As we saw above, a core-collapse supernova is the result of the collapsing core of a large mass star, when its nuclear fuel runs out and the core is no longer able to support itself by the electron degeneracy pressure of the core material. The collapse can reach velocities of 70,000 km/s and is suddenly halted by the neutron degeneracy pressure when the core mass is small enough and a neutron star forms. The ensuing shock wave sweeps up an expanding shell of stellar material which is observed as a supernova remnant.
The Crab Nebula in the constellation Taurus (already discussed above in the chapter on Pulsars) is a supernova remnant from a bright supernova recorded by Chinese astronomers in the year 1054. The nebula was the first astronomical object identified with a historical supernova explosion.
The released gravitational potential energy during core collapse is unimaginable - more energy than is produced by 100 stars like the Sun during their entire lifetimes. Most of the energy released during collapse is carried off into the outer regions of the star by neutrinos and absorbed by nucleosynthesis and kinetic energy; however a relatively small fraction of the energy triggers the accompanying Core-collapse supernova explosion. Luminosity can peak at ten billion Lsun and temporarily outshine the entire host galaxy.
|The word “Nova” comes from the Latin word for “new.” Plural is Novae.
Observers initially thought such objects to be a new star, hence the name.
What happens in detail is dependent on the mass of the collapsed core, its degenerate state and on the metallicity of the progenitor star.
Metallicity is a measure of the abundance of chemical elements other than hydrogen and helium in a star. Stars with low metalicity are generally old stars lacking enriched material from stellar explosions.
The released energy triggers many different processes of nucleosynthesis that forges heavier chemical elements, although much of those may subsequently disintegrate. Generally supernovae are thought of as the source of most heavier elements above Iron (more below). This heavy element enriched gas can be incorporated into future generations of stars and planets.
If the collapsed core is larger than about 3 Msun neutron degeneracy pressure is not enough to prevent further collapse, and a black hole forms. Often this does not generate a supernova at all. But if a black hole forms later caused by in falling material into the core, a super bright supernova may occur.
Core-collapse supernovae are very complex and subject to much ongoing research. Main factors to observe are the variations in the observed spectra and the duration of the whole event.
One of the most observed supernova is 1987a in the Large Magellanic Cloud then went off in 1987 (actually that is when it was observed; it went off 160,000 years earlier, because of the finite speed of light we saw it that much later) and has been observed ever since.