Novae are smaller than supernovae and occur in binary star systems where a white dwarf absorbs matter from its companion, triggering a nuclear explosion. Only a small amount of material is ejected compared to supernovae. White dwarfs are super-dense and composed of degenerate matter. They are the “ash” of larger stars that have fused hydrogen into heavier elements. Novae can happen multiple times in the same place.
A nova is a type of stellar explosion smaller than a supernova or hypernova. The phenomenon occurs in binary star systems, where a white dwarf absorbs a critical amount of matter from its companion, compressing the hydrogen on its surface and eventually triggering a nuclear explosion. Only about 1/10,000 solar masses of material is ejected, compared with 1.38 solar masses of material in a type I supernova and a dozen or more solar masses in a type II supernova. Although a Type I supernova also involves a white dwarf, in that case most of the star’s entire mass is molten. In the case of a nova, only a small percentage are.
A white dwarf is made up of degenerate matter, a super-dense phase of matter with the unusual property that its pressure is only weakly related to temperature. When a white dwarf’s gravitational field sucks in hydrogen gas from a nearby star, the gas fuses on the dwarf’s surface, merging with the degenerate matter. A white dwarf star packs a mass similar to the Sun into a volume similar to that of the Earth, compressing the incoming matter very tightly.
A white dwarf is the “ash” of a much larger star that has fused together much of its hydrogen into heavier elements. Usually, white dwarfs are composed of carbon and oxygen. The fusion of carbon and oxygen nuclei into heavier elements is an energy-intensive process that occurs only in the cores of very massive stars. Hence, the white dwarf does not have the ability itself to engage in further fusion reactions. But, if enough hydrogen is accumulated on its surface, it forms a shell that reaches the critical temperature and pressure for hydrogen fusion, about 20 million Kelvin. Hydrogen fuses rapidly, releasing energy in the same way as a fusion nuclear bomb (H-bomb). A nova results.
Since a nova doesn’t destroy its host star, it can happen more than once in the same place. One nova, RS Ophiuchi, has exploded six times since astronomers began observing it in 1890.
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