for National Geographic News
Astronomers have identified the smallest known black hole—an object about 3.8 times as massive as the sun.
The black hole, part of a binary star system known as XTE J1650-500, is only about 15 miles (25 kilometers) in diameter, said Nikolai Shaposhnikov, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Shaposhnikov announced the find, which was made using an experimental new technique, Monday in Los Angeles at a meeting of the American Astronomical Society's High Energy Astrophysics Division.
The black hole is located about 10,000 light-years away in the southern constellation Ara.
Scientists say the object approaches the lower mass limit at which stars can collapse into black holes. At smaller masses, white dwarf stars or neutron stars form instead.
Space "Traffic Jam"
The scientists were able to determine the black hole's size by examining nearby x-ray emissions.
Black holes themselves are unobservable, because light cannot escape from them. But one way to detect them is when they occur in binary star systems, since they pull gas away from their companion stars.
(Related: "Most Massive Stellar Black Hole Found in Binary System" [October 17, 2007].)
As that gas spirals inward, it forms an accretion disk that gets denser near the center. Shaposhnikov compared the phenomenon to rush-hour traffic funneling into New York City's Lincoln Tunnel.
"A black hole represents a traffic jam [in] the astrophysical environment," he said at an April 1 press conference. "Matter has to squeeze into the black hole."
The in-falling matter also gets very hot, emitting x-ray light that was viewed by Earth-orbiting satellites.
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