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The Question

(Submitted March 09, 1998)

You mentioned in the introduction to supernovae that they are "relatively rare in our own galaxy". I am taking an introductory astronomy course and learned that a galaxy the size of our Milky Way should have about five supernovae per century. so why haven't more supernovae been seen in our galaxy in the past century and even in the last 1,000 years?!

The Answer

This question has perplexed astronomers studying our Galaxy for years. We think, based on theory, that a certain amount of stars would explode in our galaxy during a century. This number then seems to match other galaxies reasonably well. However, for our own Galaxy there hasn't been much evidence for a supernova in hundred of years.

One answer is just that the number 5 is an average. Meaning that in some centuries you'll have a few more, and in others a few less. We may just be at one of the low points, and then in a century or two there will be more that we can see. Another explanation is that there may have been several supernovas, but they were blocked out by huge clouds of gas and dust which absorb visible light.

There could be a "deeper" answer....i.e. the formation of massive stars in our neighborhood or whole Galaxy may have been "shut off" at some point, only to have turned back on more recently. But since we see very many massive stars (the kind that become supernovae), it seems unlikely that their production would have been shut off at any time in the past or that it would ever bee noticeable to us if it had.

Steve Bloom
for Ask an Astrophysicist

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