As their name suggests, supernova remnants (SNRs) are the remains of material thrown out into space by a supernova explosion. The high speed collision between chemical elements processed in the supernova (and its progenitor) and the interstellar medium can produce some of the most beautiful objects in the sky, as demonstrated by the Cassiopeia A SNR in the constellation Cassiopeia (see Figure 1), especially at X-ray energies due to heating by the fast-moving shock wave. The spatial and spectral resolution of current telescopes, such as the Chandra X-ray Observatory, Hubble Space Telescope, and Spitzer Space Telescope which took the images shown in the figure, permit detailed investigations of SNRs. SNRs are invaluable tools for studying many aspects of supernovae, such as the final evolutionary stages of the progenitor star prior to its explosion and the explosion mechanism itself, and for understanding physical processes such as nucleosynthesis and chemical enrichment of the Universe and shocks and particle acceleration.
Supernovae generally arise from two types of sources. A Type Ia supernova is produced when a white dwarf accretes enough mass to make it unstable to gravitational collapse. White dwarfs are formed from low mass stars, and as such, they are generally very old and composed of relatively light elements, such as carbon and oxygen. On the other hand, a Type II supernova is the result of the gravitational collapse of a massive star (one with a mass greater than about eight times the mass of the Sun) after it forms an iron core and runs out of thermonuclear fuel. Such massive stars have short lifetimes, and the core of these stars forms either a neutron star or a black hole.
Supernovae only occur within the Milky Way at a rate of about two per century. While many extragalactic supernovae are detected every year, they are distant and can only be studied for a few years while they are bright. On the other hand, SNRs within our Galaxy can stay visible for many tens of thousands of years and can thus give complementary information to supernova observations. For example, there are several ways to determine the type of supernova that gave rise to a particular SNR. If a neutron star or black hole can be associated with the SNR, the supernova must have been of Type II. If the SNR is in a region where there is recent star formation or if the SNR resides near young massive stars, the supernova was likely to have been of Type II. If the SNR is far from the Galactic plane, then the progenitor star was likely to be old, and hence the supernova was of Type Ia. Finally, nucleosynthesis of chemical elements in low mass stars (which produce white dwarfs) and massive stars (which produce neutron stars or black holes) is different. Therefore by measuring the chemical elements in the SNR, the supernova type that gave rise to the SNR can be deduced.
Supernova remnants are sometimes classified as shell, plerion, or composite. Shell-type SNRs are those that are bright along the outer regions or rim (see Figure 1). This illumination is due to the supernova blast wave, with speeds of many thousands of kilometers per second, colliding with the interstellar medium. Studying these bright regions can provide insights into the physics of shock waves. For example, particles in the blast wave can be accelerated to very high energies by reflecting off magnetic fields within the shock; these high energy particles are one of the primary sources of cosmic rays detected on Earth. Plerionic SNRs are those which are bright in the central region, due to the presence of a hot plasma or a relativistic wind of particles emitted by a neutron star that is associated with the SNR. Composite SNRs are those that are bright both in the center and outer regions.
The Cassiopeia A SNR (Figure 1) is a shell-type SNR that is 11,000 lightyears away  and, from the rate at which it is expanding, about 340 years old . The blue dot near the center of the SNR is the X-ray bright neutron star that formed during the supernova explosion, and its progenitor was about twenty times the mass of the Sun . The irregular shape of the upper left side of the SNR suggests the explosion was asymmetric and could be the result of a jet of material. Asymmetries in SNRs can provide clues about the state of the progenitor star at the time of explosion or about the explosion mechanism. Instabilities of fluid oscillations or rotating magnetic fields within the progenitor star seem to be required for supernova explosions, and these mechanisms can provide preferred directions that give rise to asymmetric explosions.
More details and references on SNRs can be found in the review by Vink . An online catalog of SNRs based on radio observations is maintained by Dave Green at https://www.mrao.cam.ac.uk/surveys/snrs/. while a catalog based on high-energy observations is maintained by Gilles Ferrand and Samar Safi-Harb at http://www.physics.umanitoba.ca/snr/SNRcat/. Also images of Cassiopeia A have been taken over the past two decades, allowing a full 3D view of the evolution in time of the SNR (see http://chandra.harvard.edu/photo/2013/casa/).
 J.E. Reed, J.J. Hester, A.C. Fabian, P.F. Winkler, The three-dimensional structure of the Cassiopeia A supernova remnant. I. The spherical shell, Astrophysical Journal, 440, 706-721 (1995).
 R.A. Fesen, et al., The expansion asymmetry and age of the Cassiopeia A supernova remnant, Astrophysical Journal, 645, 283-292 (2006).
 O. Krause, et al., The Cassiopeia A supernova was of Type IIb, Science, 320, 1195-1197 (2008).
 J. Vink, Supernova remnants: the X-ray perspective, Astronomy & Astrophysics Review, 20, 49:1-120 (2012).
Author: Wynn Ho (University of Southampton)