What is a supernova remnant?
A supernova remnant is the remains of a supernova explosion. A supernova is the instantaneous release of 10^31 megatons of energy from the volatile collapse of a massive star, or the nuclear burning on a white dwarf’s surface.
At the end of a massive star’s life, when its center has been converted into iron, nuclear fusion ceases. Nuclear fusion creates thermal pressure that prevents the star’s mass from collapsing onto itself due to gravity. Without the pressure, gravity begins to pull the mass together: the matter’s electrons and protons are pushed so close together that they merge into neutrons, releasing massive amounts of energy in the form of neutrinos. The conversion of the core to neutrons releases 10^51 ergs of energy; the outer layers of the star collapse and bounce off the solid central core, violently ejecting into the interstellar medium. Depending on the mass of the star, the mass either collapses into a neutron star, a massively dense star, or a black hole, a point of singularity where gravity collapses the mass into a single point.
At the end of low mass stars’ lives, they do not collapse into neutron stars or black holes and do not create supernova explosions. They resist gravity’s collapse with electron degenerate pressure, where electrons are pushed together completely. Normally, the white dwarf remains in this equilibrium and cools down for eternity. However, if a white dwarf is in a binary system with a red giant, the dwarf can accrete matter from its companion. If enough matter falls on it, the star is no longer stable, and gravity will overcome the electron degeneracy pressure and the star will collapse. The collapse raises the temperature of the star until the core begins fusion again: this ignites a wave of explosive nuclear burning, which releases 10^52 ergs of energy.
About 99% of the energy released by a supernova is in the form of neutrinos; the rest is in the form of kinetic energy. This accelerates the stellar material through the interstellar medium, compressing and heating gas. The interstellar medium becomes enriched with stellar medium, notably heavy elements. The heated material emits light, mostly X-rays, although the small percentage of the light that is visible can outshine entire galaxies.
Online catalogues of supernova remnants:
A Catalogue of Galactic Supernova Remnants by D.A. Green and Astrophysics Group, Cavendish Laboratory, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
The Chandra Supernova Remnant Catalogue, with access to X-ray images.
The MOST Supernova Remnant Catalogue, in radio.
Scientific papers about supernova remnants: