Q&A: Miscellaneous X-ray Sources

Q:
I was intrigued by a comment in your X-Ray Astronomy Field Guide on White Dwarfs (page 2):

" the exhausted red giant will puff off its outer layer leaving behind a hot core. This hot core is called a Wolf-Rayet type star... After about a million years the WR star will collapse to form a White Dwarf star."

I though that Wolf-Rayet stars were very massive stars that would eventually explode as supernovas. How can they also be the progenitors of White Dwarfs which have a maximum mass limit of 1.4 solar masses?

A:
You are correct in your comment that classic Wolf-Rayet stars are thought to be massive stars that are rapidly evolving toward a supernova explosion. In this process the outer envelope has evaporated and the hot helium core has been exposed, and is also apparently evaporating. This process occurs with low mass stars as well. Their red-giant envelope has been expelled and the hot core is evaporating material, and lighting up the previously expelled envelope to produce a planetary nebula. Since the central stars of planetary nebulas show many of the spectral characteristic of standard Wolf-Rayet stars, but are much less massive, we refer to them as "Wolf-Rayet type" stars.