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The Question

(Submitted February 08, 1998)

Approximately how much do supernovae expand when they blow up?

The Answer

A supernova remnant can become very large, but depends on its age. There are large bubbles of hot gas within our galaxy that extend for hundreds of light years from older supernovae. The Crab Nebula, which is believed to have been formed from a supernova in 1054, is currently expanding at 1450 km/s which, after a little hand waving math, makes it roughly five light years in radius (assuming a constant expansion velocity, which is not correct in the early phase of the supernova, but is roughly correct over most of the 1,000 year age of this remnant). So had the Crab supernova been at Alpha Centauri distance when it went off, the blast wave would be coming at us or recently past us.

Jesse Allen for "Ask an Astrophysicist"

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