A supernova explosion ejects 1 - 20 solar masses of debris into interstellar space at velocities approaching 10% of the speed of light. The kinetic energy of this debris is about equal to the total energy that the Sun will radiate during its entire lifetime of 10 billion years -- about 100 times greater than the net energy radiated by the supernova. This debris will continue to expand for several thousand years, driving a blast wave (a "sonic boom") into the interstellar gas. A shell of hot compressed gas expands to a diameter of a few light-years within centuries, and to hundreds of light years after several thousand years. The expanding shell can be seen at radio, infrared, optical, and X-ray wavelengths. It remains visible for many centuries. We call it a supernova remnant (SNR). We have found hundreds of SNRs in the Milky Way.

Take a look at these images of Cas A from the Chandra Photo Album. The supernova, which occurred around 1680 AD, was barely visible because it is behind a dark interstellar cloud. In fact, it is still debated whether the historical record of the sighting of SN1680 is accurate (some say 1667). But there can be no doubt that a supernova went off there because its remnant, called Cassiopeia A, is one of the brightest radio and X-ray sources in the sky.

In fact, we can still see remnants of supernovae that occurred in prehistoric times. One famous example is the Cygnus Loop (also called the Veil Nebula). Here are some images of the entire Cygnus Loop as seen in optical, X-ray, and infrared bands. Another famous example is the Vela supernova remnant (optical, X-ray) the remnant of a star that exploded 11,300 years ago at a distance of about 1,500 light years. We can date the Vela SNR more accurately than the Cygnus Loop because it contains a pulsar, a spinning neutron star that is the remnant of the supernova explosion (we will discuss pulsars in Lesson 7).

As you can see, supernova remnants look different depending on the wavelength band in which they are observed. That is true because the radiation comes from different constituents of the compressed gas. For example, the optical radiation comes from gas heated to temperatures of order 10⁴ - 10⁵ K, while the X-rays come from still hotter (10⁶ - 10⁷ K) gas. You can see that the X-ray emission comes from inside the shell of optical and radio radiation, indicating that the hottest gas is inside the blast wave (for example, check the superposed radio and X-ray image R,X of IC433, the remnant of SN837). The infrared radiation from a SNR comes from grains that are heated by the gas. The radio emission is most interesting; it comes from cosmic ray electrons that are accelerated up to velocities nearly equal to the velocity of light. They gyrate in turbulent magnetic fields compressed by the blast, emitting synchrotron radiation, which we will discuss in the next lesson.

After 20,000 years or so, the blast slows down enough that the remnant becomes invisible. But the cavity of low density hot gas created by the explosion will persist for millions of years before the interstellar gas flows back in to fill it. Since massive stars are born in clusters, there's a good chance that another star in the same cluster will blow up before the cavity from the previous supernova is filled in -- and another, and another. The repeated action of many supernovae may cause the cavity to continue to grow, to diameters of hundreds of light years. Astronomers have observed many such superbubbles in the Milky Way and other galaxies. The compressed interstellar gas on the periphery of the superbubble may begin to fragment and collapse under its own gravity, giving rise to propagating star formation.

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Last modified February 22, 2002
Copyright by Richard McCray