Hypervelocity Stars Hint at a Supermassive Black Hole Right Next Door

Scientific American

23-05-2025

An astonishing fact only known for the past few decades is that every big galaxy in the universe has a supermassive black hole at its heart. This was suspected in the 1980s, and observations from the Hubble Space Telescope, which has peered deep into the cores of galaxies all across the sky, confirmed it. The 'normal' kinds of black holes made when stars explode range from five to about 100 times the mass of the sun, more or less. But these central galactic monsters are millions of times more massive, and some have grown to the Brobdingnagian heft of billions of solar masses.
A lot of mysteries still remain, of course, such as how they formed early in the history of the universe, how they grew so humongous so fast and what role they played in their host galaxy's formation. But one odd question still nagging at astronomers is: What's the galaxy size cutoff where this trend stops? In other words, is there some lower limit to how massive a galaxy can be and still harbor one of these beasts?
The inklings of an answer are emerging from a surprising place: studies of rare stars moving through our own galaxy at truly ludicrous speeds.
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Orbiting our Milky Way galaxy is a menagerie of smaller 'dwarf' galaxies, some so tiny and faint you need huge telescopes to see them at all. But two are so large and close that they're visible to the unaided eye from the Southern Hemisphere: the Large and Small Magellanic Clouds.
The Large Magellanic Cloud (LMC) is the bigger and closer of the two, and it's not clear if it harbors a supermassive black hole (SMBH). If such an SMBH exists there, it must be quiescent, meaning it's not actively feeding on matter. As material falls toward such a black hole, it forms a swirling disk of superheated plasma that can glow so brightly it outshines all the stars in the galaxy combined. No such fierce luminescence is seen in the LMC, so we don't know if an SMBH is there and not actively feeding or if the LMC is simply SMBH-free.
But a recent study published in the Astrophysical Journal offers strong evidence that an SMBH does lie at the center of the LMC—based on measurements of stellar motions in our own Milky Way!
The study looked at hypervelocity stars, ones that are screaming through space at speeds far higher than stars around them. Some of these stars are moving so rapidly that they have reached galactic escape velocity; the Milky Way's gravity can't hold them. In the coming eons, they'll flee the galaxy entirely. And we have good reason to believe these runaway stars were launched by SMBHs—but how?
Such a situation starts with a binary system, two stars orbiting each other. These systems contain a substantial amount of orbital energy, the sum of the kinetic energy of the two stars—their energy of motion—and their gravitational potential energy, the amount of energy released if they were to move closer together.
If the binary star approaches a third object, some of that energy can be swapped around. One star can become bound to the third object, for example, while the other star can get a kick in its kinetic energy, flinging it away. The amount of the kick depends in part on the gravity of the third object. A massive black hole, of course, has an incredibly strong gravitational field that can fling the star away at high speed.
And I do mean high speed; such a star can be flung away from the black hole at a velocity greater than 1,000 kilometers per second. S5-HVS1, for example, was the first confirmed such hypervelocity star, and it's moving at more than 1,700 kilometers per second. Feel free to take a moment to absorb that fact: an entire star has been ejected away from a black hole at more than six million kilometers per hour. The energies involved are terrifying.
We have seen a few of these stars in our galaxy, and careful measurements suggest they're moving away from the center of the Milky Way, which is pretty convincing evidence that Sagittarius A*, our own Milky Way's SMBH, is to blame.
But not all of the high-velocity stars that have been detected appear to come from our galactic center. Fortunately, Gaia, the sadly now decommissioned European Space Agency astronomical observatory, was designed to obtain extremely accurate measurements of the positions, distances, colors and other characteristics of well more than a billion stars—including their velocity.
There are 21 known hypervelocity stars at the outskirts of the Milky Way. Using the phenomenally high-precision Gaia measurements, the astronomers behind the new research examined the stars' 3D velocities through space. They found that five of them have ambiguous origins, while two definitely come from the Milky Way center. Of the 14 still left, three clearly come from the direction of the LMC.
The trajectories of these stars effectively point back to their origin, and based on our current knowledge, that origin must be a supermassive black hole. Even better, although the remaining 11 stars have trajectories that are consistent with both Milky Way and LMC origins, the researchers found that five are more likely to have come from our home galaxy and the other six are more likely to have come from the LMC.
So there could be nine known hypervelocity stars plunging through our galaxy that were ejected by a supermassive black hole in another galaxy.
Using some sophisticated math, the team found that the most likely mass of the black hole is 600,000 or so times the mass of the sun. This isn't huge for an SMBH—it's very much on the low end of the scale, in fact—but then, the LMC is a small galaxy, only 1 percent or so the mass of the Milky Way. We know that the mass of a black hole tends to scale with its host galaxy's mass (because they form together and affect each other's growth), so this lower mass is consistent with that.
If this is true, then our satellite galaxy is shooting stars at us! And there may be more of them yet to be found, hurtling through space unseen on the other side of our galaxy, or so far out that they're difficult to spot and even harder to study. And all this helps us get a clearer—but still quite hazy!—sense of just how far down the galactic scale we can expect to find big black holes.
Black holes are funny. Most people would worry about falling into one, as well as a host of other terrors, but now you can add 'having to dodge intergalactic stellar bullets' to that list.