Supernovae and Shocks

A supernova results from the explosion of a star.  After the explosion, a remnant remains that consists of ejected material and the interstellar material swept up by an associated expanding shock.  There are really two main routes to produce a supernova:

  1. Fusion ceases in the core of a massive star.  Without the outward pressure from energy generated through fusion, the star collapses inwards under its own gravity to form a neutron star or black hole.
  2. A white dwarf in a binary system can accumulate mass from its companion star.  If the white dwarf accumulates enough mass, it will undergo a thermonuclear explosion.

Regardless of how the supernova is initially produced, the resulting remnant goes through several stages:

  1. Free expansion of the ejecta.  This phase lasts so long as the mass of the ejecta is much larger than the mass of the interstellar material swept up.
  2. A shell of shocked circumstellar and interstellar gas is swept up.  This phase is called the ‘Sedov-Taylor Phase’ and is well-modeled by an analytical solution.  This phase is valid so long as the time since the blast is less than the cooling time of the ejecta and is characterized by strong X-ray emission
  3. The shell begins to cool and forms a thin, dense shell surrounding a hotter interior.  This phase is termed the ‘Snowplow Phase’ and the dynamics are dominated by internal pressure.
  4. The dense shell continues to expand outwards due to its momentum and the interior cools.
  5. Eventually, the expansion of the remnant slows to the speed of random velocities in the surrounding medium.  At this point, the remnant will merge into the general turbulent flow.

At each of these stages, different wavelengths of emitted radiation trace the remnant best. Thus, by observing at different wavelengths, astronomers can trace out the evolution of a given supernova remnant.  See the discussion of Kepler’s Supernova Remnant to learn more about what observations at different wavelengths tell us about a specific supernova.


If you are particularly interested in learning more about supernovae, you might want to check out one of these pages:

And, if you are curious about the physics of shocks.  You should be sure to check out Philip Mocz’s website (also created as a final project for the same class on the interstellar medium):

1D Euler and MHD Shocks

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