Smallest Known Black Hole Identified

02 April 2008

If you want to know the universe’s ultimate tough guys, look no further than black holes. These strange objects gobble up gas from their surroundings, and sometimes swallow entire stars. But a black hole’s gravity is so powerful that nothing, not even light, can escape its grasp.

The black hole has about 3.8 times the mass of our sun, and is orbited by a companion star, as depicted in this illustration.

But just as Olympic boxing teams have their flyweights, somewhere out there in the depths of space exists the lightest black hole in the universe. It’s still a tough guy, but it’s smaller and lighter than all other members of its kind.

Astronomers may never find the universe’s lightest black hole, but in results announced on March 31, they have come close. Nikolai Shaposhnikov and Lev Titarchuk, who work at NASA’s Goddard Space Flight Center in Greenbelt, Md., have identified the smallest known black hole in the universe. This black hole would weigh the same as 3.8 of our Suns if it could be put on a giant scale.

The Sun is a huge object, and could contain more than a million Earths. So an object weighing the same as 3.8 Suns might sound like a lot. But it’s a pipsqueak when compared to all other known black holes. Previously, the smallest known black hole would weigh about 6.3 Suns, and some black holes tip the scales at millions or even billions of times that of our Sun.

In this top-down illustration of a black hole and its surrounding disk, gas spiraling toward the black hole piles up just outside it, creating a traffic jam. The traffic jam is closer in for smaller black holes, so X-rays are emitted on a shorter timescale.

The new record holder, known as XTE J1650, formed in the center of a dying star. The star’s core was a giant nuclear reactor, generating energy by turning light elements such as hydrogen into heavier elements such as oxygen. But eventually, the reactor ran out of fuel and shut down. The core collapsed due to its own gravity and formed a black hole.

Astronomers think that this process can form black holes down to about 3 times the weight of our Sun. If a star’s core is even smaller than that when it runs out of fuel, it will form another type of object, called a neutron star. So the XTE J1650 black hole is not only the lightest known black hole, it’s close to the smallest possible size for a black hole.

Amazingly, equations from Albert Einstein predict that a black hole with 3.8 times the mass of our Sun would be only 15 miles across -- the size of a city. “This makes the black hole one of the smallest objects ever discovered outside our solar system,” says Shaposhnikov.

The measurement of the black hole's mass is due to high-precision timing observations made by NASA’s Rossi X-ray Timing Explorer satellite, shown here prior to launch.

Shaposhnikov and Titarchuk made their discovery by using NASA’s Rossi X-ray Timing Explorer, a small and low-cost satellite that launched in late 1995. Rossi is able to make extremely precise measurements of gas whirling around black holes. By timing the motion of the gas, the two astronomers were able to measure the strength of the black hole’s gravitational field, which tells them how much it weighs.

Shaposhnikov and Titarchuk are presenting their results on Monday, March 31, at the American Astronomical Society High-Energy Astrophysics Division meeting in Los Angeles, Calif. Titarchuk also works at George Mason University in Fairfax, Va., and the U.S. Naval Research Laboratory in Washington, D.C.