The Sun is more than a hundred times wider than Earth, and more than 300 thousand times heavier. But imagine cramming three Suns into a ball about the size of Colorado Springs, Tulsa, or Lubbock. That would give you a black hole the size of one recently discovered in Auriga — the smallest black hole yet seen — or not seen.

The black hole itself produces no energy, so there’s no way to see it directly. But it has a companion star. The system is known by a catalog number, which we’ll shorten to 2MASS J0521.

Most black holes are found because they’re pulling gas off the surface of a companion. The gas forms a swirling disk around the black hole. It’s extremely hot, which makes it bright — and fairly easy to find.

In this case, though, that’s not happening. Instead, astronomers discovered the black hole through its gravitational pull on the companion. The motions of the two bodies allowed the astronomers to measure the masses of both. They found that the companion is about three times heavier than the Sun, but much bigger and brighter. And the dark companion is a little more than three times the Sun’s mass. At that heft, it almost has to be a black hole. If so, it’s only about a dozen miles across — the size of a small city.

2MASS J0521 appears near Capella, the brightest star of Auriga. It’s high overhead at nightfall. Capella is one of the brighter stars in the night sky, so it’s easy to spot.

Script by Damond Benningfield

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