Astronomers have found the most massive stellar black hole known in the galaxy so far: Called Gaia BH3, it’s 33 times the mass of the Sun (OK, to be more precise, 32.7±0.82) and only about 1,900 light-years from Earth, making it the second closest black hole known, too! [link to journal paper]

Black holes this massive are posing an interesting conundrum, since we don’t really know how they form. We see them in other galaxies (well, we detect them at least) but nothing this beefy has been found in the Milky Way before. Hopefully it’ll help us understand how these bizarre objects are born.

[Note: Stellar black holes have masses up to a few dozen times the mass of the Sun. There are also supermassive black holes with millions or billions of times the Sun; Sgr A* is the one in the center of our Milky Way at over 4 million solar masses; the new black hole is the most massive stellar one found in our galaxy so far.]

Sometimes finding black holes is easy. If they happen to be eating a star or gas cloud, the material falling in piles up just outside the Point of No Return into a disk that gets super-hot, which also makes it very luminous. The disk blasts out high-energy light like X-rays, and can be among the brightest sources of X-rays in a galaxy. 

But what if the black hole isn’t actively feeding? We call ones like this quiescent, and they’re really hard to spot. They’re black. Not even light can escape from them, so they literally are the darkest objects in the Universe. How can you spot one then?

While they aren’t profligate with light, they are pretty generous with their gravity. If a black hole is in a binary system with a normal star, and that system is close to us, we can see the physical motion of the star as it goes around the black hole. Now mind you, this isn’t easy: That motion is very, very teeny, so you need an incredibly high-resolution and stable platform from which to observe it.

Enter Gaia: a space mission by the European Space Agency that scans the sky and measures (among many other things) the positions and motions of well over a billion — yes, billion with a B — stars in our galaxy. It gets ridiculously precise measurements, too. Just as importantly, it’s been doing so for about a decade, so it can track the motions of its targets long enough to get pretty precise details.

In the constellation of Aquila is a star called 2MASS J19391872+1455542 (2MASS was an infrared survey of the sky and the alphanumeric salad is the star’s coordinates on the sky). It’s similar in color to the Sun but is running out of fuel in its core, and is swelling up into a giant. That makes it bright enough for Gaia to see clearly and get good measurements. It was found using software that looks for binary stars in the data; as the two stars move around each other their positions in the sky change by a small amount in a periodic way. But this star is a little different: It shows binary motion, but it doesn’t seem to have a binary companion!

That’s pretty suspicious. The orbital motion of a star around another star depends on their masses and distances from each other. Looking at the math, the astronomers find that the star is orbiting an object with 33 times the mass of the Sun, but gives off no light. A star that massive would be booming out light, bright enough to see with the naked eye! Yet no light is seen.

So, yeah. Black hole.

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