Fast-spinning stars survive black hole encounters, revealing why their flares fade

Astronomers have long known that stars can venture too close to supermassive black holes without being instantly obliterated. Some survive these encounters, their cores remaining intact, only to return months or years later for another brush with gravitational forces that would seem to guarantee destruction. Each time they pass, they shed material—stellar skin stripped away by tidal forces—and that falling debris lights up the darkness around the black hole in a burst of radiation. These repeating partial tidal disruption events, or rpTDEs, are rare enough that finding even one is noteworthy. Finding a handful has allowed astronomers to watch the same star interact with the same black hole again and again, a cosmic laboratory of sorts.

But the observations have posed a puzzle. In roughly four of the ten known repeating systems, the flares grow progressively dimmer with each return. This should not happen, at least not according to the models that physicists had built to describe these encounters. The simulations suggested that even as a star lost less material during successive passes—a natural consequence of having already shed its outer layers—the peak brightness of the resulting flare should remain roughly constant. The material might be less abundant, but it should still arrive at the black hole's accretion disk in a concentrated enough burst to produce a similarly bright flash. For two years, researchers at Syracuse University found themselves stuck on this contradiction.

The breakthrough came from reconsidering a property of the star that had been largely overlooked: how fast it was already spinning before its first encounter with the black hole. Doctoral student Ananya Bandopadhyay, working with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin, realized that a rapidly rotating star behaves fundamentally differently from a slowly rotating one when subjected to the black hole's tidal torque. When a star approaches a black hole, the gravitational forces do more than pull material away from it—they also twist it, causing it to spin faster. But a star that is already rotating at high speed cannot be spun up as dramatically. This seemingly small difference has large consequences. The timing of the material's return to the black hole changes. Less acceleration means the stripped debris takes longer to fall back, spreading the influx over a longer period. A more gradual arrival produces a dimmer peak, which is exactly what astronomers have observed.

This finding raised a second question: Why would a star approaching a supermassive black hole already be spinning so rapidly? The answer may lie in a mechanism known as the Hills process, named for the physicist Jack Hills. Imagine two stars in a tight binary orbit around each other, their dance so intimate that tidal forces lock each one's rotation to their shared orbital period. The tighter the binary, the faster each star must spin. Now imagine this binary system wandering too close to a supermassive black hole. The black hole's gravity is strong enough to tear the pair apart. One star is ejected into the galaxy at high speed, while the other is captured into a new orbit around the black hole itself. For the captured star to end up in the extraordinarily tight orbit observed in rpTDEs—one that brings it back to the black hole every few months—the original binary must have been extremely compact. And that same compactness would have forced the captured star to spin rapidly before it was ever torn away from its companion.

The elegance of this explanation is that it accounts for multiple mysteries at once. The rapid pre-existing rotation explains the fading flares. The need for a very tight binary to produce the observed orbital configuration explains why these stars end up in such extreme orbits in the first place. And the Hills mechanism, which has been theorized for decades, finally has observational support. Coughlin notes that this same process may have shaped the unusual stellar populations orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. Some of the stars now dancing around our galaxy's central black hole may have arrived there through the same violent binary disruption that explains the fading flares in distant galaxies. The work, published in The Astrophysical Journal, represents a significant step forward in understanding not just these rare repeating events, but the broader physics of how stars and black holes interact across the cosmos.