over hundreds of billions of years, white dwarfs will cool to temperatures at which they are no longer visible. However, over the universe's lifetime to the present (about 13.7 billion years) even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins.

As a class, white dwarfs are fairly common; they comprise roughly 6% of all stars.(RECONS estimate)

 

...

 

Formation

 

Almost all small and medium-size stars will end up as white dwarfs, after all the hydrogen they contain is fused into helium. Near the end of its nuclear burning stage, such a star goes through a red giant phase and then expels most of its outer material (creating a planetary nebula) until only the hot (T > 100,000 K) core remains, which then settles down to become a young white dwarf which shines from residual heat.

A typical white dwarf has half the mass of the Sun yet is only slightly bigger than Earth; this makes white dwarfs one of the densest forms of matter (109 kg·m−3), surpassed only by neutron stars, black holes and hypothetical quark stars. The higher the mass of the white dwarf, the smaller the size. There is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun). If this limit is exceeded, the pressure exerted by electrons is no longer able to balance the force of gravity, and the star collapses to a neutron star. Carbon-oxygen white dwarfs avoid this fate by undergoing a runaway nuclear fusion reaction (leading to a Type Ia supernova explosion) prior to reaching the limiting mass.

Despite this limit, most stars end their lives as white dwarfs since they tend to eject most of their mass into space before the final collapse (often with spectacular results—see planetary nebula). It is thought that even stars eight times as massive as the Sun will in the end die as white dwarfs, cooling gradually to become black dwarfs.

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