Tiny Black Hole Found in Our Galaxy

NASA scientists have identified the smallest, lightest black hole yet found.

The new lightweight record-holder weighs in at about 3.8 times the mass of our sun and is only 15 miles (24 kilometers) in diameter.

"This black hole is really pushing the limits," said study team leader Nikolai Shaposhnikov of NASA's Goddard Space Flight Center in Greenbelt, Md. "For many years astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question."

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The low-mass black hole sits in a binary system in our galaxy known as XTE J1650-500 in the southern hemisphere constellation Ara.

NASA's Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in 2001, and astronomers soon realized that the system harbored a relatively lightweight black hole. But the black hole's mass had never been precisely measured.

Black holes can't be seen, but they're identified by the activity around them, which also helps astronomers estimate a size of the region inside the activity, and how much mass mut be in that confined region to generate all the surrounding activity.

More specifically, astronomers weigh black holes by using a relationship between the apparent size of the black hole and the X-rays emitted by the torrent of gas that swirls in the black hole's disk before taking its fatal plunge.

As the hot gas piles up near the black hole, it radiates X-rays. The intensity of the X-rays varies in a pattern repeated over a nearly regular interval.

Astronomers have long suspected that the frequency of this signal, called the quasi-periodic oscillation, or QPO, depends on the mass of the black hole.

As the black hole gets bigger, the zone of swirling gas is pushed farther out, so the QPO ticks away slowly. But for smaller black holes, the gas sits closer in and the QPO ticks rapidly.

Shaposhnikov and his colleague Lev Titarchuk of George Mason University used this method to "weigh" XTE J1650-500 and found a mass of 3.8 suns.

This value is well below the previous record holder GRO 1655-40, which tips the scales at about 6.3 suns.

This new mass measurement could help shed light on what the smallest star that will produce a black hole is.

Astronomers know that some unknown critical threshold, possibly between 1.7 and 2.7 solar masses, marks the boundary between a star that generates a black hole upon its death and one that produces a neutron star.

Knowing this boundary would help scientists understand the behavior of matter when it is scrunched to extraordinarily high densities.