Astronomers have started monitoring about a million massive stars to see if any suddenly vanish, seemingly without a trace. Such a disappearing act would support a theory that some massive stars simply implode when they die, rather than exploding in brilliant supernovae or gamma-ray bursts.

As a massive star ages, it accumulates iron in its core. Eventually, this iron core grows so massive that it is crushed by its own gravity, forming a black hole.

Sometimes the process is accompanied by a supernova, when the star's outer layers explode outwards to produce a brilliant flash of light at visible wavelengths. In rare cases, black hole births are even more spectacular, with the star firing out powerful jets of high-energy radiation as it dies - a phenomenon known as a gamma-ray burst.

But as many as half of black hole births may happen more stealthily, with no explosion to mark the event. A new survey led by Christopher Kochanek of Ohio State University in Columbus, US, may detect these events by watching massive stars suddenly wink out.

'No guarantees'

The survey, which uses the 8.4-metre Large Binocular Telescope (LBT) on Mount Graham, Arizona, US, took its first images in early 2008. It is monitoring about 1 million red supergiant stars - massive stars in the final stage of their lives - in 30 nearby galaxies.

The team plans to take images of the galaxies twice per year, watching for the sudden disappearance of the red supergiants. By watching 1 million stars, the team hopes to catch about one stellar death per year in their survey, which will last five years.

"There's no guarantee that you'll find these things - because it could just be that they all do a supernova at some level," Kochanek told New Scientist. "But that's no reason not to give it a try."

Giant outbursts

Stan Woosley of the University of California in Santa Cruz, US, who does theoretical studies of stellar deaths, agrees. "My own view is that it is impossible to make a black hole without some sort of electromagnetic display," he told New Scientist.

But perhaps there are cases where the radiation comes out in the form of X-rays or gamma rays, he says, rather than the visible light astronomers normally look for when searching for supernovae. "In any case, it is worth looking."

If a star seems to disappear, the team will try to confirm the formation of a black hole by looking for X-rays emitted by stray bits of matter falling into the black hole, Kochanek says.

In addition to clarifying what proportion of black holes are born without fanfare, the survey may also detect rare, giant outbursts from massive stars that are close to going supernova. Such an outburst in the 1840s temporarily made a star called Eta Carinae the second brightest star in the sky.

Journal reference: Astrophysical Journal (in press)