Mystery of "Dark" Star Explosions Solved?

Ker Than in Pasadena, California
for National Geographic News
June 8, 2009

Cosmic "dust bunnies" hiding in corners of distant galaxies are taking the dazzle out of some of the universe's brightest explosions, new research suggests.

Gamma ray bursts are brilliant flashes of light thought to happen when very massive stars explode. Each blast normally sends out beams of invisible gamma rays and x-rays followed by an hours-long afterglow of visible light.

(See a picture of a gamma ray burst that was visible to the naked eye.)

But for years astronomers have puzzled over so-called dark gamma ray bursts—those that send out plenty of gamma rays and x-rays but have little to no afterglow.

One popular theory has been that dark gamma ray bursts happen so far away that the expansion of the universe has stretched the wavelengths of visible light into the invisible infrared part of the spectrum.

Another idea is that dust around the explosion is absorbing visible light but letting other kinds of radiation through.

"Whatever the cause, it was like hearing the foghorn without seeing the lighthouse," study co-author Joshua Bloom, of the University of California, Berkeley, said in a statement.

Dark, Dusty Corners

To test the two theories, Bloom and colleagues used NASA's Swift satellite and telescopes at California's Palomar Observatory to seek out galaxies where dark gamma ray bursts occur.

The team reasoned that if dark gamma ray bursts happen very far away, visible light from their host galaxies should also be redshifted and therefore invisible to optical sensors.

But after examining 14 dark gamma ray bursts, the researchers noted that they could see visible light from stars in 11 of the host galaxies.

"We think we've solved most of the mystery of what makes [dark gamma ray bursts] dark," study co-author Daniel Perley, also of UC Berkeley, said today at a press conference.

"We conclude that a large majority of dark bursts are the result of dust obscuring the optical light."

Closer looks at the hosts themselves, though, revealed that the galaxies do not appear to be unusually dusty. This means that the dark bursts must be hidden within otherwise undetectable dust clouds.

The finding supports theories that very massive stars form in dusty places, and it suggests that dark, dusty corners of the universe have been concealing previously unknown numbers of star births and deaths.

Findings presented today at the 214th meeting of the American Astronomical Society in Pasadena, California.

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