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The Question

(Submitted March 06, 2005)

I read that gamma-ray bursts are caused by hypernova explosions of Wolf-Rayet stars in the distant universe. Assuming this is correct, wouldn't these massive stars seem to be more prevalent in a younger Universe? Under what conditions might these stars form, and how long would formation take? If formed in the earliest Universe, wouldn't these stars be less likely to contain initial impurities of heavier elements?

The Answer

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Our page tells you that the Wolf-Rayet stars are massive stars near the end of their lives. Astronomers are not completely sure how these stars form, but it is possible that most extremely massive stars (more than forty times the mass of the sun) become WR stars for a short time near the ends of their lives. The mean lifetime of WR stars is estimated at only around 100,000 years and the formation time (from an existing star) is comparable. The reason there were probably more WR stars earlier in the universe is because it is believed that there were more massive stars early on. Stars early in the universe would initially contain fewer heavier elements than stars today, but heavy elements are produced inside WR stars and then distributed throughout their galaxy by the strong winds that WR stars generate. Thus WR stars are an important contributor to the enrichment of galaxies by heavy elements.

Hans Krimm for "Ask an Astrophysicist"

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