Astronomers have found the closest known black hole candidate to Earth, a quiet beast nearly ten times as massive as the Sun just 1,570 light-years away.*. It was found thanks to the antics of its star-like Sun-like companion, but how it has become is a headache [link to paper].
There are probably a hundred million black holes in our own galaxy, the Milky Way alone, but finding them is difficult. The simplest way is if they are in a binary system with a companion star and are close enough to each other that the material is ripped from the star and falls into the black hole. As it builds up on the outside of That Last Big Step, the material heats up and emits a lot of high-energy radiation, essentially heralding the existence of the black hole.
But if the black hole is alone, or in binary with the farthest orbiting star, it is quiescent, making it difficult to detect. The problem with black holes is that they are Blacktherefore they can only be detected by their immense gravity.
However, this can reveal them if the conditions are right.
In the case of this new one, they were. But even then, it’s a difficult discovery.
Illustration of a star about to be torn apart by an intermediate-mass black hole. The material falls into the missing hole, creating many X-rays and allowing astronomers to learn a lot about the black hole. Credit: ESA / Hubble, M. Kornmesser
The Gaia spacecraft is a European Space Agency mission to map the positions of nearly two billions stars. Not only does it get their positions in the sky with phenomenal accuracy, it also measures their movements and colors.
If two stars orbit each other, each will trace a very small ellipse in the sky. In most cases this physical motion is too small to see, but for relatively close binary systems with a large orbital separation, Gaia can detect this change in a star’s position over time.
There are nearly 170,000 such binary stars in the Gaia database. Astronomers here looked for those with unusually large motion – indicating that one of the stars is indeed very massive – but where the overall system light was weaker than expected. Massive stars are bright, so if binary motion indicates one is massive but severely sub-luminous, it could be a black hole.
They found a handful of promising candidates, but upon closer inspection they found only one that looked solid: the star Gaia DR3 4373465352415301632 – they call it Gaia BH1 (“Gaia Black Hole 1”) for convenience. It is a star in the constellation Ofphiuchus, with colors and luminosity indicating that it is very similar to the Sun, albeit a little colder and less massive. Gaia directly measures its distance via parallax 1,570 light years from us, which is consistent with its brightness.
If it weren’t for the fact that the star is forming a tiny ellipse in the sky, it would be completely insignificant. There are no other stars nearby that could be related to it, yet there it is, moving back and forth across the sky with a period of 185.6 days.
They obtained the star’s spectra using several ground-based telescopes and used them to measure the star’s Doppler shift as it orbited its invisible companion. The speed of the star is quite high, well over one hundred kilometers per second, which indicates that the companion object is indeed very massive. They find that it has a mass of 9.8 ± 0.2 times that of the Sun., and this is the deciding factor. Such a massive normal star would be extraordinarily bright, many thousands of times brighter than the Sun-like star, completely submerging it.
The fact that the object is very massive but completely dark is really only explainable if it is a black hole. Technically it’s not confirmed, so we have to call it a candidate, but given the data I’d bet a fair amount of money it’s a black hole.
Illustration showing a quiet black hole on a starry background. Credit: NASA, ESA, D. Coe, G. Bacon (STScI)
It is also a strange system. We know of other black holes orbiting normal stars, but none of them are 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 formed.
Initially it would have been a star with about 20 times the mass of the Sun, which is a powerful monster. It would run out of fuel in its core only a few million years after its birth, while the lower-mass star was still on its way to settle to become a normal star itself. Then the huge star would have swelled into a huge red supergiant, large enough to physically swallow the smallest star. This is called the common envelope phase of a track. In general, the outer layers of the more massive star are ejected by the movement of the lower-mass star and, in turn, the two stars end up approaching each other, although in this case the second was probably too light to do so. in an efficient way.
It’s hard to get a star that far from that scenario; normally they would be a few million miles away. At some point the most massive star explodes as a supernova, losing some mass, which can move the second star farther … but not enormously.
It is possible that there was a third star in the system and that it could fiddle with the orbits enough to explain the system, but that situation is complicated and has a narrow set of parameters to work. It is also possible that the system was born in a star cluster and the gravitational influence of the other stars during the close passages could insert the binary into what we see today.
The big question now is: how common are systems like this? One was previously seen in a nearby galaxy – Kareem El-Badry, the Principal Investigator on the Gaia BH1 paper, also worked to find this precedent, but the fact that this is close to us implies that similar systems are common. The galaxy is 120,000 light-years in diameter, so if these things are rare, then the closest one is that close would be extremely unlikely. If they are common, Gaia is likely to find many more in the future; the longer you observe the same stars over and over the easier it is to detect their movements.
It is ironic to know of thousands and thousands of black holes in distant galaxies millions and billions of light years away, because they are really huge and give off enormous amounts of energy, yet the vast majority of black holes in our own galaxy are completely invisible to us. .
Now. They are invisible now, but we are continually improving in finding them. They are dark, but they cannot hide forever.
* I will notice that this is still a very long road in human terms: 15 quadrillion kilometres! – so it’s not a danger to us.