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Brief Introduction to Supernovae

A brief introduction
on Supernovae
The explosion of a star as a Supernova is certainly one of the most energetic events observable in the whole universe, and their radiation covers the whole band from radio to gamma-rays, accompanied with neutrino emission.
The last supernova to be seen in our galaxy was seen in 1604 by the famous astronomer Kepler. The brightest and closest since then was supernova 1987A in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way. As it is possible to understand from the picture of SN 1987A on the right (before and later the explosion), they are extremely bright, rivalling, for a few days, the combined light output of all the rest of the stars in the galaxy. For this reason, the first astronomers that observed "new" stars appearing in the sky called them "Stellae Novae".

The physics involved in generating these catastrophic events must be both extreme and exotic... and therefore very interesting! In addition, these explosions give us the opportunity to better understand the evolution and the structure of the interior of a star, and they have a key role in the element enrichment of the universe, producing a significant fraction of the iron-group elements now present in the universe, as well as contributing significantly to the abundances of elements heavier than iron. Supernovae are also used by cosmologists as "standard candles" to measure distances in the universe and to determine the expansion rate of the universe. Finally, they can be used to probe obscured starburst regions of distant galaxies, where thousands of new stars are forming, hided by huge amount of dust.

Supernovae fall into at least two different types, whose evolutionary history and progenitors are quite different. Type Ia SNe are supposed to  result from mass transfer inside a binary system consisting of a white dwarf star and an evolving giant star, which results in a termonuclear explosion. Type II supernovae are, in general, single massive stars which come to the end of their lives in a very spectacular way, after the collapse of their core. Here you can find a picture summarizing the SNe zoo, with the present day classification based on their spectral appearence and the different light curves shapes.

SNIa Thermonuclear Supernovae

Type Ia SNe are observed in all types of galaxies, including the elliptical ones, as the SN in the image on the left. In addition their spectra and their light curves are quite homogeneous and ddon't show signs of hydrogen. For these reasons the origin of a Type I supernova is supposed to be an old, evolved binary system in which at least one component is a white dwarf star. White dwarf stars are very small compact stars, very similar to one another, which have collapsed to a size about one tenth that of the Sun, and 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). The pair of stars loses angular momentum until they are so close together that the matter in the companion star is gradually accreted by the white dwarf. The mass transferred from the giant star increases the mass of the white dwarf to a value higher than the critical limit whereupon the nuclear burning of the carbon igites in an explosive way due to the degenerate structure of the star, and blows the star to bits. What we see in a Type Ia SN, than, is nothing more than the fallout from a thermonuclear explosion. The subsequent energy released is from the radioactive decay of the nickel through cobalt to iron.

SNII Core-collapse Supernovae

This type of events is not observed in elliptical galaxies, but generally in the spiral arms of galaxies and in HII regions, where star formation is active. This support the idea that massive stars, with masses higher than 8 times the mass of the Sun, are the progenitors of this class of SNe.
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 kilometre 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 2 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 which are responsible for the shape of the light curve after the first two weeks.When the shock reaches the star's surface the temperature reaches 200 thousand degrees and the star explodes at about 15000 kilometres/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.  

If you want to learn more on supernovae, check this list of Supernova and Supernova Remnant pages on the WWW

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