Read: About that monstrous black hole we’re all orbiting
The black hole resides at the center of a galaxy known as Messier 87, named for the 18th-century French astronomer who discovered it. Messier is one of the biggest nearby galaxies. The black hole at its center has a mass 6.5 billion times that of the sun.
The photographic evidence of a long-unseen cosmic force is an extraordinary achievement in science. Messier 87 is located about 55 million light-years from Earth. The electromagnetic radiation there—the kinds of signals scientists seek to detect—took a very long time to reach the planet, longer than any human beings have been around. By the time it arrived, they had figured out how to peer back into the depths and snap a picture.
Exciting as it is, the photo actually doesn’t capture the black hole itself, nor its interior. Astronomers aimed their telescopes at the event horizon, the invisible boundary thought to surround all black holes. When something crosses this barrier, it doesn’t come back. In the photos, the event horizon has cast a shadow on the bright, hot gas swirling at the galactic center. Just before the cosmic material crosses over, it heats up and glows. The black hole appears in silhouette, a slightly elongated ball, ringed by a halo of fire.
If the photo looks fuzzy around the edges, consider the size of the shadow. Heino Falcke, one of the astronomers involved in the effort, predicted that from here, the shadow of the black hole inside Messier 87 would be 20 to 40 microarcseconds across. All you need to know about microarcseconds is that 10 of them are equivalent to the size of a coin on the moon, viewed from Earth.
These scales made black holes elusive. As massive as they are, as strong as their pull can be, not so long ago, they were enigmatic enough that plenty of scientists didn’t think they even existed.
The modern theory of black holes has its beginnings, as many wonders of gravity do, with Albert Einstein and his general-relativity equations, published in 1915, which described gravity as a distortion of space and time. The physicist Karl Schwarzschild ran with Einstein’s equations and came up with an idea of his own: Based on the principles of general relativity, matter could become squeezed into a tiny point of infinite density, a locus known as a singularity. The singularity would warp the space around it, creating a spherical region with an invisible barrier, from which nothing could escape.
Einstein actually resisted the idea, but the evidence piled up. In the 1930s, the astrophysicist Subrahmanyan Chandrasekhar, just 19 years old at the time, upended theories about star formation with his mathematical calculations showing that some massive stars collapse into dense, light-trapping objects—when they run out of fuel and die. The term black hole entered the literature in the late 1960s, just a few years before astronomers found their first evidence of one, in the early 1970s, in the constellation Cygnus. Before the decade was over, astronomers detected a radio signal coming from the supermassive black hole at the center of our very own galaxy, known as Sagittarius A* (pronounced as “a-star”).