Smallest Black Hole
The astronomy record books are filled with objects that are bigger, farther, and faster than anything else: the biggest planet, the farthest galaxy, the fastest star. Yet sometimes, astronomers learn more about how things work by studying records at the other extreme: the smallest.
An example is a black hole in a star system known as XTE J1650-500. It's the smallest and least-massive black hole yet discovered -- less than four times the mass of the Sun. That's important because it's close to the expected limit for how small a black hole can be.
Black holes like J1650 are born when the core of a heavy star collapses. But not all of these stars create black holes. Some give birth to neutron stars instead.
The dividing line depends on mass. The gravity of a smaller core isn't strong enough to make a black hole. The dividing line probably comes in at around two or three times the mass of the Sun: anything less massive becomes a neutron star, while anything more massive becomes a black hole.
Until now, the smallest known black holes were about six or seven times the mass of the Sun. So finding a black hole that's even smaller helps zero in on the lower limit. It also reveals more about the physics of the matter and space around these lightweight black holes.
J1650 is in Ara, the altar. From the southern half of the U.S., the constellation peeks above the horizon in late evening, and remains in view for a few hours.
Script by Damond Benningfield, Copyright 2008
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