for National Geographic News
A strange black hole locked in a tight orbit with a huge star in a nearby galaxy could be the most massive stellar black hole known, astronomers say.
Stellar black holes form when a weighty, dying star's core collapses, creating a region with such a strong gravitational pull that even light can't escape.
Astronomers also believe that supermassive black holes with masses equal to millions of suns exist at the centers of many galaxies, including our own Milky Way.
The newly described stellar black hole has a mass almost 16 times that of our sun, said lead study author Jerome A. Orosz, a professor of astronomy at San Diego State University.
"Before this, the most massive known stellar black hole was GRS 1915+105, with a black-hole mass estimated to be 14 plus or minus 4 solar masses," Orosz said. But the mass of GRS 1915+105 has recently come into question, he added.
Orosz's team also found that the black hole's companion star has a mass about 70 times that of the sun—making it the most massive star in a binary system containing a black hole.
"The present-day companion is no more than about three million years old," Orosz said. "The model suggests that it will die in another two to three million years, forming another black hole."
Orosz and colleagues report their puzzling new observations in this week's issue of the journal Nature.
Since black holes can't be seen directly, astronomers look for the intense radiation released as hot matter spirals into a singularity.
Researchers can then detect the existence of a black hole in a binary system by observing a visible star in orbit around its invisible companion.
The mass of the black hole in such a system can then be deduced by measuring its gravitational tug on the star.
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