A Black Hole Is ‘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.

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.