Main sequence stars more massive than 8 solar masses are unable to lose sufficient mass to become white dwarfs. Their higher luminosities drive all evolutionary stages faster. The core is quickly converted to helium and then helium reactions produce a carbon‐oxygen core more massive than 1.4 solar masses, the limit at which electron pressure can balance gravity (in other words, the central temperature and density increase until ordinary gas pressure provides the balance against gravity). As electron pressure plays no role in stopping further core contraction, subsequent stages of core contraction can proceed, each momentarily halted by establishment, first in the core and then in outwardly moving spherical shells, of various thermonuclear reactions converting lighter elements into heavier elements (see Figure ). Such a star will quickly evolve to an onion‐shell configuration marked by conversion of sulfur, silicon, and magnesium in the core to iron and similar elements (see Figure ). Exterior to this core is a layer of these elements at cooler temperatures and unable to convert to iron. In a higher layer, carbon and oxygen are reacting to produce sulfur, silicon, and magnesium. This is overlain by an inert region that in turn is overlain by a shell of helium converting to carbon and oxygen. Further out is a hydrogen‐burning shell moving outwards into the envelope of the star.
- Evolutionary tracks of high‐mass stars.
Onion shell model. Nuclear reactions are limited to the densest and hottest central region of the star.
The conversion of a stellar core to iron is a serious problem for a star, because iron is a minimum energy nuclear configuration— nuclear reactions involving iron require the input of energy. During the evolution of a star, energy was released (and radiated away) in converting lightweight elements to iron, thus to convert iron back to lightweight elements requires recapture of this energy. Similarly, the conversion of iron into heavier elements also requires energy. The star now has no thermonuclear energy source in the core, but the core is losing energy outwards because energy always flows from high‐ temperature to low‐temperature regions. The only source of energy is gravitational, and now the core begins to contract in free‐fall collapse. Within a tenth of a second, the central temperature reaches 200 billion K and the density 10 12 g/cm 3.
The catastrophic collapse is the result of two factors. Gravitational contraction dumps an immense amount of energy into the material. But as the core temperature exceeds 10 billion degrees Kelvin, the photons that are produced (black body radiation) are of such short wavelength (Wien's law), they are able to interact with and destroy the iron nuclei:
The photodissociation of iron rapidly absorbs the energy released by gravitation, and the collapse is accelerated. When compressed enough, electrons are pressed into nuclei and helium ceases to exist:
This reaction also cools the core not only because the conversion of protons to neutrons requires energy to occur, but because the neutrinos (ν) carry away additional energy, thus accelerating the collapse. The loss of electrons that contributed to supporting the core through their degeneracy further aids the collapse.
If the collapsing core is not too big, neutronization of the material can stop the collapse. Like electrons, at high enough density neutrons can exert a neutron degeneracy pressure. The neutrons find themselves in an overcompressed state. The core thus rebounds almost instantly, sending a shock wave outward into the star.
Exterior to the collapsing core, stellar layers still rich with light elements have also been falling inward. The shock wave from the rebounding core reverses their infall and blasts this material toward the surface. This expansion is augmented by absorption of energy from the neutrinos pouring out of the core (normally neutrinos do not interact, but here absorption of energy is significant as the matter immediately outside the core is incredibly dense and the production of neutrons has produced a very high neutrino flux). These layers heat up and runaway thermonuclear reactions ensue. With immense numbers of neutrons mixed into the material from the core, the full gamut of chemical elements is produced. The core collapses and initiation of the type II supernova explosion is rapid, occurring on a time scale of minutes.
As observed from outside, the inner core region is revealed when, a few hours after initiation of core collapse, the shock wave reaches the exterior of the star. The outer envelope is blown away at thousands of kilometers per second, and the star, originally very bright, increases by 100,000 to 1,000,000 times in luminosity (see Figure ). Several solar masses of material may be blown away, including large quantities of newly formed heavy elements. Some 14 supernovae of this type (Type II) have been observed in the Milky Way Galaxy over the last 2,000 years, mostly by Chinese astronomers. These include SN 1054, which produced the Crab Nebula, visible 23 days in broad daylight; SN 1572, observed by Tycho Brahe; and SN 1604, observed by Kepler. SN 1987 in the Large Magellanic Cloud was the last naked‐eye supernova. Other supernovae remnants, the expanding clouds of heavy element rich gas, are the Vela Nebula and the Cygnus Loop. This type of supernova is seen in other galaxies in association with young stellar populations. This is consistent with Type II supernovae precursors being young, evolved high‐mass stars.