A white dwarf is a small, dense star that forms when a main-sequence star burns all its hydrogen and helium fuel but lacks the pressure to fuse carbon and oxygen. It has a mass of 0.5-0.7 times the Sun but a volume comparable to Earth. White dwarfs are the final state of 97% of known stars and make up 6% of stars in our solar neighborhood. They are not very bright and do not have their own energy source. The first white dwarf was observed in 1783.
A white dwarf is a relatively small and dense type of star that forms when a main-sequence star burns all of its fuel of hydrogen and helium, but lacks the pressure and heat needed to fuse carbon and oxygen. A white dwarf typically has a mass between 0.5 and 0.7 times that of the Sun, but a volume comparable to that of the Earth. The white dwarf is the final state of stellar evolution for 97% of known stars.
The transformation of a star into a white dwarf begins when a main-sequence star, around the mass of our Sun, burns up all of its hydrogen fuel and begins to be forced to fuse helium into carbon and oxygen. As its core begins to build up with carbon and oxygen that cannot be fused, fusion must occur on a shell outside the core. The core’s immense gravity pulls the hydrogen together and causes it to fuse much faster than before, increasing the star’s luminosity by a factor of 1,000 – 10,000 and increasing its radius to something comparable to the orbit of Mars.
When all of the hydrogen in the star is fused, gravity takes over and the star begins to fall in on itself. If the star is massive enough, a supernova could occur. Otherwise, the excess material floats away to form a planetary nebula and only the super-dense core, which is the white dwarf, remains. Because a white dwarf doesn’t have its own energy source, the only heat it produces is a leftover from its helium fusion days. After billions of years, white dwarfs are expected to cool down to become black dwarfs, lifeless stellar shells, although the age of the universe (13.7 billion years) has not yet been old enough for this to happen.
White dwarfs make up 6% of all stars in our solar neighborhood. Since no nuclear reactions occur in their nucleus, they are not very bright, although they are observable with powerful telescopes. Sirius B, the mate of its more famous partner, Sirius A, also known as Dog Star, is a white dwarf. The first white dwarf was observed by Friedrich Herschel on January 31, 1783, in a binary system, Eridani B and C.
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