Type II Supernovae

A Type II supernova is comparable to a Type Ia in luminosity (the Type Ia is on average slightly more luminous but both produce the luminosity of billions of Suns) and in frequency (about once every 50 years in a galaxy like ours). However, they result from completely different phenomena. The Type Ia supernova energy comes from nuclear reactions; the Type II supernova instead derives its energy from gravity.
Massive Stellar Cores
The key to a Type II supernova is again the Chandrasekhar limiting mass. Although to this point we have emphasized the role of this limiting mass for white dwarfs, in fact it applies to any mass that is being supported by electron degeneracy pressure against gravity. As we shall now see, massive stars late in their lives have cores that are supported by degeneracy pressure and therefore there is a maximum mass beyond which they become unstable gravitationally. Consider the following schematic picture of the central few thousand kilometers of a massive star late in its life.

Note that the core region shown blown up in this figure is a tiny part of the whole star. It is only a few thousand kilometers in diameter, while the entire star is a supergiant with a radius that may be comparable to the size of the inner Solar System.

An Onion with an Iron Core
A massive star can go through a sequence of shell burnings, fusing heavier elements up to iron. But because iron and nearby nuclei are the most stable in the Universe, the star cannot derive energy from fusion of iron to heavier elements. Thus, late in its life the massive star develops a layered structure in its center, with concentric shell sources involving the fusion of hydrogen, helium, and so on up to silicon, with an iron core accumulating at the center. As the above figure illustrates, the star is a bit like an onion with an iron core.

Neutrinos

Neutrinos are central to the Type II mechanism. The spectacular light show and expanding supernova remnant carry only about one percent of the energy released in the explosion. The rest is in an enormous flux of neutrinos emitted from the core of the doomed star.

These neutrinos are important for several reasons, but two are critical. First, it is thought that heating by the neutrinos is responsible for keeping the shock wave from stalling in the core. Second, the burst of emitted neutrinos is a critical test of the Type II mechanism, since it is difficult to conceive of a process other than gravitational core collapse that could produce such a burst. As we shall see, the detection of a burst of neutrinos from the nearby supernova SN 1987A is the evidence that makes us certain that the core collapse mechanism is correct.

Degeneracy Pressure
The iron core cannot produce energy from fusion, so it must be supported primarily by electron degeneracy pressure. But there is a limit to the mass of the iron core that the degeneracy pressure can support--the Chandrasekhar mass mentioned above and discussed earlier in conjunction with the stability of white dwarfs. Thus, the star is stable initially, but the iron core grows steadily from the shell source around it. If the star is massive enough (probably 8-10 solar masses for the entire star while on the main sequence is the lower limit) at some point the core can no longer support the rest of the star and it collapses.

The Collapse of the Core
When the collapse begins, it proceeds with catastrophic speed. It is accelerated by neutrino emission and by disintegration of the core iron nuclei by very high energy photons so that the entire sequence happens in a fraction of a second! The core collapses until the density in the center reaches density comparable to the matter found in nuclei. Because matter at that density is highly incompressible, at that point the core rebounds and the rebound quickly forms a shock wave. The shock wave, assisted by a huge flux of neutrinos pouring out of the core, travels outward initially at speeds of a few percent of the speed of light. It reaches the surface some hours later, blasting the outer envelope of the star off in a tremendous explosion.
What Happens to the Core?
The crushed core that remains behind suffers one of two fates. If its mass is below about 2-3 solar masses, it collapses and cools to become an incredibly dense neutron star that is stabilized by neutron degeneracy pressure. If, on the other hand, the mass of the core is larger than this, calculations indicate that nothing, not electron degeneracy pressure, not neutron degeneracy pressure--nothing--can stop the collapse. In that case the core of the star collapses to a point of infinite density and surrounds itself with a strange region of spacetime called a black hole. We shall discuss both neutron stars and black holes in Chapter 22.