The Astronomy Cafe - Ask the Astronomer

Are supernovae the same as novae, but just brighter?

They are quite different, and typically differ by a factor of a million in their power. Novae can be either recurrent or non-recurrent. That means they can either reappear many times or be a 'one shot' event for that star. About 40 novae per year occur in the Milky Way compared to one supernova every 40 years. Novae seem to be binary stars in which one of the companions is a dense stellar remnant like a white dwarf. The other member of the system is an evolved red giant star millions of miles across, perhaps even bigger than Earth's orbit. Some of the gas from the evolved star makes its way over to the dense companion forming an orbiting accretion disk. Gas from this disk then leaks onto the surface of the companion and builds up until it reaches some critical mass. The surface of the dense companion then explodes in thermonuclear violence as this fresh new gas is consumed and ejected to produce an expanding cloud. The gas disk can be disrupted by this explosion, but over the years it rebuilds and the process repeats itself as a 'recurrent nova'. There are also one-shot novae that do not seem to reoccur.

Supernovae come in two major flavors: Type I and Type II. The Type I supernovae are commonly found in elliptical galaxies and are believed to have as their precursors a nova-type binary system. They are associated with stars that make up the oldest, low mass stars in systems such as globular clusters. Evidently what has happened here is that the star has evolved into a red giant, and some of its gas was absorbed by a dense companion star, probably a white dwarf or even a neutron star. But instead of a gentle nova-like outburst, the companion star was completely blown apart by the detonation. Type II supernovae come from the most massive stars in a galaxy that are at least 8 to 10 times more massive than the Sun. In this case, the star evolves until its available thermonuclear fuels can no longer support the star. Through a complex process we are now only beginning to understand fully, the entire star explodes, and ejects nearly all of its mass into space in a powerful, high velocity shock wave. These supernovae occur mostly in the arms of spiral galaxies about once every 50 years. The last one observed near us was SN1987A in 1987, located in the Large Magellanic Cloud some 160,000 light years away, and probably shouldn't count for the Milky Way's quota. We might be surprised by a supernova in your lifetime.

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