There’s no limit on how big a black hole can get — keep funneling in stars and gas, and it keeps getting bigger. In fact, the largest black holes yet discovered are close to 20 billion times the mass of the Sun.
But there is a lower limit on the size of a black hole — about three times the Sun’s mass. And a black hole of about that size lurks in the constellation Scorpius.
It’s part of a binary system that consists of the black hole plus a puffy companion star. The black hole steals gas from the companion. The gas forms a rapidly spinning disk around the black hole. And some of the gas is funneled into “jets” that squirt away at close to the speed of light.
As the gas spirals closer to the black hole, it gets so hot that it emits X-rays. The X-rays push away the infalling gas, which shuts down the jet and causes the X-rays to drop. But the inner part of the disk quickly heats up again, then plunges toward the black hole, restarting the jet. The whole process takes less than a minute, and produces “pulses” of energy like a beating heart. The length of the beats reveals the mass of the black hole — about three times the mass of the Sun.
Such black holes form when the core of a massive star collapses. Any core that’s less than about three times the mass of the Sun will form not a black hole, but a neutron star. So the black hole in Scorpius is about as small as one can get — the smallest black hole yet discovered.
Script by Damond Benningfield, Copyright 2012
For more skywatching tips, astronomy news, and much more, read StarDate magazine.