Hit and run. An inconspicuous high-velocity star (circled) is the prime suspect in the star-slaughter.

CSI Cassiopeia

Astronomers have found the perpetrator of a cosmic crime committed 432 years ago. The guilty star, quite similar to our own sun, first killed its partner and then fled the crime scene. The discovery beautifully confirms the commonly accepted theory behind the formation of a particular type of supernova.

When Danish astronomer Tycho Brahe witnessed a stellar explosion in the constellation Cassiopeia in November 1572, he didn't know much about astrophysics. But from his meticulous brightness estimates, astronomers now conclude that he saw a type Ia supernova. These are thought to arise when a normal star spills its outer layers of gas on its compact white dwarf partner. As a result, the white dwarf becomes too massive to support itself, and it shatters into smithereens in a titanic thermonuclear explosion.

At least, that's the theory--but it had never been confirmed beyond doubt. Now, astronomer Pilar Ruiz-Lapuente of the University of Barcelona and her colleagues claim to have identified the murderous star, which survived the ordeal but was flung into space at a breakneck speed of almost 500,000 kilometers per hour after its partner's demise. Studies using the 4.2-meter William Herschel Telescope at La Palma, Canary Islands, the 10-meter Keck Telescope at Mauna Kea, Hawaii, and the Hubble Space Telescope revealed a more-or-less sunlike star at the known distance of the supernova (some 9200 light-years) but with an unusually high space velocity. Although there is no evidence that the widow star swept up remains of its exploded white dwarf partner, it seems to be swollen to about twice the size of the sun, probably because of the supernova's energy, the researchers report in the 28 October issue of Nature.

The theory about how type Ia supernovas blow up is pretty well established, and it's hardly surprising that there is indeed a surviving companion star, says theoretical astrophysicist Stanford E. Woosley of the University of California, Santa Cruz--"but it's certainly nice to see," he says. Learning more about type Ia supernovas may also help cosmologists better understand the universe at large, Woosley adds: Observations of remote supernovas of this particular kind are used to calibrate the expansion history of the cosmos.

Related sites
The team's Web page on the discovery
More on Tycho's supernova
Supernova primer

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