Astronomers have discovered the closest known black hole to Earth, a serene monster with a mass nearly ten times the mass of the Sun 1,570 light-years away from us.*. He was found by the actions of his sun-like star companion, but how did he become a head scratcher [link to paper].
There are likely hundreds of millions of black holes in our Milky Way alone, but finding them is difficult. The easiest way is if they are in a binary system with a companion star, and they are close enough to each other, the matter is stripped from the star and falls into the black hole. As it piles up outside that last big step, the matter heats up and emits a lot of high-energy radiation, essentially declaring the presence of the black hole.
But if the black hole is lonely, or in a binary with a star orbiting far away, it is extinct, making it difficult to detect. The thing about black holes is that they blackso it can only be detected by its immense gravity.
However, this can expose them if the conditions are right.
In the case of this new one, they were. But until then, it’s a tough find.
Artwork of a star about to be torn apart by a medium-mass black hole. The matter falls into the elliptical hole, which produces a lot of X-rays and allows astronomers to learn a lot about the black hole. Credit: ESA/Hubble, M. Kornmesser
The GIA spacecraft is the European Space Agency’s mission to map the locations of approximately two Billion stars. Not only does it make their positions in the sky reach colossal accuracy, it also measures their movements and colors.
If two stars orbit each other, they will each trace a very small oval in the sky. In most cases, this physical motion is too small to be seen, but for relatively close binary systems with a wide orbital distance, Gaia can detect this change in the star’s position over time.
There are approximately 170,000 oscillating binary stars in the Gaia database. The astronomers here looked for stars with unusually large motion – indicating that one of the stars is indeed very massive – but where the overall light from the system was weaker than expected. Massive stars are bright, so if the motion of the binary suggests that one is massive but under intense light, it could be a black hole.
They’ve found a handful of promising candidates, but after more careful analysis, they’ve found only one that looks solid: the star Gaia DR3 4373465352415301632—they called it Gaia BH1 (“Gaia Black Hole 1”) for convenience. It is a star in the constellation Ophiuchus, with colors and brightness indicating that it is very similar to the Sun, although slightly cooler and less massive. Gaia directly measures the distance across the parallax as 1,570 light-years from us, which corresponds to its brightness.
Were it not for the fact that the star makes a small oval in the sky, it would be completely unnoticeable. There are no other nearby stars that can be associated with it, yet they move back and forth across the sky for a period of 185.6 days.
They obtained the star’s spectra using several different ground-based telescopes, and used them to measure the star’s Doppler shift as it orbited its unseen companion. The star’s speed is very high, well over a hundred kilometers per second, which indicates that the companion object is indeed very massive. They found that it has a mass of 9.8 ± 0.2 times the mass of the SunAnd this is the breaking point. A normal star of this mass would be exceptionally bright, thousands of times brighter than a Sun-like star, and completely engulfing it.
The fact that the object is so massive but completely dark can only be explained if it is a black hole. Technically, it’s not confirmed, so we have to call it a candidate, but looking at the data, I’d bet a decent amount of money that it’s a black hole.
Artwork showing a serene black hole on a background filled with stars. Credit: NASA, European Space Agency, de Co, J. Beacon (STScI)
It’s a strange system, too. We know of other black holes in orbit with ordinary stars, but they are not widely separated; The star is about as far from the black hole as Mars is from the sun. This is actually a problem, because of the way the black hole is formed.
At first it was a star with a mass about 20 times the mass of the Sun, a powerful beast. Its core would have run out of fuel a few million years after its birth, while the less massive star was still stabilizing to be a normal star. Then the massive star swells into a massive red giant, Big enough to physically swallow the smaller star. This is called common envelope binary stage. In general, the outer layers of a more massive star are ejected by the motion of the star of lower mass, so the two stars end up getting close to each other, although in this case the second would probably be too lightweight to do so efficiently.
It’s hard to dismiss a star from this scenario; Usually the distance between them is a few million kilometers. At some point, the more massive star explodes as a supernova, and loses some of its mass, which could move the second star far, far away…but not dramatically.
It is possible that there is a third star in the system, and that could mess with the orbits enough to explain the system, but this situation is tricky and has a narrow set of parameters to work with. It’s also possible that the system was born in a star cluster, and the gravitational influence of other stars during close passes could push the duo into what we see today.
The big question now, how common are systems like this? One has previously been seen in a nearby galaxy—Karim Al-Badri, principal investigator on the Gaia BH1 paper, also worked on finding this previous galaxy as well—but the fact that this galaxy is so close to us suggests that systems like this are common. The galaxy is 120,000 light-years across, so if these things were rare, the closest galaxy would be extremely unlikely. If it’s popular, Gaia is likely to find more in the future; The longer he observes the same stars over and over, the easier it will be for him to detect their motions.
Paradoxically, we know thousands and thousands of black holes in distant galaxies millions and billions of light-years away, because they are really massive and release huge amounts of energy, and yet the vast majority of black holes in our galaxy are completely invisible to us.
Currently. They are invisible Currently, but we always get better at finding it. They are dark, but they cannot hide forever.
* I’ll note that this is still pretty long in human terms – 15 quadrillion kilometres! – So there is no danger to us.
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