In the Milky Way, new stars arise from cosmic gas enriched by previous generations of stars that unleashed all kinds of heavy metals—which, in astronomy parlance, is anything heavier than helium—when they exploded in supernovas. Stars that are richer in these materials, astronomers believe, unleash more winds from their surface over their lifetime. The more winds they produce, the more mass they lose. This attrition “has been notoriously hard to constrain,” says Vicky Kalogera, an astrophysicist at Northwestern University who studies black holes. Astronomers thought, based on what they understood about stellar metallicities—a gorgeous term for the abundance of heavy metals in massive stars—that the biggest black hole an environment such as the Milky Way could produce would max out at about 15 times the mass of our sun. The existence of Cygnus X-1 suggests that this fundamental fact of our galaxy is incomplete. “Massive stars can potentially hold on to a lot more mass before they collapse, and essentially take that with them as they collapse into the black hole,” Carl Rodriguez, an astrophysicist at Carnegie Mellon University who was not involved in the new research, told me.
Read: What Earth owes to black holes
Cygnus X-1 might be one of the biggest known black holes in our galaxy, but it’s far from the biggest black hole in the cosmos. A mass of 21 suns is cute compared with the size of other black holes out there. The supermassive black holes at the centers of galaxies, even our own, are several million times more massive than the sun. The gravitational-wave observatories LIGO and Virgo have discovered another class of black holes, many millions of light-years away, that can coalesce into even bigger objects. When one pair of black holes, with masses 65 and 85 times that of our sun, collided, dispatching ripples across space for us to detect, they morphed into a black hole 150 times heavier than the sun. “The big black holes that LIGO has seen were made in regions long ago, far away, when there weren’t so many heavy elements,” Stan Woosley, an astrophysicist at UC Santa Cruz who was not involved in the research, told me. The stars they used to be had retained their bulk, and when it was time, they took it with them into the darkness.
Black holes are so far beyond our everyday experience, even for the astronomers that study them—who, like the rest of us, deal with the mundane details of existence—that it is easy to imagine these objects as sinister cosmic beasts preying on innocent particles of light, or, if you’ve seen Interstellar, as the portal that allows Matthew McConaughey to travel to a realm beyond time. Last spring, the discovery of a black hole just 1,000 light-years from Earth—almost on our doorstep, in astronomical terms—prompted half-joking panic from a public growing ever more wary of ominous-sounding headlines. But black holes are not so bad, really, and they’re not a threat to Earth. The supernova explosions that create many of them blast material into space that can find its way into other stars and, eventually, planets. Supermassive black holes blow bubbles of radiation so immense that they influence the future of star formation in their galaxies. The cosmos, including our small piece of it, would not be the same without them.
Miller-Jones and the other scientists suspect that they’ve got the right estimate for Cygnus X-1 now—no more surprises. But attempting to decipher black holes can often feel like a game of galactic whack-a-mole. “Every time you have some new bit of information, or answer one question,” Rodriguez said, “three more appear.”