How Do Astronomers Find Black Holes? They Look at the Space Gas!


Black holes are so-called because they are, essentially, black. A black hole is a place in space with such a strong gravitational pull that light can’t even get out. Because of this, astronomers can’t “see” black holes. But they’re able to find them by observing the behavior of gas and stars in outer space.

On rare occasions, two black holes can merge. Scientists can find mergers using gravitational wave detectors, but they can’t see exactly where they happened. With a “light signature,” say authors of a new article in The Astrophysical Journal Letters, researchers could learn about the event in much more detail. The authors describe a situation that might let scientists observe mergers in the future. Authors on the study include Professors D. Barry McKernan and K. E. Saavik Ford, both of Borough of Manhattan Community College and The Graduate Center.

Scientists think that collections of black holes can sometimes be found orbiting supermassive black holes in the center of galaxies. The smaller, orbiting black holes occasionally pair off and orbit each other as they simultaneously orbit the larger black hole, sometimes breaking apart to orbit a different partner. In rare cases, these pairs can merge into one.

According to the new paper, if a large amount of gas is pulled toward the central black hole, scientists might be able to see a sort of flash when such a merger happens. When gas falls toward a supermassive black hole it forms a disk around the hole, and this disk would also surround the smaller, orbiting black holes. When two of the smaller black holes start to merge their momentum increases, and gas in their area would start to feel their pull more strongly.

“When the merger product tries to take ‘its’ gas with it—big problem,” Ford said. “The gas attached to the merger product collides with the rest of the gas in the disk and it heats up, creating a potential flare.”

But this flare would be the opposite of a problem for astronomers. Under the right conditions researchers might be able to observe this flare using powerful telescopes, and “see” two black holes merging for the very first time.