About twice a century for galaxies like the Milky Way, and since there are about 30 billion of these spiral galaxies in the visible universe, that works out to perhaps 1 billion every YEAR. We don't see most of these because their optical light is probably hidden by galactic dust clouds in their own host galaxies. The above Hubble Space Telescope photo of a supernova in a distant galaxy shows that they are often faint, and unless you are looking at the right galaxy at the right time, you can miss them completely. They are only bright enough to be seen for a few months at most.
Copyright 1997 Dr. Sten Odenwald
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