By Ron Cowen
To make a truly spectacular stellar explosion, you gotta have heft. And according to a motherlode of data on a supernova dubbed SN 2007bi, the exploding star originally tipped the scales at more than 200 times the mass of the sun. That would make the star more massive than any known in the Milky Way.
If the researchers are correct in their mass determination, the supernova belongs to a type never before observed and predicted to be common only in the early universe. The presence of such an exploding star in the modern-day universe — the explosion was observed in 2007 as it appeared in a nearby dwarf galaxy a relatively recent 1.6 billion years ago — suggests that astronomers may have to revise their models of how extremely massive stars live and die, several researchers say. Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues report their observations in the Dec. 3 Nature.
Several lines of evidence suggest that SN 2007bi was the explosion of a behemoth. The brightness of any exploding, massive star is governed by the glow generated by the radioactive decay of nickel-56, which is forged during the explosion. The supernova took about 70 days to reach its peak brightness, an indication that the glow from radioactive material had to punch through a huge amount of ejected material before it could be seen. In addition, the extraordinary brightness and duration of SN 2007bi, which lingered for about 1.5 years, indicates that the explosion generated a large amount of radioactive nickel — about three solar masses. The exploding star also had to be massive enough to gravitationally capture all that ejected nickel. Otherwise the supernova would not have been so bright.