Explosive Star Doesn't Fit the Mold
This image, dated January 12, 2008, shows both SN2007uy and SN2008D (the bright spot near the northern tip of the galaxy) two days after the discovery of SN2008D.
Credit: Science/AAAS

A supernova spotted earlier this year may actually represent a cosmic event closer to energetic gamma ray bursts, rather than classic stellar explosions.

European researchers now suggest that the supernova known as SN 2008D resulted from a massive star collapsing into a black hole. That event produced a five-minute long burst of X-rays, which NASA?s Swift telescope detected on January 9, 2008.

"Our observations and modeling show this to be a rather unusual event, to be better understood in terms of an object lying at the boundary between normal supernovae and gamma-ray bursts," said Paolo Mazzali, an Italian astrophysicist at the Padova Observatory and Max-Planck Institute for Astrophysics.

Stars that were about eight times more massive at birth than our sun end their relatively short life in a cataclysmic explosion and collapse into either neutron stars or black holes. The massive exploding stars emit a short cry of agony in the form of light, X- or gamma-rays, with gamma-rays being the most energetic.

Mazzali?s team found that the early behavior of the supernova indicated that it was a highly energetic event for a supernova, although not quite as powerful as a gamma-ray burst. Theoretical models show that the original star was at birth as massive as 30 times the Sun, but had lost so much mass that at the time of the explosion the star had a mass of only 8 to10 solar masses. The likely result of the collapse of such a massive star is a black hole.

The original star also shed much of its hydrogen and helium-rich outer layers before exploding, characteristics normally associated with gamma-ray bursts. However, Mazzali?s team saw a helium signature still lingering in the explosion?s aftermath, which suggests that the star did not quite reach the level of a gamma-ray burst.

This presents an alternative explanation to one detailed earlier in the journal Nature by another group of astronomers. That team suggested that X-rays were detected only because stargazers caught the star in the act of exploding, and that the event was a more typical supernova.