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Black holes can form from vanishing stars

RENNES, France, April 24 (UPI) -- Instead of forming amidst the gigantic explosions that momentarily can outshine entire galaxies, new findings suggest black holes also can form in the dark, scientists reported Thursday.

The investigators said such a murky origin for these cosmic monsters could help explain why many gamma ray bursts -- the invisible but most powerful eruptions of energy since the Big Bang -- seem to emerge from darkness.

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"Research on black holes is a way to explore the limits of the observable universe," astrophysicist Felix Mirabel, of the National Research Council of Argentina and the Atomic Energy Commission of France, told United Press International.

Black holes are pits in space and time with gravitational pulls so powerful nothing can escape -- not even light. Although the black holes themselves cannot be seen, astronomers think they have detected millions of these gravitational giants by looking for X-rays emitted from the tumultuous energy outbursts that occur because black holes rip apart stars, planets and anything else that crosses their path.

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According to current theory, black holes form when stars burn out, with the stellar remnants collapsing under their own weight to form dimensionless but infinitely dense points. Astrophysicists have long thought these collapses trigger catastrophic explosions known as supernovae, with the rapid inrush of matter releasing more energy in a few moments than generated by billions of stars.

"But, although there are many theoretical models, we have almost no observational evidence on how black holes are actually formed," Mirabel explained.

Mirabel and his colleague Irapuan Rodrigues decided to look for clues to black hole origins by investigating one of the most famous black hole candidates, Cygnus X-1. Discovered more than 30 years ago, Cygnus X-1 was the first X-ray emitter found in the constellation, Cygnus, or The Swan.

Back then, cosmic high-energy X-ray sources remained complete mysteries to astronomers, but research eventually suggested although Cygnus X-1 was relatively puny in size -- about eight to 10 times the mass of the sun -- it nevertheless was massive enough to persuade researchers it was a black hole.

Satellite and ground-based telescopes helped Mirabel and Rodrigues track Cygnus X-1's motion through our galaxy. In findings published online Thursday by the journal Science, the duo found it was zipping through space at roughly the same speed as the stars surrounding it.

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That means Cygnus X-1 "didn't receive a kick from a supernova explosion," Mirabel said. The star it formed from simply vanished, "all of a sudden in a few seconds. You would just have seen a luminous star -- and it would have been a bright, massive star -- suddenly disappear from your vision," he explained.

The finding could help reveal more about gamma ray bursts, deadly flashes of radiation that can unleash more energy in a few seconds than the sun can over its entire 10-billion-year lifespan. Scientists suspect supernovae cause many gamma ray bursts, but a large number of these explosions are "dark" and are not linked to any bright light.

Astrophysicist Sylvain Chaty, of University Paris 7, in Cedex, France, plans to look for more of these quietly formed black holes. He said more black holes might be formed in this manner than expected, "which would explain why we see fewer black holes that we should in our galaxy."

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(Reported by Charles Choi, UPI Science News, in New York)

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