This Black Hole Probably Shouldn’t Exist
New observations have prompted astronomers to rethink their theories about some of the universe’s invisible objects.
There’s a spot in space, thousands of light-years from here, that might best be described as a cosmic amusement park. A supergiant star, so hot that it glows electric blue, and a black hole spin around each other at extraordinary speeds, orbiting so closely that some of the star’s material is pulled toward the black hole. The stellar particles swirl around the invisible object in a tilt-a-whirl of luminous reds and oranges. What the black hole doesn’t swallow, it hurls into space, producing intense jets of radiation."There is a lot going on,” James Miller-Jones, an astrophysicist at Curtin University in Australia, told me.
Astronomers have studied this distant environment, and particularly the black hole, for decades. In fact, the black hole is the first ever discovered, after some years of research led to the breathtaking realization that this weird object couldn’t be anything but an invisible pit of inscrutable depths.
But when Miller-Jones and a team of researchers directed their attention to it a few years ago, they noticed something weird. According to their recently published findings, the black hole, the system’s main attraction, is much more massive than they thought. Which is particularly strange because, based on what astronomers currently understand about these kinds of objects and the way they form, this black hole probably shouldn’t exist.
Black holes are some of the most mysterious objects in the universe, in our own Milky Way galaxy and many light-years beyond, and they often surprise the researchers trying to understand them. On the scale of scientific discoveries, Miller-Jones’s finding is much closer to a huh moment than a eureka moment. A familiar black hole showed it still has secrets. The accidental discovery is a reminder that astronomers are still trying to understand some of the most basic forces in our galaxy.
The black hole, named Cygnus X-1, resides in a swan-shaped constellation called Cygnus. The invisible object was once a blue star itself. Its existence as a black hole began millions of years ago, when that star reached a breaking point, running out of fuel and collapsing. Our experience with Cygnus X-1 began a little more recently, in the 1960s, in the skies over New Mexico. Researchers had launched rockets carrying Geiger counters toward the boundary of space in an attempt to pick up X-rays coming from the moon. Instead they detected a set of mysterious emissions coming from well beyond our solar system, and traced one of them back to the constellation Cygnus.
Telescope observations eventually revealed that Cygnus X-1 was a murky presence near its companion star, and had a surprising amount of mass. Something that big should be a star, scientists thought, and yet they couldn’t see it. It had to be a black hole. But at the time, black holes—perplexing points in space where gravity is so strong that nothing can escape from them—were still the stuff of complicated theories of physics, not reality. Even the term black hole was new. Nobody had found one before. The data in support of this hypothesis accumulated, and by 1990 the scientific community had reached a historic consensus. Cygnus X-1 was indeed a black hole. The X-rays that had helped reveal its existence were coming from the hot gas swirling around it.
Today, astronomers know that black holes are everywhere, in the centers of most galaxies, including our own, and sprinkled throughout. They find evidence of the invisible objects using indirect methods. A carousel of stars rotating around a seemingly empty spot in space? A black hole might be in there. Gravitational waves—ripples in the fabric of space-time—rolling past Earth? They may have been produced by the force of two black holes smashing into each other. Astronomers have even managed to photograph a black hole, in a way: In the picture, taken by a collection of telescopes stationed across the world, the supermassive black hole in the center of a neighboring galaxy has cast its shadow on the glowing material around it.
There are still many unknowns, and even the most familiar objects, like Cygnus X-1, can still confound scientists. The latest research updates the black hole’s size from 15 times the mass of our sun to 21 times that of our sun. To the untrained eye, this is a small, almost negligible, jump. But to astronomers, the revised estimate means they must revisit their theories on massive stars and the black holes they become.
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.
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.”