Supernova Remnant Formation

Many of the gaseous structures in the interstellar medium were formed by the explosions of massive stars (most with mass greater than 15 times that of the sun). The supernova explosion emits about 10^49 ergs of optical energy, most of which gets deposited in the kinetic energy of the ejecta. This ejecta (the remnant of the supernova) goes through a three phase evolution. In the first phase, free expansion, the front of the expansion is formed from a hydromagnetic shock wave interacting with the ambient interstellar medium. This phase is characterized by constant temperature and velocity until reverse shock waves eventually heat the interior of the supernova remnant (SNR). During the second phase, known as the Sedov or Intermediate Non-Radiative Expansion Phase, the SNR material slowly begins to decelerate by 1/r^(3/2) and cool by 1/r^3. Once the temperature reaches 10^6 K, radiative cooling dominates. The third phase, the Snowplow phase, begins at this point and velocity now decreases as 1/r^3. Outward expansion stops and the SNR starts to collapse under its own gravity. After millions of years, the SNR will be absorbed into the interstellar medium due to Rayleigh-Taylor instabilities breaking material away from the SNR's outer shell.

During its long evolution, the SNR produces a large amount of ionizing radiation, particularly in the soft x-ray part of the spectrum. These x-rays are created by the interaction of the interstellar medium ambient with the ejecta from the supernova explosion. From x-ray observations, basic information about an SNR's initial energy, spectra, age, and ambient density can be obtained. Observational evidence of SNRs has shown the existence of high-temperature plasmas, of which x-rays are characteristic. Because of this, use of very sensitive and high resolution x-ray instruments would permit the quantitative investigation of both conditions of high temperature plasmas and the nature of shock waves.

For more information, on SNR evolution, see: Chevalier, R.A. 1977. Ann. Rev. Astron. Astrophys. 15: 175-96
For more information on SNR and soft X-rays, see: Gorenstein, P., Tucker, W.H. 1976. Ann. Rev. Astron. Astrophys. 14: 373-414
or connect to Jonathan Keohans's Supernova Homepage
For more general information on SNRs, see: Astronomy, The Cosmic Perspective by Zeilik and Gaustad
Special Thanks to Rob Klinger, Penn State