What do you get when you blow up a really big star? Everything it turns out.

Supernovae are vast explosions in which an entire star is destroyed. They are mostly seen in distant galaxies as 'new' stars appearing close to the galaxy of which they are members. They are extremely bright, rivalling, for a few days, the combined light output of all the rest of the stars in the galaxy.

As most observed supernovae occur in very distant galaxies they are too faint even for the largest telescopes to be able to study them in great detail. Occasionally they occur in nearby galaxies and then a detailed study in many different wavebands is possible.

The last supernova to be seen in our galaxy, the Milky Way, was seen in 1604 by the famous astronomer Kepler. The brightest since then was supernova 1987A in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way. The brightest supernova in the northern sky for 20 years is supernova 1993J in the galaxy M81 which was first seen on 26 March 1993.

Supernovae fall into two different types whose evolutionary history is different. Type I supernovae result from mass transfer inside a binary system consisting of a white dwarf star and an evolving giant star. Type II supernovae are generally single massive stars arriving at the end of their lives in a very spectacular fashion.

Type II supernovae

The structure of all stars is determined by the battle between gravity and radiation pressure arising from internal energy generation. In the early stages of a star's evolution, the energy generation in its centre comes from the conversion of hydrogen into helium. For stars with masses of about 10 times that of the Sun this continues for about ten million years.

After this time all the hydrogen in the centre of such a star is exhausted and hydrogen ‘burning’ can only continue in a shell around the helium core. The core contracts under gravity until its temperature is high enough for helium 'burning', into carbon and oxygen, to occur. The helium 'burning' phase also lasts about a million years but eventually the helium at the star's centre is exhausted and it continues, like the hydrogen 'burning', in a shell. The core again contracts until it is hot enough for the conversion of carbon into neon, sodium and magnesium. This lasts for about 10 thousand years.

This pattern of core exhaustion, contraction and shell 'burning' is repeated as neon is converted into oxygen and magnesium (lasting about 12 years), oxygen goes to silicon and sulphur (about 4 years) and finally silicon goes to iron, taking about a week.

No further energy can be obtained by fusion once the core has reached iron and so there is now no radiation pressure to balance the force of gravity. The crunch comes when the mass of iron reaches 1.4 solar masses. Gravitational compression heats the core to a point where it endothermically decays into neutrons. The core collapses from half the Earth's diameter to about 100 kilometres in a few tenths of a second and in about one second becomes a 10 km diameter neutron star. This releases an enormous amount of potential energy primarily in the form of neutrinos which carry 99% of the energy.

A shock wave is produced which passes, in two hours, through the outer layers of the star causing fusion reactions to occur. These form the heavy elements. In particular the silicon and sulphur, formed shortly before the collapse, combine to give radioactive nickel and cobalt.

When the shock reaches the star's surface the temperature reaches 200,000 degrees and the star explodes at about 15,000 km/sec. This rapidly expanding envelope is seen as the initial rapid rise in brightness. It is rather like a huge fireball, which rapidly expands and thins allowing radiation from deeper in towards the centre of the original star to be seen. Subsequently most of the light comes from energy released by the radioactive decay of cobalt and nickel produced in the explosion.

Type I supernovae

Type I supernovae are even brighter objects than those of type II. Although the explosion mechanism is somewhat similar the cause is rather different.

The origin of a Type I supernova is an old, evolved binary system in which at least one component is a white dwarf star. White dwarf stars are very small compact stars which have collapsed to a size about one tenth that of the Sun. They represent the final evolutionary stage of all low-mass stars. The electrons in a white dwarf are subject to quantum mechanical constraints (the matter is called degenerate) and this state can only be maintained for star masses less than about 1.4 times that of the Sun.

The pair of stars loses angular momentum until they are so close together that the matter in the companion star is transferred into a thick disc around the white dwarf and is gradually accreted by the white dwarf. The mass transferred from the giant star increases the mass of the white dwarf to a value significantly higher than the critical value whereupon the whole star collapses and the nuclear burning of the carbon and oxygen to nickel yields sufficient energy to blow the star to bits. The subsequent energy released is, as in the Type II case from the radioactive decay of the nickel through cobalt to iron.

After the explosion

The evolution of the supernova after the explosion is one in which the ejected material continues to expand in a shell around the progenitor site while, in Type II supernovae, the central neutron star remains. The material continues to travel outwards for thousands of years until they impinge on gas and dust clouds in the surrounding interstellar space. There the ejected gas will mix with the interstellar material and eventually may be incorporated into a new generation of stars.

The origin of the elements

Theories of the Big Bang have successfully predicted the abundances of the light elements. The first stars were composed of hydrogen, helium and a very small amount of lithium and beryllium and almost nothing else. Some of these stars became supernovae and distributed the 'heavy' elements, made in their interiors, into the interstellar environment. Subsequent generations of stars have further increased the proportion of 'heavy' elements such as carbon, oxygen, phosphorus and iron.

It is a sobering thought that all the heavy elements that we encounter were formed in this way in the centres of stars and that without such violent explosions we would not exist.

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