The nature of the Supernova explosion is another story entirely from that of the relatively tranquil nova process. Unlike the novae, whose essential condition remains unaltered after the blow-up, the supernova may suffer a complete collapse of its stellar core resulting in a super-dense star or even a black hole. (see Pulsars, Black Holes, Neutron Stars).
Supernovae are exploding stars reaching extreme luminosity (-18 Absolute Magnitude maximum), and a supeenova may outshine the combined luminosity of the entire galaxy in which it appears! Supernovae are also quite rare. There have been but a handful of supernovae within our galaxy in recorded history. Most supernovae are found in external systems or galaxies, and to date more than 400 such supernovae have been discovered. One of the best known appeared in M.31 (the Andromeda Galaxy) in 1885. There are at least two types of supernovae: SN I and SN II. Type I SN are powerful and brilliant, while Type II SR are faint and much less energetic. It is now believed that Type I SN are formed by the members of double-star systems. The cause of a supernova outburst is still the subject of intensive investigation (even controversy), but it is agreed that the onset of the explosion is ultimately related to instabilities in the structure of the star that arise when the supply of nuclear fuel in the central parts of the star is exhausted (see section on Evolution of Stars for more detail).
These instabilities occur only in stars whose mass is greater than about 1 1/2 times that of our Sun. Less massive stars, including the Sun, begin to contract when their nuclear fuel is consumed. In time, the pull of gravity is balanced by the pressure of degenerate electrons, an incompressible electron fluid that finally emerges because no two electrons can occupy the same energy state. When this stable configuration is reached, the star is called a white dwarf (which see), and gradually dies "not with a bang, but a whimper" as scientists delight in quoting. With stars of 1.5 solar masses, the density and temperature in the central core exceed the critical values beyond which stability is possible. The star collapses under the influence of gravity and an explosion occurs. In supernovae the outer shells of the collapsing star are ejected at ultrahigh velocity. In some, if not all, cases, a dense relic is left behind -- rotating neutron star or perhaps even a black hole! The resulting magnetic field on the surface of a neutron star can be more than a thousand billion times stronger than the average magnetic field on the Sun.
We have not observed any Supernovae.in our galaxy in over 300 years. Tycho Beahe wrote in De Stella Nova in 1573 about the supernovae that appeared in 1572: "...it was brighter than any other fixed star, including Sirius and Vega. It was even brighter than Jupite and maintained approximately its luminosity for almost the whole of November. On a clear day it could be seen ... even at noon."
The list below (with one exception) shows some of the major bright supernovae discovered in external galaxies. Figure A. shows some of the remnants of supernovae that have been discovered in our own galaxy. Many of these remnants are listed in the sections on Radio and x-ray sources. Supernovae remnants are often strong emitters of energy in the radio and x-ray frequencies Supernovae release gravitational energy in several forms. There is the radiant energy emitted in the early phases of the explosion. The matter simultaneously ejected carries away translational kinetic energy. The neutron star that survives is endowed with an enormous amount of rotational kinetic energy. As mentioned, it is believed that Type l supernovae are members of double-star systems. Their early evolution is similar to that of a single massive star. When they reach the white-dwarf stage, however, matter is transferred suddenly from the companion star, adding matter to the white dwarf and pushing the mass beyond the critical limit of 1.44 solar masses. At that point the core of the white-dwarf collapses violently, releasing energy as a supernova, leaving behind a binary system of an ordinary giant star and an x-ray source.
For a star much more massive than the Sun, the supernova evolution is different. The star also fuses hydrogen into helium in its core for a few hundred million years and, when the hydrogen is almost exhausted, the core contracts, the outer layers of the star expand, and the star becomes a red giant. Hydrogen continues to be burned in a shell around the core, as the core itself contracts until it heats up enough to fuse helium into carbon. When the helium is nearly exhausted, the core begins to burn the carbon. At that point, one of two conditions can occur. The ignition of the carbon could induce instabilities that would detonate the star as a SN II, leaving behind nothing but an expanding gaseous remnant. Or, if the carbon is safely ignited, the extraordinarily high temperatures in the core could generate neutrinos at an ever-increasing rate sapping the stars energy, causing the core to plunge to a total collapse. In this event, a final burst of neutrinos might carry away so much of the red giant's rotational momentum, that it would blow off the entire outer envelope of the star. An explosion of this kind would leave behind a gaseous remnant, in the center of which would be a pulsating pulsar a rapidly rotating neutron star) or a black hole.
Copyright (c) 1997-99 Michael Erlewine