A Wolf-Rayet Star is one of the final stages in the life of a very massive star. We’re talking stars with more than 20 times the mass of the Sun. They’re already lived short, violent lives, but in the Wolf-Rayet stage, they blast out huge amounts of stellar material into space.
These stars start out with 20 times the mass of the Sun or more. Once they use up their hydrogen fuel, they swell up to become red supergiants. If a star is truly massive, it makes this final transition into a Wolf-Rayet star.
Wolf-Rayet stars were first discovered in 1867 by the astronomers Charles Wolf and Georges Rayet. They knew they were looking at something unusual because the spectral signature of the stars. It wasn’t until 1929 that astronomers realized that the strange signature to the stars came from the fact that they’re ejecting gas at enormous velocities: 300-2400 km/s. While a star like the Sun loses about 10-14 solar masses every year to its solar wind, a Wolf-Rayet star loses 10-5 solar masses every year.
It’s believed that the Wolf-Rayet stage of a star’s evolution stars when heavier elements forged through nuclear fusion reach the surface of the star. This sets off the powerful winds mentioned earlier. In fact, the winds are so strong they completely obscure the surface of the star itself. Astronomers aren’t seeing the star, they’re seeing the winds. The winds shorten the star’s life significantly, and it’s believed that this is the final stage in a star’s life before it finally detonates as a supernova. Of course, astronomers haven’t actually caught a star going through this yet.
There are about 230 known Wolf-Rayet stars in the Milky Way, with another 100 in the Large Magellanic Cloud, and 12 in the Small Magellanic Cloud.