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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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