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A white dwarf is what stars like our Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material, creating a planetary nebula. Only the hot core of the star remains. This core becomes a very hot (T > 100,000K) young white dwarf, which cools down over the course of the next billion years or so. (That is, unless it is accreting matter from a nearby star). Many nearby, young white dwarfs have been detected as sources of soft (i.e. lower-energy) X-rays ; recently, soft X-ray and extreme ultraviolet observations have become a powerful tool in the study the composition and structure of the thin atmosphere of these stars. A typical white dwarf is half as massive as the Sun, yet only slightly bigger than the Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars.

Brown dwarfs are objects which have a size between that of a giant planet like Jupiter and that of a small star. In fact, most astronomers would classify any object with between 15 times the mass of Jupiter and 75 times the mass of Jupiter to be a brown dwarf. Given that range of masses, the object would not have been able to sustain the fusion of hydrogen like a regular star; thus, many scientists have dubbed brown dwarfs as "failed stars".

A white dwarf is what is produced when a low to medium mass star dies. They become red-giants during their helium-burning process after-which they shed their outer layers and become planetary nebulas. When its degeneracy pressure is exceeded the limit to which it cannot support this mass it may explode into a supernova. Now, a neutron star ( A Neutron star is made up entirely (almost) of neutrons, containing roughly or as much mass as our Sun but packed into a sphere only about 10km across.), is the remnants of a supernova explosion. As for a brown dwarf they are failed stars that never heated up enough to explode into a normal star.

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