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The Question

(Submitted February 16, 1998)

I just heard the term 'hypernova'. Do hypernovae really exist? Is it true that there was a recently discovered one? How does a hypernova form, in contrast to supernovae & black holes?

The Answer

A hypernova is a possible explanation for gamma-ray bursts. It can be thought of as a "failed supernova" -- a massive star whose core collapses but which doesn't quite blow itself apart. The idea is that the star's core collapses because it has run out of fuel and can no longer produce enough pressure to withstand gravity. The central part of the star collapses, forming either a neutron star or a black hole. In a supernova the resulting shockwave blows off the outer parts of the star. In the case of a hypernova the shock wave doesn't blow off the outer layers of the star. The material of the outer layers falls onto the central black hole or neutron star converting its gravitational potential energy to heat and radiation. This can result in a much higher luminosity than a supernova. This is why hypernovae were proposed as a possible explanation for gamma-ray bursts. The X-ray afterglow from a gamma-ray burst has been found to be more luminous than a supernova. Whether hypernovae actually exist is still an open question.

Damian Audley
for Ask an Astrophysicist

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