|MadSci Network: Astronomy|
There are two types of supernovae. Type I yields about 10^42 joules of energy and type II yields about 10^43 joules. Here 10^42 means 10 to the 42th power, or a 1 followed by 42 zeros. One ton of TNT is equal to 4.18 X 10^9 joules so you see that a type I supernova is about 2 X 10^32 tons of TNT and a type II is 10 times that. The destructive force of an explosion is hard to measure. It depends a lot on what is being destroyed. For instance, to vaporize a piece of ice of, say, ten tons you need about 4.2 X 10^9 joules which is one ton of TNT. Destroying a harder material may take much more than that. A type II supernova can sweep up about 1000 solar masses. A solar mass is 2 X 10^30 kilograms. Another way to look at this is by looking at Hiroshima after Aug. 6, 1945, when it received a bomb of 14 kilotons. One kiloton is 1000 tons of TNT. Nagasaki received 22 or 23 kilotons three days later. Today's fusion bombs have yields of the order of megatons, that is, millions of tons of TNT. I don't know what you mean by "critical mass". Nuclear weapons explode by a chain reaction in a fissile material like uranium 235 or plutonium 239. To produce the fission of most of the material, it is necessary to have a certain amount of the material concentrated on a block so that the number of neutrons needed to produce the reaction grow with time. When that happens we say that the amount of fissile material is above "critical mass". The critical mass depends on the shape of the block of fissile material, so there is no simple way to calculate the mass. Generally speaking the lower the surface of the block the lower the critical mass because less neutrons will escape the block through the surface. With respect to supernovae, the situation is completely different. A supernova explodes for a very different reason. Fissile elements like uranium or plutonium are very scarce in the universe. Normally, a star supports itself against its own gravity by the energy it generates from fusion. However, it is not possible to fuse iron nuclei together. If a star is massive enough, it will create iron in its core after several million years. At this point, it has no way to produce the energy it needs to support itself against gravity. So its core collapses to become a “neutron star,” composed entirely of neutrons. The outer regions of the star collapse until they hit the neutron star, at which point they bounce (literally) back into space. This is what we call a Type II supernova. The star needs a mass of about 10 times the Sun's mass for this to happen. It is even less clear how type I supernova go off. Apparently a star with a mass around one solar mass can go supernova, type I version, if it evolves into a white dwarf and then gets more mass from somewhere else. White dwarfs are stars that exhaust their fuel and collapse to the point where electrons and atomic nuclei are at their maximum degree of compression. If the white dwarf mass is increased beyond a limit of 1.4 solar masses, called the Chandrasekhar limit, it collapses very fast by forcing electrons and protons to combine and produce a very high amount of neutrons and neutrinos. Neutrons stay and form a neutron star, but neutrinos fly away and can blow out the outer part of the star producing the supernova event. There are many uncertainties with this picture because we do not understand the physics involved very well. White dwarfs can get this extra mass if they are part of a binary system, and their companion star expands and starts passing mass to them. Vladimir Escalante Ramirez
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