Astronomers say there's a medium-sized black hole hiding inside the globular star cluster 47 Tucanae, the second-brightest of its kind in the night sky.NASA / ESA / Hubble

When it comes to size, nearly all the known black holes in the universe fall into two categories: They’re big, or they’re really, really big. Stellar mass black holes weigh up to a few dozen times the mass of our sun, and supermassive black holes can weigh millions or billions times the mass of the sun.

But astronomers have for decades suspected the existence of something in between, intermediate-mass black holes that weigh between 100 and 10,000 suns. Several have put forth observations and measurements of these medium-sized black holes in the last decade, based on telescope data and computer simulations, but their existence has not yet been confirmed with certainty.

Bülent Kiziltan, an astronomer at the Harvard-Smithsonian Center for Astrophysics, says these medium-sized black holes are the “missing link” in the classification of black holes, and could be the ancient seeds that, in some cases, grew into the supermassive black holes that exist at the center of most galaxies.

Kiziltan and his team present their own discovery of an intermediate-mass black hole in a new study published Thursday in the journal Nature. Kiziltan says they have evidence for the existence of a medium-sized black hole at the heart of 47 Tucanae, a 12-billion-year-old ball of of thousands of stars located in the constellation of Tucana, named for the South American bird. The globular cluster is the second-brightest of its kind in the night sky, and can be seen from Earth without a telescope. The black hole, the researchers say, is the mass of about 2,200 suns, the grande in the cosmic version of coffee drinks.

Astronomers have studied Tucanae and other stars clusters for signs of black holes, where theoretical models suggest they should exist. Some of the strongest candidates come from astronomers at the University of Maryland, who in 2014 and 2015 described two potential medium-sized black holes by studying ultra-luminous X-rays coming from their home galaxies. But most results have been inconclusive, and claimed discoveries of intermediate-mass black holes—in these clusters and elsewhere in the universe—are often contested by other scientists.

Medium-sized black holes are difficult to find because astronomers don’t yet understand how they’re made. They know that stellar-mass black holes are the remains of collapsed stars, created in a single moment when their fuel is spent after millions of years. The creation of supermassive black holes, the glue that holds together galaxies, remains a mystery—but scientists can at least detect them.

Astronomers can find black holes in a couple of ways. They can examine the bright streams of radiation that matter emits as it gets gobbled up by a black hole, or use infrared images to track the gravitational pull of a black hole on the objects orbiting near it. The latter method has the advantage of revealing how large the hole is—the greater the gravitational tugs, the bigger the hole.

47 Tucanae is a tricky place to search for black holes, Kiziltan says. Its stars are heavily concentrated at the center, making optical observations of individual stars that could be near a black hole difficult. There’s also no collection of gas at the heart of the cluster, so there’s nothing for the black hole to consume, a process that would produce X-rays that can be detected by telescopes. Kiziltan and his team say that over the course of 47 Tucanae’s evolution, its medium-sized black hole behaved like a cosmic spoon, stirring the matter inside so that bigger stars were drawn to the center and smaller ones got pushed to the edges. The black hole also flung pulsars—rotating stars that emit a bright beam of energy—to the outskirts of the cluster. Kiziltan and his team used mathematical models and computer simulations to create hundreds of replicas of the cluster, each accounting for the existence of the black holes in different sizes. The replica that best explained the distribution of matter inside 47 Tucanae led them to conclude the existence of a medium-sized black hole.

Kiziltan’s work will be subject to the same scrutiny as previous finds. “Some of them will be skeptical, because they will try to look into the lines of evidence,” he says. “But this is how science works.”

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