Supernova
Milky Way candidates

The next supernova in the Milky Way will likely be detectable even if it occurs on the far side of the galaxy. It is likely to be produced by the collapse of an unremarkable red supergiant, and it is very probable that it will already have been catalogued in infrared surveys such as 2MASS. There is a smaller chance that the next core collapse supernova will be produced by a different type of massive star such as a yellow hypergiant, luminous blue variable, or Wolf–Rayet. The chances of the next supernova being a type Ia produced by a white dwarf are calculated to be about a third of those for a core collapse supernova. Again it should be observable wherever it occurs, but it is less likely that the progenitor will ever have been observed. It is not even known exactly what a type Ia progenitor system looks like, and it is difficult to detect them beyond a few parsecs. The total supernova rate in the Milky Way is estimated to be between 2 and 12 per century, although one has not actually been observed for several centuries.

Statistically, the most common variety of core-collapse supernova is type II-P, and the progenitors of this type are red supergiants. It is difficult to identify which of those supergiants are in the final stages of heavy element fusion in their cores and which have millions of years left. The most-massive red supergiants shed their atmospheres and evolve to Wolf–Rayet stars before their cores collapse. All Wolf–Rayet stars end their lives from the Wolf–Rayet phase within a million years or so, but again it is difficult to identify those that are closest to core collapse. One class that is expected to have no more than a few thousand years before exploding are the WO Wolf–Rayet stars, which are known to have exhausted their core helium. Only eight of them are known, and only four of those are in the Milky Way.

A number of close or well-known stars have been identified as possible core collapse supernova candidates: the high-mass blue stars Spica and Rigel, the red supergiants Betelgeuse, Antares, and VV Cephei A; the yellow hypergiant Rho Cassiopeiae; the luminous blue variable Eta Carinae that has already produced a supernova impostor; and both components, a blue supergiant and a Wolf–Rayet star, of the Regor or Gamma Velorum system. Mimosa and Acrux, two bright star systems in the southern constellation of Crux, each contain blue stars with sufficient masses to explode as supernovae. Others have gained notoriety as possible, although not very likely, progenitors for a gamma-ray burst; for example WR 104.

Identification of candidates for a type Ia supernova is much more speculative. Any binary with an accreting white dwarf might produce a supernova although the exact mechanism and timescale is still debated. These systems are faint and difficult to identify, but the novae and recurrent novae are such systems that conveniently advertise themselves. One example is U Scorpii.
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