At the violent hearts of galaxies, where supermassive black holes reign, some stars defy the expected fate of annihilation — returning again and again to shed a little of themselves with each pass, their light growing dimmer like a candle burning low. Researchers at Syracuse University have found that a star's pre-existing spin, likely inherited from a torn-apart binary partnership, holds the key to this fading pattern. In understanding why these survivors grow quieter with each encounter, science draws a thread connecting the exotic to the familiar, reaching even toward the black hole at the
Why Stars Survive Black Holes' Repeated Attacks—And Grow Dimmer
A star that was already spinning rapidly before its first encounter
Mark
So a star can actually survive being attacked by a black hole multiple times?
Mimi
Yes. If it passes close enough to be torn apart but not so close that it's completely destroyed, it loses some material but keeps its core intact. That core stays in orbit and comes back for another pass, sometimes months or years later.
Mark
And each time it comes back, it produces a visible flare?
Mimi
Right. The material stripped from the star falls toward the black hole, heats up, and releases energy. We can see that light from Earth. But here's where it gets strange: in some systems, each successive flare gets dimmer, even though the star keeps losing material.
Mark
Why would it get dimmer if material keeps falling in?
Mimi
That's what stumped researchers for years. The simulations predicted the flares should stay roughly the same brightness. The key turned out to be the star's rotation. The black hole doesn't just tear the star apart—it also spins it up faster with each encounter.
Mark
How does that affect the brightness?
Mimi
If a star is already spinning fast before the first encounter, it can't be spun up much more. That means the material stripped away falls back toward the black hole more slowly, over a longer time. Less material arriving more slowly produces a dimmer flare.
Mark
But why would a star already be spinning fast?
Mimi
Likely because it came from a binary system—two stars orbiting each other very tightly. When that pair got too close to the black hole, the black hole tore them apart and captured one. Stars in tight binaries are tidally locked, forced to spin at the same rate they orbit each other. The tighter the binary, the faster the spin.
Mark
So the same event that captured the star also gave it the spin that explains the dimming?
Mimi
Exactly. One mechanism—the Hills process—explains the rapid rotation, the tight orbit, and the fading flares all at once.
The Pulse
- Astronomers had caught stars surviving repeated brushes with supermassive black holes, but a stubborn puzzle remained: why did roughly four in ten of these systems produce flares that grew progressively fainter with each pass?
- Existing simulations predicted peak flare brightness should stay roughly constant — tidal forces spin the star faster after each encounter, compressing the fallback of stripped material into a brighter burst — yet the universe stubbornly disagreed.
- After two years of contradiction, doctoral student Ananya Bandopadhyay identified the missing ingredient: a star that arrives already spinning rapidly, leaving little room for the black hole to spin it up further, so each successive stripping event produces a weaker, dimmer flare.
- The Hills mechanism — a supermassive black hole tearing apart a binary star pair and capturing one member — elegantly explains both the rapid pre-existing spin and the tight orbits observed, tying multiple mysteries to a single origin.
- The findings carry implications beyond distant galaxies, potentially illuminating the strange stellar populations orbiting Sagittarius A*, the black hole anchoring our own Milky Way.
At the violent hearts of galaxies, where supermassive black holes reign, some stars defy the expected fate of annihilation — returning again and again to shed a little of themselves with each pass, their light growing dimmer like a candle burning low. Researchers at Syracuse University have found that a star's pre-existing spin, likely inherited from a torn-apart binary partnership, holds the key to this fading pattern. In understanding why these survivors grow quieter with each encounter, science draws a thread connecting the exotic to the familiar, reaching even toward the black hole at the center of our own galaxy.
At the center of most galaxies sits a supermassive black hole whose gravitational grip shapes everything around it. When a star wanders too close, the expectation is total destruction — yet astronomers have found something far stranger: some stars survive. They return on repeating orbits, each pass stripping away outer layers and producing a burst of light, growing dimmer with every encounter. These repeating partial tidal disruption events, or rpTDEs, have puzzled astrophysicists for years.
The basic mechanics are well understood. A black hole's gravity pulls harder on a star's near side than its far side, shearing away material without fully destroying the star. That torn material spirals inward, heats up, and flares. Wide-field sky surveys have made it possible to track the same star through multiple such encounters. But about four of the roughly ten known repeating systems showed something the models could not explain: the flares were getting progressively fainter. Simulations insisted peak brightness should hold roughly steady — tidal forces spin the star faster after each pass, compressing the returning material into a concentrated burst that maintains a similar peak rate. Yet the observations told a different story.
