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The universe has so many black holes that it’s impossible to count them all. There may be 100 million of these intriguing astral objects in our galaxy alone. Nearly all black holes fall into one of two classes: big, and colossal. Astronomers know that black holes ranging from about 10 times to 100 times the mass of our sun are the remnants of dying stars, and that supermassive black holes, more than a million times the mass of the sun, inhabit the centers of most galaxies. But scattered across the universe like oases in a desert are a few apparent black holes of a more mysterious type. Ranging from a hundred times to a few hundred thousand times the sun’s mass, these intermediate-mass black holes are so hard to measure that even their existence is sometimes disputed. Little is known about how they form. And some astronomers question whether they behave like other black holes. Now a team of astronomers has accurately measured — and thus confirmed the existence of — a black hole about 400 times the mass of our sun in a galaxy 12 million light years from Earth. The finding, by University of Maryland astronomy graduate student Dheeraj Pasham and two colleagues, was published online August 17 in the journal Nature. Co-author Richard Mushotzky, a UMD astronomy professor, says the black hole in question is a just-right-sized version of this class of astral objects. “Objects in this range are the least expected of all black holes,” says Mushotzky. “Astronomers have been asking, do these objects exist or do they not exist? What are their properties? Until now we have not had the data to answer these questions.” While the intermediate-mass black hole that the team studied is not the first one measured, it is the first one so precisely measured, Mushotzky says, “establishing it as a compelling example of this class of black holes.” A black hole is a region in space containing a mass so dense that not even light can escape its gravity. Black holes are invisible, but astronomers can find them by tracking their gravitational pull on other objects. Matter being pulled into a black hole gathers around it like storm debris circling a tornado’s center. As this cosmic stuff rubs together it produces friction and light, making black holes among the universe’s brightest objects. Since the 1970s astronomers have observed a few hundred objects that they thought were intermediate-mass black holes. But they couldn’t measure their mass, so they couldn’t be certain. “For reasons that are very hard to understand, these objects have resisted standard measurement techniques,” says Mushotzky.