MadSci Network: Astronomy

Re: What is the primary difference between a nova and a supernova?

Date: Thu Sep 28 10:07:37 2000
Posted By: Philip Plait, Astronomer/Programmer
Area of science: Astronomy
ID: 968385668.As

A nova and supernova are two different beasts. A nova is actually a binary system, usually a normal star like the Sun and a small dense white dwarf. They orbit closely, and the white dwarf can actually draw hydrogen gas off the normal star. The fierce gravity of the white dwarf compresses the material, and if enough builds up on the surface it can fuse into helium, releasing a sudden and tremendous amount of energy. We see the star brighten quickly, and fade over time.

There are two types of supernovae. One is very similar to a nova: a binary system with a normal star and a white dwarf. If the gas coming from the normal star comes at just the right rate, it can pile up more efficiently than in a regular nova. When it finally fuses, the release of energy can be much larger than a nova, and can actually blow the white dwarf apart. To be honest, no one is completely sure how this works. Some models involve two white dwarfs coalescing.

The other type of supernova involves a star with a mass greater than about 8 times the Sun's. There are a great many details involved, so I'll be brief. The star fuses hydrogen into helium in the core. When hydrogen runs out, the helium fuses into carbon, the carbon into oxygen, and so on, until the core is mostly iron. Unlike the previous elements, iron does not release energy when it fuses. When the pressure builds enough to fuse iron, it robs the core of heat and electrons, both of which are needed to support the mass of the star. The core collapses, a flood of neutrinos is released, and the outer layers of the star explode outward in a supernova.

The rotation of the star complicated matters greatly, and again, to be honest, no one is sure how this affects the explosion. It has only been a couple of years that computer models became sophisticated enough to include something like rotation, and so it's still too new to understand it completely.

I do not think a star can rotate quickly enough to prevent core collapse. The gravity of the core is fantastically strong, and in general a star does not rotate quickly enough to come anywhere near preventing collapse. After the core collapses it spins much faster due to conservation of angular momentum, but that it too late to prevent collapse in the first place.

There are a great many websites about this. Marcos Montes has compiled a fantastic list of links to tutorials, catalogs, research groups and articles about supernovae. I have a series of webpages about the history of Supernova 1987A, and one of them gives more details about the explosion of a massive star.

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