NASA scientists have spotted a black hole with a diameter of only 24 kilometers (15 miles) and a mass of only 3.8 times our sun.
"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."
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 must be in that confined region to generate all the surrounding activity. More specifically, astronomers can 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 into the black hole's disk from its companion star.