A Black Hole Is ‘Almost on Our Doorstep’

The invisible point of darkness resides in a double-star system just 1,000 light-years away.

The star system HR 6819, made up of two stars and a black hole (whose invisible orbit is indicated in red) (ESO)

In the night sky, far south of the equator, there’s a curious collection of faint constellations embedded in the tapestry of stars. They do not bear the names of myths and legends, because the ancient Greeks couldn’t see them from the Northern Hemisphere. These constellations were charted later, in the mid-18th century, by a French astronomer who sailed south, and he named them in honor of some rather mundane objects of his own time: a telescope, a microscope, a pendulum clock, an easel, various other tools and chisels. “It looked like somebody’s attic!” an American astronomer later remarked.

And just like a cluttered attic, this corner of sky has been hiding something truly remarkable.

Astronomers have discovered a black hole in one of the constellations, the suitably named Telescopium. At just 1,000 light-years away, the black hole is closer to our solar system than any other that astronomers have found to date. A thousand light-years might sound distant to us, but in cosmic proportions, it’s very close.

“On the scale of the Milky Way, it’s in our backyard,” Thomas Rivinius, an astronomer at the European Southern Observatory (ESO) in Chile who led the new research, told me. “Almost on our doorstep.”

For comparison, consider some of the best-known black holes in astronomy, the ones usually intriguing enough to make headlines. The black hole at the center of our Milky Way galaxy is more than 25,000 light-years away, and the black hole that astronomers captured in unprecedented detail last year lies 55 million light-years away, in another galaxy altogether. This one, by contrast, is so close that, on a clear night in the Southern Hemisphere, far from light pollution, the pair of stars that orbit the black hole can be seen with the naked eye. From here, the stars appear as a single pinprick of light.

So if this black hole is, at least in astronomical terms, right there, how has it eluded astronomers for so long?

Well, there’s the obvious: Black holes are invisible. The way to find the darkest points in the universe is to look for luminous clues around them. Most of the black holes that astronomers have found in our galaxy—a few dozen—were spotted because they were devouring nearby stars, pulling material into their maws and past a point of no return. That process is so luminous that not only can black holes be detected from Earth, but they’re actually quite difficult to avoid. “Sometimes they become the brightest objects in the sky,” says Erin Kara, an astrophysicist at MIT who studies black holes and was not involved in the latest discovery. In fact, some black holes emit so much radiation while they feed that telescopes can’t look at them without frying their electronics, Kara says.

A telescope’s view of the constellation Telescopium (ESO / Digitized Sky Survey 2)

The newly discovered black hole doesn’t fit into this category. It resides within a two-star system, but it isn’t close enough to either to ruin their day. Astronomers didn’t go looking for the black hole either; they started examining this system, known as HR 6819, years ago as part of a study of stars that orbit in pairs. When they analyzed the data, they noticed that there was something unusual about HR 6819, particularly the behavior of the inner star. The star’s velocity was so extreme that astronomers suspected a third object was lurking nearby and flinging it around. (The team put this work on hold for several years, after Stanislav Stefl, the astronomer who suggested the missing object could be a black hole, died in a car accident in 2014.)

The astronomers worked out the mass an object must have to jostle the star so much, and their calculations suggested that the object would measure four times the mass of our sun—nearly the same size as the inner star itself. “An object of that mass, you can’t hide it,” Rivinius said. Unless it’s invisible.

The animation at the top of this story shows the arrangement of the two stars and their black hole. Although it appears as if the inner star (whose orbit is shown in blue) and the black hole (in red) are chasing each other, the objects are orbiting each other. The inner star completes a swift loop every 40 days, while the outer star traces a wider orbit around.

Don’t worry: Despite its proximity to Earth, the black hole is no danger to us. It’s a blip compared to the one at the center of our own galaxy, which has a mass 4 million times that of our sun. And, as far as humanity is concerned, it’s not close enough to pose any kind of threat. “One has to be very close to it to be sucked in,” Rivinius said.

There are many more like it. Black holes are the by-products of aging stars that exploded in spectacular fashion at the end of their lifetime. Such supernovas can, briefly, outshine entire galaxies, but nearby, companion stars can survive the cataclysm, which explains why HR 6819 still exists.

Astronomers estimate there are hundreds of millions of black holes in our galaxy. The latest discovery gives them hope that there are others lurking around nearby stars, perhaps even some of the most familiar points of light in our sky. “It’s important to emphasize that it’s the closest we’ve found yet,” says Sera Markoff, an astrophysicist at the University of Amsterdam and a member of the team that produced last year’s historic black-hole photo. “There might be closer ones.”

The general assumption in astronomy is that we humans don’t live anywhere special in the universe, and whatever we encounter here, in our cosmic neighborhood, we should expect to find elsewhere. Dietrich Baade, an ESO emeritus astronomer and one of the authors of the new research, compares the likelihood to seeing hummingbirds in a tropical city.

“If I am in an ordinary hotel and I have breakfast on the terrace and I see a hummingbird flying around, then I can be sure that there must be many more hummingbirds in the area, because my hotel is not in a special place,” Baade says.

There are likely other black holes orbiting “nearby,” hidden in dark crevices around bright stars. Some may not be orbiting alongside stars at all, but drifting along in the darkest crevices of space, without any bright beacons to illuminate their existence like a cosmic flashlight shining on a forgotten box in an attic.