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.
What is a Type Ia Supernova?
Just as there are different types of stars, there are different types of supernovae. They are classified empirically based on the elements identified in their spectrum. The core collapse supernovae described above are called Type II if they display hydrogen, Type Ib if they show helium, and Type Ic if neither hydrogen nor helium are present (these are arbitrary choices of representative letters). Although these categories were initially defined based on observational evidence, astronomers now understand the physical differences of the progenitor stars and their explosions that give rise to these classifications. As described above, a massive star becomes like an onion with the heaviest element, iron, fused in the center, and concentric shells of lighter elements out to helium and hydrogen. Since Type Ib do not show hydrogen but do show helium, this indicates that at the time of core collapse, the star did not have a hydrogen shell. Similarly, Type Ic have neither a hydrogen nor a helium shell, and their spectra show heavy elements such as iron from the core. How could this be? In massive stars that burn hotly and brightly, radiation pressures are large enough to blow the outer layers off the star. In more massive stars, more mass is lost from the outer shells - thus it is expected that stars of 8 to 20 solar masses become Type II, and more massive stars become Type Ib and Ic. This hypothesis has been confirmed for some of the nearest such supernovae, when the massive star visible in pre-explosion images has disappeared. There is one more empirical classification of supernovae called Type Ia. As with the Type Ic, the Type Ia do not show hydrogen or helium, but they do have remarkably strong silicon absorption lines, and also show iron. All Type Ia are very bright, and have similar intrinsic luminosities - this means they all release the same amount of energy, and a lot of it. These characteristics indicate they are not caused by a star's core collapse, but are thermonuclear explosions of 1.4 solar mass carbon-oxygen white dwarf (COWD) stars. A star which is initially 2-8 solar masses is not hot enough to fuse elements heavier than carbon and oxygen. At this stage the star cools, shrinks, loses most of its mass during a planetary nebula phase, and becomes a COWD star. These stars are very dense - the mass of the sun but the size of Earth - and only stable when less than 1.4 solar masses. However, if a COWD has a binary companion it may accrete matter, and grow. At the critical mass, a thermonuclear runaway reaction fuses most of the material to radioactive nickel in a matter of seconds, which then decays to iron. The remaining material is burned into lighter elements like silicon. Although COWD stars are too faint for direct confirmation as the progenitor, they are the only known physical scenario which simultaneously explains the brightness, similarity, and spectra of Type Ia supernovae.
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