|MadSci Network: Astronomy|
Well, this is a tough one! There are several ways to interpret your question, but I'll think of it this way: how many supernovae have occurred that we know about?
Before the invention of the telescope, not too many supernovae were known. The first recorded supernova was in 1066. Oddly, the Chinese astronomers took note of it, but no western ones did! The next recorded one was in 1181, again by the Chinese. The next was in 1054. Again, it was observed by the Chinese, but there is evidence it was seen by American Indians as well. This was a very bright supernova, easily visible in full daylight, and today can be seen as the Crab Nebula, one of the most studied objects in the sky.
The great astronomer Tycho studied another one in 1572, and another great astronomer named Kepler observed one in 1604. The next one I could find in my research was in 1885, and occurred in the nearby Andromeda Galaxy. This was the first supernova ever seen outside our own Galaxy. The last naked eye supernova was 1987A, which occurred in a satellite galaxy to the Milky Way.
Since the invention of the telescope, there have been more supernovae seen every year. Last year, for example, 82 were found! I could not find an exact number, but it seems safe to say that over a thousand supernovae have been seen, and probably many more. Since we don't know just when and where they may occur, they are almost always discovered by accident. Recently, some telescopes have been built that automatically search the skies for anything that changes from night to night. They find asteroids, comets, variable stars, and the occasional supernova.
There has not been a supernova seen in our own Milky Way galaxy for hundreds of years. When is the next one? No one knows. There are many good candidate stars for explosion, such as Betelgeuse in Orion, or Eta Carina (check out the Hubble Space Telescope Eta Carina pagefor lots of pictures and info!). When these stars blow, it will be quite a show!
There is a vast amount of information about supernovae on the web. My web page has a good starting point, with pictures and descriptions of supernova 1987A and links to other sites as well.
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