Massive star mystery: Do they explode?

By Robin Lloyd

updated 12:57 p.m. ET April 7, 2008

Some of the most massive stars might not explode as supernovae, a new study suggests. Rather, researchers speculate, they simply collapse into black holes or if they do generate explosions, they're not as intense as the deaths of less massive stars.

The claim has been questioned by some astronomers, however.

How stars die depends in part on their mass. Our sun, for example, will swell into a red giant before becoming a white dwarf. More massive stars explode in massive supernova explosions. The new finding suggests that stars of around 20 to 30 times the mass of the sun might not explode at all, but rather collapse to form black holes.

The research, which also asserts that stars as small as seven times the mass of the sun still apparently have enough fuel to "go supernova," is based in part on a study of several pre-explosion images of stars taken by the Hubble Space Telescope. The team also looked at supernova SN2006bc, imaged as it faded several months after explosion.

Seeking supernova precursors
Astronomers from Queen's University Belfast in Northern Ireland, led by astronomers Stephen J. Smartt and Mark Crockett, requested the image of SN2006bc as part of a long project studying the massive exploding stars — supernovae. Exactly which types of star will explode and the lowest mass of star that can produce a supernova has not been known precisely. NASA and other sources frequently cite eight solar masses as the lower limit for supernova generation.

As part of an 10-year search for elusive supernova precursor stars, every time a supernova is discovered in a nearby galaxy, Smartt and his team begins a search of earlier Hubble images of the same galaxy to locate the star that later exploded. Often they're looking for one of hundreds of millions of stars in the galaxy. This is a little like sifting through days of surveillance camera footage to find one frame showing a suspect, Smartt said.

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If the astronomers find a star at the location of the later explosion, they can work out the mass and type of star from its brightness and color. Several such stars have been identified before they exploded, and the Queen's team studied the nature of five of them, Smartt said.

Red supergiant stars of up to 30 solar masses are expected to explode as Type II-P supernovae, stars that form as a result of a stellar core collapse and subsequent violent explosion.

However, in their analysis, the team found no very massive stars that had exploded, suggesting that the red supergiant stars between 18 and 30 solar masses may instead collapse to form black holes either without producing a supernova or by producing one that is too faint to observe, Smartt said.

The supernovae findings were detailed last week at the Royal Astronomical Society National Astronomy Meeting in Belfast.

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