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High mass stars go through a similar process to low mass stars in the beginning, except that it all happens much faster. They have a hydrogen fusion core, but much of the hydrogen fusion happens via the CNO cycle. After the hydrogen is exhausted, like low mass stars, a helium core with a hydrogen shell forms, then a carbon core, with helium and hydrogen shells. Then unlike low mass stars, they have enough mass that gravity contracts the core raising the temperature and carbon can fuse into neon, then neon into oxygen, then oxygen into silicon, then iron. Each stage of burning lasts a shorter time than the previous one. For example, in a 25 solar mass star, hydrogen burning would take about 7 × 106 years, helium burning 7 × 105 years, carbon burning, 600 years, neon burning 1 year, oxygen burning 6 months and silicon burning one day.
Iron cannot release energy by fusion because it requires a larger input of energy than it releases. So the iron core continues to be subjected to gravity, which pushes the electrons closer to the nuclei than the quantum limit allows, and they disappear by combining with protons to form neutrons, giving off neutrinos in the process. Once this process starts, in a fraction of a second, an iron core the size of the earth and with a mass like our Sun, collapses into a ball of neutrons a few kilometers across. This gravitational collapse releases an enormous amount of energy, more than 100 times what our Sun will radiate over its entire 10 billion year lifetime. This energy blows the outer layers of the star off into space in a giant explosion called a supernova (plural: supernovae.) The ball of neutrons left behind is called a neutron star and is incredibly dense. In some cases the remaining mass is large enough that gravity continues to collapse the core until it becomes a black hole.
The explosion sends a shock wave of the star's former surface zooming out at a speed of 10,000 km/s, and heating it so it shines brilliantly for about a week. This shock wave compresses the material it passes through and is the only place where many elements such as zinc, silver, tin, gold, mercury, lead and uranium are produced. Over several months the gases cool and fade in brightness and join the debris of interstellar space. This debris has in it all of the elements that were created in the star's core. Millions or billions of years later, this debris may be incorporated into new stars. The fact that the Earth contains elements that are produced only in supernovae is evidence that our solar system, planet and bodies contain material that was produced long ago by a supernova.
The crab nebula is a remnant from a supernova that went off in 1054 A.D. When Betelgeuse explodes as a supernova it will be more than 10 times brighter than the full moon in our sky. It is only 640 light years away, and could have already become a supernova, but the light from it just hasn't reached us yet.
Supernovae occur in stars with at least 8 solar masses.