For two years, researchers at Syracuse University sat with this contradiction. The resolution, led by doctoral student Ananya Bandopadhyay, required a new ingredient: a star that was already spinning rapidly before its first encounter. A fast-spinning star cannot be spun up much further by tidal forces, so the fallback timing of stripped material changes little between passes. As progressively less material is shed, the peak fallback rate genuinely drops — and the flares fade, exactly as observed. The simulations finally matched reality.
This answer immediately raised another question: why would such a star arrive spinning so fast? The likely explanation is the Hills mechanism — a binary star system torn apart by a supermassive black hole, with one star ejected and the other captured. Stars in a very tight binary become tidally locked, rotating as fast as they orbit each other. The compact configuration needed to place a captured star on the short orbits seen in rpTDEs would naturally produce a rapidly spinning star, precisely the kind needed to explain the dimming pattern. Rapid rotation, tight orbit, and fading flares all emerge from a single underlying event. The same physics may also shed light on the peculiar stellar populations surrounding Sagittarius A*, the black hole at the heart of our own galaxy.
At the center of most galaxies sits a supermassive black hole, an object so massive it outweighs the sun by millions or billions of times over. The gravitational environment around these cosmic anchors is among the most extreme in the universe. When a star wanders too close, the outcome seems inevitable—the black hole should tear it to shreds. Yet astronomers have discovered something stranger: some stars survive these encounters. They return again and again, each time shedding material and flaring with light, only to grow dimmer with every pass.
These repeating partial tidal disruption events, or rpTDEs, have puzzled astrophysicists for years. The mechanism itself is well understood. A black hole's gravity pulls harder on the near side of a star than the far side, creating a tidal force that can strip away the star's outer layers without destroying it entirely. The torn material spirals inward, heating up and releasing energy as it falls toward the black hole. Each time the star returns to its orbit and makes another close pass, fresh material gets stripped away, producing another burst of light. Wide-field surveys that scan the sky repeatedly have made it possible to catch these events and track the same star through multiple encounters.
But the observations revealed a puzzle that theoretical models could not explain. About four of the roughly ten known repeating systems showed a pattern: the flares grew progressively fainter with each encounter. This should not happen, at least not according to the simulations. Even if less material was being stripped from the star during each passage, the models predicted the peak brightness of the resulting flares should remain roughly constant. For two years, researchers at Syracuse University sat with this contradiction, unable to reconcile what the universe was showing them with what their equations predicted.
The breakthrough came from recognizing a detail that had been overlooked. When a black hole's tidal forces pull material away from a star, they do more than just tear it apart. They also apply a twisting force—a torque—that causes the star to spin faster after each encounter. This increased rotation changes the timing of how stripped material falls back toward the black hole. Even though less material is being shed, it returns in a more concentrated burst, compressed into a shorter time window. That concentrated flow maintains a similar peak fallback rate, which is why the flares stayed roughly equally bright in the models. The mystery remained unsolved.
The answer, according to doctoral student Ananya Bandopadhyay and her colleagues, required what she called "a new ingredient": a star that was already spinning rapidly before its first close encounter with the black hole. If a star begins its dance with the black hole already rotating at high speed, it cannot be spun up nearly as much during subsequent passages. Without that large increase in rotation, the time required for stripped material to fall back toward the black hole stays relatively constant. As progressively less material is stripped away, the peak fallback rate decreases. The flares become fainter with each encounter, matching what astronomers actually observe. The simulations finally reproduced reality.
This raised a new question: why would a star approaching a supermassive black hole already be spinning so fast? The answer may lie in a process called the Hills mechanism. Imagine two stars orbiting closely around each other, a binary system bound by their mutual gravity. When this pair ventures too close to a supermassive black hole, the black hole's gravity tears the binary apart. One star is ejected into space. The other is captured into orbit around the black hole. Stars in a very tight binary become tidally locked, meaning each star rotates on its axis at the same rate the pair orbits each other. The tighter the binary, the faster the rotation must be. To leave a captured star on the short orbit observed in rpTDEs, the original binary would have to be extraordinarily compact. That same tight configuration would naturally produce a rapidly spinning, tidally locked star—exactly the kind of star that can explain the fading flares.
This mechanism ties together several puzzles at once. The rapid rotation, the tight orbit, and the dimming flares all emerge from a single underlying phenomenon: the tidal destruction of a binary system and the capture of one of its stars. From a theoretical standpoint, this represents a significant step forward in understanding the physics of these extreme systems. The implications may extend beyond distant galaxies. The same Hills mechanism could explain some of the unusual stellar populations now orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. The same physics that governs the fate of stars light-years away may also shape the environment in our own cosmic neighborhood.
Notable Quotes
We were puzzled by this for two years— Ananya Bandopadhyay, doctoral student at Syracuse University
Each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars— Eric Coughlin, associate professor at Syracuse University
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