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The Question

(Submitted August 22, 2000)

When a star of adequate mass ceases to have enough fusion to support its self, collapses and goes supernovae just exactly WHERE does the energy come from that causes the explosion? The answer provided on your site simply stated that the star collapsed and energy was produced. So where does it come from?

The Answer

The short answer is that the energy released in a supernova explosion comes from the gravitational energy released as the star collapses. Think about an object falling from a very high building. You may know that this object as it is falling is transforming its potential energy into kinetic energy --- The same happens for a star iron core which collapses: it transforms its gravitational energy to heat and motion --- When the collapse is stopped by the formation of a neutron star able to sustain its own gravitational pressure because of the fermion pressure, there is a "bounce" and the star explodes.

Now the detailed answer is much, much more complicated than that and in fact, is a field of research by itself.

The large amount of gravitational energy available from the collapse of the core of the star is indeed transformed to heat and kinetic energy but the problem was that this is not the part which explode. The part which goes flying off the surface is a relatively loosely bound envelope of matter (hydrogen and heavier elements up to silicon)--- How the heat from the gravitational collapse of the core is transfered to the rest of the star was a heated discussion (no pun intended :-)) --- The solution of the "transfer" of energy may lie in neutrinos, these elusive particles which are supposedly massless, neutral and react very weakly with matter. These neutrinos are ejected carrying most of the energy of the gravitational collapse and in the process they give a little (around 1%) of their energy to the envelope which then "explodes."

The detection of a burst of neutrinos (7 total compared to the billions ejected in the explosion) right before the detection of SN1987A is considered to be the most impressive confirmation of the theory.

For more information you may want to check scientific articles dated from around the discovery of the neutrino burst (check "Scientific American" and "Sky and Telescope" index for neutrinos and SN 1987A).

We hope this answers your question.

Koji and Ilana for
the "Ask an Astrophysicist" team.

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