At some point not too long ago, on cosmic time scales at least, one star in a large star cluster had a really bad day. At a certain point in its orbit through the cluster, it strayed too close to a black hole and began to be disrupted. As it spiraled closer, it was messily torn apart more and more until it wasn’t so much a star as a cloud of hot, excited debris and the radiation given off just as some of that shredded star fell into the black hole. Thanks to the simple fact that the black hole shredded the star instead of devouring it all at once, scientists were left with a unique chance to use that spiraling debris to measure the mass of that black hole.

This isn’t a straightforward process. When a planet orbits a star, the bulk of the mass is in the star, and the planet’s motion is easy to see separate from the star’s motion. The rotation rates of the star and planet really have close to nothing to do with how the objects move. With this shredded star and invisible black hole, things are a lot more complicated. 

First off, that black hole can’t be directly observed. And instead of a single moving object, there is a spinning disk that doesn’t all spin together. And thanks to relativity, that disk’s motion does not reflect normal Newtonian orbital dynamics and a lot of relativity. This system is trying really hard to hide its secrets. That said, a team of researchers led by Sixeng Wen has published a paper in The Astrophysical Journal that uses that swirling debris disk and a whole lot of math to calculate both the spin and mass of the black hole.

And that mass is pretty interesting. This black hole in a star cluster appears to be intermediate in size, between the mass of black holes that form directly from collapsing stars and supermassive black holes in the centers of galaxies that can be millions of solar masses in size. 

How the universe created supermassive black holes is a bit of a mystery, and one of the possible solutions is the merger of stellar-mass black holes until they are intermediate-mass, and this object in a nice dense star cluster where mergers statistically should happen is fifteen to twenty thousand solar masses, making it totally intermediate and exactly what we’re looking for.

And this system, which is only visible because it mostly ate a star and left remains behind for us to see, hints at the possibility that star clusters are full of intermediate-mass black holes – ones that are generally invisible because they haven’t been messy eaters. 

While I’m not sure that a universe full of invisible black holes is an existential win, it is a scientifically super cool thought. And it is the thought we’re going to leave you with today.

More Information

The University of Arizona press release

“Mass, Spin, and Ultralight Boson Constraints from the Intermediate-mass Black Hole in the Tidal Disruption Event 3XMM J215022.4–055108,” Sixiang Wen, Peter G. Jonker, Nicholas C. Stone, and Ann I. Zabludoff, 2021 September 6, The Astrophysical Journal