In the simplest formation scenario for a supernova remnant (SNR), the energy of the supernova explosion would be deposited equally in all directions and the SNR would be a uniform spherical shell. However, differing progenitor and explosion conditions, density variations in the interstellar medium and Rayleigh-Taylor instabilities result in 3 main types of SNR.

E0102-72 is a shell-type supernova remnant in the Small Magellanic Cloud. Credit: NASA/CXC/SAO

Shell-type remnants As the name suggests, shell-type remnants emit most of their radiation from a shell of shocked material. We view this as a bright ring, since when we look at the edges of the three-dimensional shell, there is more shocked material along our line of sight than if we look elsewhere in the shell. This is known as limb brightening.

The Crab nebula is one of the most famous Supernova Remnants. It is the prototype for Crab-type remnants. Credit: Malin/Pasachoff/Caltech

Crab-type remnants Alternatively named plerions, these remnants are named for the prototype – the famous Crab Nebula. They are powered by a pulsar located at their centre and, in constrast to shell-type remnants, they emit most of their radiation from within the expanding shell. This means that they appear as a filled region of emission rather than a ring of emission.

IC433 is an example of a thermal composite remnant. In this image, blue corresponds to the X-ray emission and red the radio emission. Credit: Jonathan Keohane

Composite remnants These remnants are a cross between the other two remnant types, and appear either shell-like or Crab-like, depending on the wavelength of the observations. In general, thermal composites appear shell-like at radio wavelengths and Crab-like in X-rays, while plerionic composites appear Crab-like at both radio and X-ray wavelengths, but also show shell structures.