For the last four hundred years the only supernovae we've seen have been in other galaxies. When is the Milky Way due its next supernova?
One thousand years ago, a very bright new object appeared in the constellation of Lupus, the Wolf. It was reputed to be two to three times the size of the disc of Venus, and a quarter of the brightness of the full moon. Its dramatic appearance was recorded by observers in China, Switzerland, Iraq, Egypt, Japan and, almost certainly, in North American rock paintings.
These stars are the best candidates for Type II supernovae. This type of supernova occurs when large stars use up all of their hydrogen fuel and without an energy source they collapse under their own weight. This causes a colossal explosion in the form of a shockwave that rebounds outwards, blasting away the outer layers of the star and ultimately carving out 'supernova bubbles' in the sheets of interstellar gas. Like earthquakes, it usually isn't possible to predict when a star will go supernova, but we have thousands of supernova candidates that we know of. When we look at stars like Betelgeuse in Orion, which are in their final stages of evolution, we can only say that they'll go supernova somewhere in the next 10,000 to 100,000 years. In astronomy that's actually a
very short space of time, but it's far longer than human timescales.
A much better method used by astronomers is to look at the frequency of supernova in other galaxies, and then scale them to the Milky Way. By looking at other galaxies, we can get a rate for the Milky Way of one supernova every fifty to a hundred years. That's now comfortably inside the human timescale. We can look at tens of thousands of galaxies, and what we ﬁnd in the nearby ones is that Type II supernovae are the most commonly occurring, by volume. What we also know is that there definitely hasn't been a Type II supernova in our Galaxy since the 1980s when modern day detectors came of age. This is because Type IIs release enormous amounts of neutrinos when they explode, and we haven't detected any yet.
So when will the next supernova explode? It could be tomorrow, or it could be in a hundred years time. We simply just don't know.
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