A black hole, as any fan of science fiction knows, is what forms from the burned-out remains of certain stars.
When the stars come to the end of their life-cycles they blast away much of their mass in a titanic explosion called a “supernova.” If the star were massive enough to begin with, the remains would collapse down into a tiny point with a gravitational field around it so strong that not even light escapes–a black hole.
Though, we can’t study anything that’s already inside a black hole, matter that is about to fall in can still be seen. Furthermore, this matter tends to heat up dramatically on its way in, making it even easier to detect. In fact, there’s a disk of matter around a black hole that heats up so much it radiates X-rays, which can be picked up by telescopes in orbit around earth. Still, the disk can be hard to spot.
Enter the lucky black hole eclipse.
NASA’s Chandra X-Ray Observatory was watching a black hole in a distant galaxy called NGC-1365 when the hole was eclipsed by a cloud. Don’t think of the kind of water vapor cloud we see in our sky; think of an enormous mass of dust and gas out in deep space.
When the cloud moved in front of the hole, the X-rays disappeared, and then reappeared as the cloud passed by. Knowing the size of the cloud and its speed, enabled the scientists to measure how large the X-ray disk must be.
How big was it? Well, the disk was fairly small, by galactic standards. If it were centered on the Sun, it would only stretch out past Mars.