I love astronomy stories that take a sudden left turn, where you expect to find one thing and then oops, it’s really another.

For example, about 600 million light-years from Earth lies a decently big galaxy. It has no name, but it appears to be a lenticular galaxy, one that’s lens-shaped. It’s a disk similar to the Milky Way, but lacks gas, dust, and spiral arms, so in some ways it’s more like an elliptical galaxy. Its mass in stars is about 85 billion times that of the Sun, about the same as the Milky Way.

All big galaxies has a supermassive black hole in their centers, and the mass of the black hole scales with the mass of the galaxy — the bigger the galaxy, the heftier the black hole. In this case, theoretical models show the central monster in this unnamed galaxy is likely about 200 million times the mass of the Sun (that’s a lot bigger than Sgr A*, the Milky Way’s black hole, which is only about 4 million, but it’s well known ours is underweight, though we don’t know why).

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That’s all well and good, but then something quite spectacular happened. In August 2024, the Zwicky Transient Facility (an observatory that scans the skies looking for objects that change in brightness or move across the sky) detected a sudden increase in brightness coming from the galaxy. It was quickly confirmed, and by October astronomers had identified the cause of the flare: it was a tidal disruption event, or TDE, the worst acronym in the history of astronomy. Why is this a rotten name? Because it indicates a black hole that’s ripped apart a star, shredding its guts in a mind-vaporizingly explosive event and spewing them billions of kilometers in to space.

Yeah. “TDE” doesn’t really cut it, does it?

TDEs are spectacularly violent. About half the star’s mass falls into a disk around the black hole, while the other half gets flung into a wide elliptical orbit around the black hole. The disruption itself produces a vast amount of energy, as you might expect when you crack open a star and explode its material hither and yon. In the case of this particular TDE, called AT2024tvd, at its brightest it was blasting out light at a rate ten billion times that of the Sun, almost as bright as the entire host galaxy!

These events can tell us a lot about the black hole that spawned all the chaos, as well as its host galaxy, so these events are studied carefully when they occur. That’s when things got weird [link to journal paper]; when they pointed Hubble Space Telescope at the galaxy in January 2025, the TDE was clearly seen… about 2,600 light-years from the galactic core. That’s way, way off-center! This offset was confirmed by Chandra X-ray Observatory images as well as radio observations using the Very Large Array. It’s essentially impossible for a supermassive black hole to be that far away from the galaxy’s middle, so this led astronomers to draw a very cool conclusion: this was the consequence of a second black hole!

Given the characteristics of the TDE, astronomers calculate the second black hole is likely around a million solar masses (though it could be as low as 100,000 and as much as ten million). This is still far less than what’s expected for the galaxy’s central black hole, so it seems clear it really is a second massive black hole in the galaxy, thousands of light-years from the central one.

How did it get there? The most likely explanation is that the host galaxy collided and merged with a second, smaller galaxy and this black hole was the one at its center. It can take a long time, billions of years, for the consumed galaxy’s black hole to fall to the center, so that’s consistent. It’s odd that the galaxy itself doesn’t show any obvious signs of a collision, but if it happened long ago it could have settled down since then. Another possibility is that the second black hole used to be much closer in to the galaxy’s center, but was involved in an encounter with a third black hole, and it wound up being gravitationally slingshot (slungshot? slingshotted?) away from the center. Either way, yegads. Tossing around massive black holes is a terrifying hobby.

Two off-center TDEs have been seen before in other galaxies; they were discovered when those galaxies were giving off anomalously high amounts of X-rays. In both those cases the black holes were in small satellite galaxies to the main galaxy, whereas this new one does appear to be coming from the galaxy itself. 

There’s another cool thing here, too. A TDE happens because the black hole rips apart a star due to tides; the star wanders too close to the black hole, and the side of the star closer in feels a much stronger gravitational pull than the far side. This effect, what we call tides, is what rips the star apart. But that means the black hole can’t be too massive. If it gets too big, then the star is too small compared to the size of the black hole, and the near and far sides don’t feel enough of a difference in gravity to be torn apart. The star will either orbit the black hole and stay intact, or just fall in.

The upper limit to a black hole’s mass to create a TDE in a sun-like star is about 100 million times the Sun’s mass. That fits with what’s seen here. Black holes with substantially less mass, like in the million solar mass range, are typically very hard to detect, but they are really good at making TDEs, so these events let us study such mini-monsters. We can determine the black hole mass, the rate at which TDEs occur, the environment around the black hole, and more.

It’s a little weird, I’ll admit, to get excited about the unimaginably violent death of a star because we can learn so much from them… but then historically astronomy has always done that; we look at exploding stars like supernovae, and slowly dying stars that create planetary nebulae, too. It’s the only way we can learn about what happens at the end of a star’s life, whether it happens naturally or because of an outside influence.

We understand now that such stellar deaths create the elements needed to harbor life in the Universe, so in a sense there’s a poetry to it. The Universe certainly doesn’t care about any of this, but we do. We can make meaning from it, and I think that’s wonderful.

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