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The Question
(Submitted June 03, 1998)
Recently my teacher asked us if we could try to find any info on the
Wolf-Rayet stars. She said that she read it in an article that had to do
with stars. I've checked around, but I can't find anything on this topic.
Do you know of any places I can check to see if I can help her? I've
read about Wolf (345?), which is a star, but nothing on Wolf-Rayet stars.
I appreciate the time you take to try and answer the many questions.
The Answer
Wolf-Rayet refers to a type of star, sometimes abbreviated
"WR star". WR stars are hot, luminous, and contain atmospheres whose
thickness is comparable to the size of the star. (Most stellar
atmospheres are proportionally much thinner.) WR stars are also losing
mass in the form of a wind at a high rate of between about 10-6 and
10-5 of a solar mass per year. By comparison, our Sun loses about
10-14 solar masses per year in its solar wind (a solar mass is
about 2 X 1033 grams). The strong winds from WR stars are due to
the fact the radiation pressure (the force of light pushing on the
gases) in the atmosphere is quite strong.
You should be able to learn more about WR stars in a mid-level
college astronomy text. (The topic may be too specialized for an
introductory text.)
One of our local Wolf-Rayet experts, Dr. Michael Corcoran,
offers the following more detailed information:
Wolf-Rayet stars (named for their discoverers) are very large, massive
stars (stars which are about 20 times bigger than the sun) nearly at the
end of their stellar lives. As these stars age, material which the
stars have cooked up in their central nuclear furnaces (like carbon and
oxygen) gradually reaches the surface of the star. When enough material
reaches the surface, it absorbs so much of the intense light from the
star that an enormously strong wind starts to blow from the star's
surface. This wind becomes so thick that it totally obscures the star
- so when we look at a Wolf-Rayet star, we're really just seeing this
thick wind. The amount of material which the wind carries away is very
large - typically, a mass equivalent to that of the entire earth is lost
from the star each year. The mass loss is so large that it
significantly shortens the star's life, and as you can imagine has
important effects on the space surrounding the star too. We think that
very massive stars become Wolf-Rayet stars just before they explode as
supernova (though no one has yet seen such a star explode).
J.K. Cannizzo
(for "Ask an Astrophysicist")
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