Q&A: Miscellaneous X-ray Sources
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?
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.