The Milky Way’s Center Is a Cornucopia of Black Holes

Astronomers have detected evidence of a handful of the likely thousands of light-gobbling objects in the middle of the galaxy.

An illustration of a supermassive black hole digesting the remnants of a star
An illustration of a supermassive black hole digesting the remnants of a star (NASA / JPL-Caltech)

For the last few years, Chuck Hailey has had the center of the Milky Way hanging in his office at Columbia University. The picture, pinned above his desk, shows a bright orange and yellow blob—the glow of cosmic gas as it gets devoured by the supermassive black hole at the center of the galaxy. Astronomers captured this glow using X-rays, a versatile type of radiation that’s good for seeing through galactic gas as well as human bodies.

Some of the X-ray emissions astronomers detect coming from the galactic center come from this black hole, known as Sagittarius A*, but not all. The rest comes from a population of other, smaller black holes clustered in the region. Astronomers have long predicted the presence of such black holes at the center of the galaxy, which they said could number in the thousands.

Now, Hailey and his colleagues have found the first evidence of these black holes.

Using archival data from the Chandra X-ray Observatory, a telescope orbiting Earth, Hailey and a Columbia-led team of scientists picked out 12 X-ray sources within a few light-years of the galactic center that they believe are some of these smaller black holes. Their research was published Wednesday in Nature.

The dozen black holes are small as far as black holes go, about 10 times the mass of our sun. They are infinitesimal, of course, compared to the supermassive black hole they surround, which is about 4 million times the mass of the sun. Here’s what Chandra captured (the blue dots are the newly discovered black holes):

Courtesy of Nature and Hailey et al.

Astronomers found the smaller black holes by looking for their victims. When a black hole orbits together with a star, in a formation known as a binary system, the black hole feeds on its companion, sucking the starlight into the shadows. The slow process of devouring the companion star produces a glowing disk of hot gas that can be seen by powerful telescopes through X-rays. This is why Hailey and his team were able to spot these black holes out of all the thousands out there. Without cannibalized stars to light the way, we can't see black holes with current technology

“A black hole all by its lonesome doesn’t do much of anything, and even when it eats some gas or dust, it’s very inefficient at converting its food to X-rays or any other form of radiation,” Hailey says. “But when a black hole, or even a neutron star or a white dwarf, when they have a stellar companion—that is, a living star—that star will shed gas and form a disk around the black hole.” Those disks are very good at producing the X-rays detectable by powerful technology.

Hailey and his team predict there may be as many as 10,000 of these small black holes lurking in our galaxy’s center. They make this estimate based on astronomers’ observations of similar black holes with stellar companions closer to Earth—oh yeah, there are black holes floating around the entire Milky Way—and their X-ray emissions. If these black holes were in the galactic center, they’d be too dim for us to spot from here. “We’re seeing the tip of the iceberg,” Hailey says. “We’re actually missing most of them just because the center of the galaxy is so far away.”

The new detection makes Hailey wonder just how many of these black holes are floating around without a star companion, completely invisible to our telescopes. It’s actually quite rare for a black hole to successfully lasso a star with its gravity and start chomping down on it, so there must be many more out there without a companion than with one, Hailey said.

If the thought of thousands of black holes swirling about the center of your galaxy scares you, don’t worry. These mysterious objects are no threat to our little planet. But let’s say—just for fun—you found yourself at the galactic center, hurtling through the region alongside particles of cosmic dust. In that situation, it would actually be more advantageous to get swallowed up by the supermassive black hole instead of one of the smaller ones. Both would kill you, of course. But the bigger black hole would make the journey less painful, says Sera Markoff, an astrophysicist at the University of Amsterdam and a member of the Event Horizon Telescope, a project that’s striving to photograph a black hole for the first time.

Every black hole is surrounded by an invisible boundary called the event horizon, beyond which nothing, not even light, can escape. Anything that crosses that horizon is a goner. If you were able to hover over the event horizon, extreme tidal forces—the fierce gravitational tug of the black hole—would stretch your body to resemble a strand of spaghetti. The smaller the black hole, the more concentrated the tidal forces, and the greater the tugging, Markoff says. “So the tidal force of a supermassive black hole is gentle”—relatively speaking, of course—“and when you pass through the event horizon, you wouldn’t really feel it. You wouldn’t get shredded until much farther in,” she says.

While the center of a galaxy would destroy any living creature that ended up there, from a cosmic perspective, it is a place of creation. Hailey likens it to a nursery for cultivating celestial objects. “It’s sort of like fertile soil in which to grow big stars, which then collapse into these black holes and then grab another star,” he says. This process produces the fertilizer for everything else in the universe, expelling the heavy elements that help seed new stars, planets, and, in some lucky places, life.