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The Question

(Submitted February 23, 1998)

How are supernovae detected and how can I find out when the last one occurred?

The Answer

Most bright supernovae (SNe) are discovered by amateur astronomers. People who know the sky really well go out and look at many galaxies with their small telescopes to see if any look unusual. Then they contact an observatory, which then notifies the astronomical community via e-mail.

In addition to the amateur astronomers there are a number of new automatic searches which find many more, fainter, supernovae. We saw an announcement in February for supernova 1997fg. This implies that at least 163 SNe were discovered last year. Most of them were very faint, and reported a dozen or so at a time.

In 1987 there was a supernova in a companion galaxy to ours, that is still a very interesting object (SN1987A). This was the last "nearby" (i.e. less than a million light years away) supernova.

The last known supernova in our galaxy was in 1680. However it was so dim that it was thought to be a star, and historically shows up only as an anomaly in the sky charts of John Flamsteed. It is now extremely bright in radio waves and X-rays and is known as "Cassiopeia A."

Most recently, X-ray astronomers and gamma-ray astronomers of the Max Planck Institute for Extraterrestrial Physics in Garching/Germany, have discovered a young supernova remnant which is exceptionally close to Earth (Nature, Vol. 396, 12 November 1998). The remnant is just 700 light years away and it is claimed to have been created about 700 years ago when a star exploded in the southern sky in the constellation Vela ("sail")

Good luck,
Jonathan Keohane, David Palmer & Karen Smale
for Ask a NASA scientist

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