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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 a High-Energy Astronomer
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