Only a tiny minority of stars — about one in a few hundred — are massive enough to actually die suddenly; the rest blow off their outer layers and contract down to a white dwarf over a period of tens of thousands of years.

But the massive stars are disproportionately bright, and so are much more likely to be the ones we see! While there may only be around nine thousand stars visible to the naked eye, there are dozens of naked eye candidates for the next supernova within our galaxy. It’s very difficult to tell by looking at a single star what stage of life it’s in, and how close it is to going supernova. A star like Eta Carinae or Betelgeuse, for instance, may have already exploded and ended its life… or it may continue to stick around for hundreds of thousands of years as it continues to burn through its fuel. There’s no cataclysmic “it’s about to blow” signal, and in the case of Eta Carinae, a recent outburst (a huge mass ejection) in the 19th century may have delayed its eventual supernova explosion by a longer timespan than human beings have existed.

On average, a star destined for a supernova remains in this indeterminate, giant phase of its life for between one and ten million years, typically. Although there are many theories about what we could look for when a star gets “close” to going supernova, the reality is that the last one we observed in our galaxy occurred over 400 years ago, the most recent remnant discovered is over a century old, and very little is known about the progenitor star that exploded in a satellite galaxy of our own in 1987: the nearest supernova to be seen by humans in action since 1604.