A massive rogue roaming our galaxy may be a black hole

Some think this unseen interstellar wanderer may be a hefty neutron star instead

illustration of an isolated stellar-mass black hole

Here’s an artist’s illustration of a small black hole roaming our galaxy.

NASA and G. Bacon/STScI

A solitary and massive celestial object is wandering our galaxy a few thousand light-years from Earth. It’s not too big, but its mass is greater than our sun’s. Astronomers suspect it might be the first loner black hole in the Milky Way found with a mass similar to our sun’s. Or it could prove to be one of the heaviest neutron stars known.

This wanderer first revealed itself in 2011. It wasn’t seen. Astronomers instead found it when its gravity briefly magnified the light from a more distant star. Back then, no one was sure what it might be. Now, two teams of astronomers have analyzed images from the Hubble Space Telescope. They’re still not entirely sure what the weighty object is, but they’ve narrowed down the list of candidates.

One group suspects this mysterious rogue is a black hole roughly seven times as massive as the sun. Make no mistake, its 94 authors say: “We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole.” They describe it in a paper due out soon in the Astrophysical Journal

Not so fast, says another team of 45 scientists. They think it’s a bit lighter — a mere two to four times the weight of our nearest star. If true, that would make it an unusually lightweight black hole — or a curiously hefty neutron star. This group will share its findings in an upcoming issue of Astrophysical Journal Letters.  

Both neutron stars and stellar-mass black holes can form when massive stars — ones with at least several times the heft of our sun — collapse under their own gravity. This happens at the end of those stars’ lives. Astronomers now believe that about a billion neutron stars and roughly 100 million stellar-mass black holes lurk in our galaxy.

Neither of these types of objects is easy to spot. Neutron stars are tiny — only about the size of a city. They also produce little light. Black holes, regardless of their size, emit no light at all. To detect these objects, then, scientists typically observe how they affect what’s around them.

“The only way that we can find them is if they influence something else,” explains Kailash Sahu. He’s an astronomer at the Space Telescope Science Institute in Baltimore, Md.

The massive mystery

To date, scientists have detected nearly two dozen stellar-mass black holes. (These are puny compared to their supermassive cousins that sit in the center of most galaxies, including our own.) Researchers found those relatively tiny black holes by observing changes in some of their neighbors. Sometimes, a black hole and a normal star will get caught in a spiral. Think of it as a dance.

But it’s a dangerous dance, as the black hole rips away matter from that companion star. As the star’s material falls onto the black hole, it emits X-rays. Telescopes orbiting Earth can detect that radiation. But scientists will find it hard to know how big a black hole was before it started dining on the star. And since birthweight is a key characteristic of a black hole, looking at black holes that are eating stars can confuse the picture. That’s why, Sahu says, “If we want to understand the properties of black holes, it’s best to find isolated ones” — like the new loner.

For more than a decade, researchers have been scanning the heavens for such isolated black holes. Hoping to spot these rogues, the scientists have looked for distorted starlight.

Einstein’s theory of general relativity states that the gravity associated with any massive object — even an unseen one — will bend the space in its vicinity. That bending magnifies and distorts the light of background stars. Astronomers refer to this as gravitational lensing. By measuring changes in the brightness and apparent position of stars, scientists can calculate the mass of a traveling object that’s acting like a lens. That technique has already turned up several exoplanets.

In 2011, researchers announced that they had spotted a star that suddenly got more than 200 times brighter. Those observations, made using telescopes in Chile and New Zealand, couldn’t nail down whether the star’s apparent position was also changing. And that information would be key to pinning down the mass of the object that was acting like a lens. If it’s a heavyweight, its gravity would distort space so much that the star would appear to move. Even a “big” shift in the star’s position, however, would have been very small and hard to detect. And it’s hard to see fine details in images captured by telescopes on Earth’s surface. (Our planet’s turbulent atmosphere just blurs them out.)

To get around this problem, two independent teams of astronomers turned to Hubble. Orbiting high above the pesky atmosphere, this telescope can capture extremely detailed images.  

Both groups found that the star’s location shifted over the course of several years.

The team led by Sahu now think the star’s apparent motion was caused by an object roughly seven times as hefty as the sun. A star that massive should have been blazingly bright in the Hubble images. But the researchers saw nothing. To be so heavy and dark, the mystery object must be a black hole, the team now concludes.

Astronomer Casey Lam led a group of researchers that came to a different conclusion. Lam works at the University of California, Berkeley. She and her colleagues calculated that the mass of the lensing object was smaller. It was probably closer to two to four times that of our sun. It that case, they said, it could be a black hole — or a neutron star.

In any case, it’s an intriguing object, says astronomer Jessica Lu at the University of California Berkeley. She’s a member of Lam’s team. The mystery object, says Lu, is either one of the most massive neutron stars ever discovered or one of the least massive black holes. “It falls within this strange region we call the ‘mass gap.’”

However one looks at it, the new results are thrilling, says Will M. Farr. He’s an astrophysicist at Stony Brook University in New York who did not take part in either new analysis. He says that to be working “at the real forefront of what’s measurable is very exciting.”

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