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Supernova [Q&A]


Krishna of Canara College asks, how do supernovae occur?

There are two kinds of Supernovae. One that occurs due to the build-up of accreting matter on the surface of a white dwarf, and another that occurs due to a collapse in the core of a giant star. The former is thought to occur as a thermonuclear deflagration event when the accretion pushes the white dwarf mass close to or above the Chandrasekhar Limit (see below). These are called Type Ia supernovae, and you can see movies of high-quality simulations of these explosions made by the Flash Center at UChicago. Type Ia supernovae are extremely useful because they can be used as standard candles to figure out cosmological distances.

But usually when people ask about supernovae, they mean the core collapse type. A star exists because it balances the contractionary forces of gravity with thermal pressure at the core. This thermal pressure is generated by the energy released during nuclear fusion. When the nuclear fuel is exhausted, that is, when the star can no longer produce energy by fusing nuclei, gravity overwhelms the star and the core contracts. Stars like the Sun end up as compressed Helium nuclei held up by electron degeneracy pressure (much like cores of planets) and live out their lives as white dwarfs. But if the mass of the core is greater than 1.4 solar masses (the Chandrasekhar Limit), electron pressure is insufficient to combat gravity.

Massive stars fuse more massive nuclei all the way up to Iron. Their cores are much more massive than the Chandrasekhar Limit. When eventually the nuclear fuel runs out, there is no longer enough energy being generated to maintain the thermal pressure to hold up the star against gravity, and the core falls inward. The physics of what happens next is quite complicated (see review by Woosley & Janka 2005, Nature Physics, v1, p147/arxiv:0601261), but briefly, a great amount of gravitational potential energy is released, the core collapses until it is halted by short range nuclear forces, and then “rebounds”, heating the rapidly infalling material. At this same time, a huge number of neutrinos are released because of inverse beta-decay, which creates a bubble of radiation that halts the collapse of the star and blows away all the outer layers. This is what we see as a Supernova.

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