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.

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.

Overblown claims?

Other astrophysicists, however, said these claims were exaggerated, raising questions about the assertion that very massive stars don't go supernova.

"My main concern is whether they have enough statistics yet to confirm this," said Adam Riess, an astrophysicist at Johns Hopkins University and at the Space Science Telescope Institute in Baltimore, which oversees Hubble's science operations. "It's a little like concluding that there cannot be a much more advanced civilization than ours because they have not contacted us yet. An argument by absence. But it's very plausible."

Crockett said it is possible that massive stars do actually produce supernovae that have simply gone unobserved as yet, "but as the number of events in our sample increases this possibility becomes increasingly unlikely."

Weidong Li, a research astronomer at the University of California, Berkeley, agreed that almost all supernova precursors have a mass lower than 25 solar masses (25 times the mass of the sun), so many groups have questioned the ultimate fate of very massive stars.

"However, very massive stars are much rarer than less massive stars, so the fact that we detect about a dozen less massive progenitors doesn't mean that stars more massive than 25 solar mass will go to black holes without a supernova explosion, or too faint to observe," Li said.

In fact, supernovae that likely come from very massive stars have already been observed, Li said, though their precursor stars have not (because the supernova sites have not been observed with the Hubble Space Telescope or big ground-based telescopes). He noted SN 2006gy, as an example, an object that he and his colleagues have published research on.

"This supernova is likely the result of a supernova explosion of a very massive star, with greater than 60 solar mass," Li told SPACE.com.

Image details

The Hubble image showing supernova SN2006bc is part of a sharp view of the spiral galaxy NGC 2397. This classic spiral galaxy has long prominent dust lanes along the edges of its arms, seen as dark patches and streaks silhouetted against the starlight. Hubble's resolution allows the study of individual stars in nearby galaxies.

Located nearly 60 million light-years from Earth, the galaxy NGC 2397 is typical of most spirals, with mostly older, yellow and red stars in its central portion, while star formation continues in the outer, bluer spiral arms. The brightest of these young, blue stars can be seen individually in a high-resolution view from the Hubble's Advanced Camera for Surveys (ACS).

The images were obtained on Sept. 14, 2006, with the ACS.