What do you call the big explosion that ends the life of a massive star?

Answer 1

The name of the explosion that happens when a massive star dies is called a supernova (-novae, plural). Some stars that have an unusually energetic explosion or are extremely large stars that are subclassified as hypernova (-novae plual).

The reason this explosion only occurs with massive stars (At least 20 solar masses) is because larger stars, when they start to run out of fuel, begin fusing much heavier elements than the smaller star is capable of. The difference between the two is a white dwarf and a neutron star or gravitational singularity.

A star's core is extremely dense, so dense that the only way for a star to counterbalance it's own gravitational pull from collapsing the star into itself is to fuse helium. The star uses the helium to create pressure that causes enough outward pressure on the star that it balances out the star's own gravity.

When a large star is running out of fuel, it starts to collapse. However, this collapse causes the star to become hot enough to fuse the helium that built up in the core when it was young into carbon and oxygen. Once it starts running out of helium, it collapses a little bit again but now hot enough to fuse the carbon in the core into neon, then the neon into more oxygen and, then the oxygen into silicon and finally silicon into nickel which decays into iron. At this point the star is dead, the only thing preventing total collapse is electron degeneracy. If the star's Fe-Ni core is small, under 1.4 solar masses, it will be just a dead star that's hot enough to emit light and solar winds will blow away it's "atmosphere," forming a planetary nebula. This phase is called the red giant.

If the Fe-Ni core is over 1.4 solar masses, it will be too dense and collapses into itself. This collapse happens extremely quickly and when it's a few hundred kilometers in radius, it stops shrinking instantly due to electron degeneracy and all the matter it was taking with it is shot back like a shockwave, That is a supernova. Even denser results in a gravitational singularity with a large accretion disc of the star's remnant matter.