| MadSci Network: Astronomy |
Hi Scott,
There are a couple of different types of "explosions" in stars. There are novae and supernovae. I'll answer your question about novae here. If you really wanted to know about supernovae, then we can answer that later.
A nova is basically caused by a nuclear explosion on the surface of an unusual type of star. A nova outburst can only occur in a special type of star system. There must be two stars orbiting each other (we call this a binary star system). One of the stars is fairly normal, perhaps like our Sun. The other star, however, is a white dwarf.
A white dwarf is a collapsed object, which we sometimes call a "dead star." Stars like our Sun support themselves against gravity by generating heat in their centers using nuclear fusion reactions. In fusion reactions, lighter elements are converted into heavier elements, releasing energy. This only works until the star runs out of light elements in its center. When the Sun runs out of light elements, it will contract fairly gracefully into a white dwarf. In a white dwarf, the matter is packed very tightly together, closely enough that the repulsion of the atoms can resist gravity. Don't worry, this won't happen to the Sun for another 5 billion years.
A binary system that will cause nova outbursts would look kind of like this picture . The normal star is so close to the white dwarf that some of its gas is being pulled off by the gravity of the neutron star.
The gravity at the surface of the white dwarf is extremely strong, about 100 thousand times stronger than gravity at the surface of the Earth. This means that the gas that falls onto the white dwarf becomes very hot and highly compressed.
Because the gas comes from a normal star, it also has a lot of light elements, just the sort that undergo fusion reactions. So, when enough hot, dense gas piles up on the surface of the white dwarf, it triggers a burst of fusion reactions. These start right at the surface of the white dwarf, where the gas is hottest. As this gas undergoes fusion reactions, though, it heats up the surrounding gas, causing that gas to undergo fusion, leading to a runaway. On Earth, we see this as a nova. Initially, we see the star system brighten by a factor of at least a hundred (usually more), usually over a period of about a week. They fade gradually, as the gas involved in the explosion expands and cools off. Years later, we can use telescopes to see the gas that is thrown off by the explosion as a nebula around the star, like in this picture , which was taken by the Hubble Space Telescope.
