In the very center of our Milky Way Galaxy sits a monster: a black hole 24 million kilometers across with 4 million times the mass of our Sun. Called Sgr A*, it likely formed with the Milky Way, both growing to their current size over billions of years.

One way galaxies grow is to consume other galaxies. Sometimes they’re about the same size, but most of the time bigger galaxies swallow and merge with smaller ones (since dwarf galaxies are much more abundant). Sometimes these smaller galaxies have big black holes at their hearts, too, and as the two galaxies collide and coalesce the smaller black hole will fall toward the big one, orbiting it for a while and eventually falling into it.

Did the same happen for Sgr A*? Oh, it almost certainly has, though evidence for such a thing is difficult to come by.

But a team of Chinese astronomers thinks they may have found some evidence for it, and the collision and merger would have happened pretty recently on a cosmic timescale. Their smoking gun? High velocity stars, of all things. [link to journal paper]

You may know I write a weekly column for Scientific American called The Universe, and in May 2025 I wrote an article about these speedy stellar bullets and their connection to black holes. Over the years astronomers have spotted a few dozen stars moving way, way faster than they should. Almost all the stars in the Milky Way orbit the galactic center, with their orbital speed depending mostly on their distance from the center (similar to planets orbiting the Sun). But some are moving at much higher velocities than expected, and in fact a handful are screaming through space at such high speeds that the galaxy’s gravity can’t hold on to them. They’re doomed to eventually fly out into intergalactic space.

What can kick these stars to such high speed? Well, the answer’s not a surprise since I’ve already said it: black holes.

If two stars orbit each other as a binary pair, and they fall toward a black hole, the complicated interaction between their individual orbits around each other and their orbit around the black hole can dump a lot of kinetic energy (the energy of motion) into one star, causing it to be flung away, while the other star can be captured into a tighter orbit around the black hole, or even fall in. 

But there’s a problem. Modeling of the physics shows that a lot of the stars ejected should be moving away at speeds higher than 700 kilometers per second. However, observations have found very few such hypervelocity stars. If you plot the number of stars versus velocity, the graph actually peaks around 700 kps, then drops off in number at higher speeds. Why don’t we see those super-fast ones?

The astronomer propose that some time ago, a smaller black hole orbited Sgr A*, probably around 15,000 times the mass of the Sun. This would have been the central black hole of a galaxy that we ate, maybe one called (seriously) the Gaia-Sausage-Enceladus galaxy that merged with the Milky Way some 10 billion years ago. The smaller black hole then fell into orbit around Sgr A*.

This affected how binary stars interacted with them. A binary falling towards them wouldn’t just drop down to Sgr A* and get torn apart. Instead, statistically speaking, it’s far more likely it would pass by the smaller black hole several times first as the binary system looped around the bigger black hole. As they interacted with the smaller black hole, the orbit of the two stars in the binary pair stars would be affected, most likely getting stretched out into a long ellipse. When they were at the part of their orbit around each other when they were farthest apart they’d be moving at their slowest, and importantly they spend most of their time like that since that part of the orbit is longer and they’re moving more slowly. So, chances are, they’d be moving slowly around each other when they finally fell down into Sgr A*. The kick one star was then subsequently given by the black hole wouldn’t be as strong (the slingshot affect is highest on objects moving rapidly, and not as strong when they’re slower).

This would explain why so few stars moving faster than 700 kps are seen. The binary system didn’t make one pass of the smaller black hole, they made several, each one making it harder to give so much extra kick energy to the star that escapes. The astronomers modeled the physics based on observations, which is how they got the mass of the smaller black hole to be 15,000 solar. 

This could explain something else, too: We see a lot of stars orbiting Sgr A* pretty close in, and their origin is something of a mystery. There’s a lot of gas floating around there, and it’s been assumed that’s what made the stars, but the gas has a lot of energy (it’s hot) and I’ve always wondered how it could coalesce to form stars. But this second black hole idea explains that: many or most of them could be the other stars that didn’t get flung away during their binary disruption. The astronomers calculated how many such stars should survive the encounter to orbit the black hole and the numbers they get are a reasonable fit to what’s seen. Interesting.

So what happened to the smaller black hole? You’d think we’d see it. To fit the observations, the astronomers think it may have finally merged with Sgr A* about 10 million years ago, so it’s long gone.

This is a clever idea, and nothing in it is too wacky. We know these high-velocity stars exist, we know there should be more faster ones, we know there are stars orbiting Sgr A*, and we know galaxies merge and their black holes can fall into orbit around each other and eventually merge as well. Every step makes sense.

So is this idea right? That’s hard to say. A black hole merger could produce a recoil, like a kick, to the remaining black hole, which would move it away from the exact center of the galaxy at roughly 0.5 kps. Such a motion has been detected in Sgr A* (which is cool; I didn’t know that before reading this paper), so that’s consistent, too.

I hope more folks follow up on this, because it’s a really interesting idea. Our galaxy has had a pretty amazing history, though we don’t know a lot of the details. It would be nice to fill in more of the blanks.

Tip o’ the ergosphere to AAS Nova.

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