These spectacular explosions are rare, with a supernova in the Milky Way occurring only once every 40 or 50 years. For a short time, a supernova releases enough light to rival the light of the entire Milky Way. In both processes, a supernova marks the brilliant death of a star. This is labeled Type 1 by astronomers, and it shows no hydrogen lines in the spectrum. The white dwarf collapses to form a neutron star or a black hole, blowing off a fraction of the excess mass in a titanic explosion. However, a supernova can also occur in a binary system, when the mass transfer from a giant or supergiant pushes a white dwarf over the Chandrasekhar limit. In this the material above the core collapses, and hits the dense iron core, being blown off in the resulting rebound and heating. This is labeled Type 2 by astronomers, who distinguish it by the strong hydrogen lines in the spectrum. One type of supernova results from the death of a single massive star, when the central core has been burned to iron and can no longer support the star against collapse. Three rings of glowing gas around supernova 1987a Among novae that are closest to Earth, the cloud of expanding debris can sometimes be seen in telescopes a few years after the explosion. The energy of the detonation accelerates the gas to speeds of thousands of kilometers per second, and deposits enough energy in the gas to cause the star to brighten by factors of hundreds or thousands. Since the individual explosions blow off only a small fraction of the star's mass, maybe only 0.01%, leaving the rest intact, the process can start over. Hydrogen may accumulate on a dwarf until nova explosions occur anywhere from 100 to 10,000 years apart. This type of explosion is called a nova (plural: novae).
The gas is heated as it is compressed, and the hydrogen may ignite in nuclear reactions that brighten the surface and blow excess gas outward.
It is necessary for the star to be a white dwarf because the surface gravity of a main sequence or larger star is too small to ignite nuclear reactions.
If the larger star dumps hydrogen onto the surface of a white dwarf, the hydrogen will be compressed by the intense gravity of the white dwarf. Consider the case of a binary where the more massive star has filled its Roche lobe and the smaller star is a white dwarf of nearly 1.4 solar masses. What are the differences between a nova and a supernova? A nova involves the transfer of mass onto a white dwarf, resulting in a rapid ignition of nuclear fusion on the surface that brightens the star for a while. These rapidly brightening or "exploding" stars were important historically as examples that the heavens are not immutable, as once thought, but are actually evolving, even as we watch. In ancient Chinese records, the novae were called "guest stars." In this century, astronomers discovered from their rate of light variation and other properties that there were two types of "new stars" - the novae and a much more energetic type called a supernova (plural form: supernovae). The word nova comes from the Latin root for new and was the term used for all "new stars" that appeared in the sky in past centuries. For ancient people who were very familiar with the night sky, these events were easily noticed and could be occasions for awe or fear. They both represent the sudden and temporary brightening in the sky of a star. Novae and supernovae are yet another case of astronomical phenomena that have been given similar names - because they looked similar to the astronomers who named them - but represent a number of distinct processes.
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