The Amazing Shrinking Black Hole

Scientists have found a pint-sized black hole that is the smallest ever identified. And the new technique they debuted should help researchers refine their knowledge of how these massive objects form.

In the life cycle of stars, mass is destiny. Red dwarfs, only a fraction of our sun's mass, don't seem to die. But supergiants live only a few million years before blowing themselves up in supernovae, leaving incredibly dense cores that coalesce into black holes. In between is a range of middleweights, including a type of supernova remnant called a neutron star, which can pack a sun's worth of mass into an asteroid-sized body. Astrophysicists have wondered where neutron stars leave off and black holes begin. Theoretical models predict the threshold at somewhere between 1.7 and 2.7 times the sun’s mass.

Because these objects cannot be observed directly--and small ones are particularly difficult to detect--astronomers must measure their gravitational or energetic effects on nearby stars or surrounding material. But this method only goes so far. The lightest black hole found previously weighed about 6.3 times as much as the sun--still quite a bit larger than that threshold.

Now two astrophysicists from NASA's Goddard Space Flight Center in Greenbelt, Maryland, have used a new technique to find an even smaller black hole. It's a relative pipsqueak, weighing only 3.8 solar masses and orbiting a binary partner about 10,000 light-years away from Earth in the constellation Ara. The object is only about the size of Manhattan. The researchers spotted it by using the Rossi X-ray Timing Explorer spacecraft to detect the periodic x-ray bursts emitted when compressed and superheated incoming material reaches the edge of the black hole.

"With this new record-holder, we've advanced closer to the boundary between black holes and neutron stars," says Nikolai Shaposhnikov. He and co-author Lev Titarchuk reported their results this week at an American Astronomical Society meeting in Los Angeles and have submitted a paper on the find to the Astrophysical Journal.

The results are "particularly intriguing" because they confirm the lower end of the mass range for black holes predicted by the models, says theoretical astrophysicist Vicky Kalogera of Northwestern University in Evanston, Illinois. The new detection method is not yet widely accepted by the astrophysical community, she says, but Shaposhnikov and Titarchuk applied it to several other black holes, whose masses scientists had determined with the conventional detection method, and the results have been consistent. "So we have no instance where the two methods disagree," Kalogera says.

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