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Supermassive black holes are some of the most mysterious entities in the universe. They weigh millions to billions of times more than our Sun and are usually found at the centers of large galaxies. The black hole at the core of the Milky Way, Sagittarius A*, has about four million solar masses. Since black holes don’t emit light, astronomers can only detect them indirectly by observing how they influence nearby stars and gas.
In a recent study published in The Astrophysical Journal Letters, Eric Coughlin, an assistant professor of physics at Syracuse University, along with colleagues, explains what occurs when a star ventures too close to a black hole and gets torn apart. Instead of vanishing instantly, the star is stretched into a long, thin stream of debris by the black hole’s gravity. Over time, this debris wraps around the black hole—a process driven by Einstein’s General Theory of Relativity, as classical Newtonian gravity doesn’t produce this effect. When parts of this debris stream collide, they release a burst of energy, causing some material to spiral inward and be absorbed by the black hole. These events generate intense radiation, briefly outshining the entire galaxy, which can contain roughly a trillion suns in brightness.
Such events are called tidal disruption events, or TDEs. They provide a rare window into studying supermassive black holes like Sagittarius A* in other galaxies. “Studying TDEs gives us a chance to learn more about these hidden black holes,” Coughlin explains.
TDEs have long intrigued scientists because each flare is like a unique signature. By analyzing how the brightness rises, peaks, and diminishes, researchers can infer properties such as the black hole’s mass and spin. However, understanding exactly how these flares form has been challenging due to the complexity of accurately simulating the processes involved.
This is changing thanks to advanced high-resolution simulations. A team led by Lucio Mayer at the University of Zurich, including Coughlin, utilized a technique called smoothed particle hydrodynamics. This method models the star as a collection of particles that interact hydrodynamically, governed by the Navier-Stokes equations—the same principles describing water flow.
Using tens of billions of particles, the team captured the detailed behavior of the disrupted star’s gas for the first time. Their findings reveal that instead of dispersing chaotically, the debris tends to form a narrow, coherent stream that follows a predictable orbital path around the black hole before ultimately crashing into itself. This confirms a long-standing theoretical prediction. Earlier, lower-resolution simulations often misrepresented the stream’s structure, showing a more chaotic spread and overestimating fluid dissipation.
By deploying powerful supercomputers and leveraging graphics processing units (GPUs), the team achieved a much clearer view of the debris stream’s shape and behavior. Their models also uncovered the influence of black hole properties—specifically, its mass, spin rate, and the orientation of that spin relative to the debris’ orbital plane. These factors can affect the timing, brightness, and duration of the resulting flare.
If the black hole is rotating, it causes additional distortions in spacetime, leading to an effect known as “nodal precession”—which can cause the debris stream to shift out of its initial plane. This shifting can delay when the flare begins, as the debris misses self-collision points, possibly causing multiple orbit crossings before a collision actually occurs. Such complexities might explain why observed TDEs vary so much—some flare up quickly and fade fast, others unfold more slowly, and some display behavior that’s hard to classify.
While the mass of the black hole may account for some of this diversity, the new simulations suggest that the black hole’s spin could play a major role. TDEs turn otherwise invisible black holes into observable phenomena—stars are torn apart, debris collides and emits light, revealing the black hole’s hidden presence. With improved simulations and more advanced telescopes, astronomers are becoming better equipped to interpret these signals with greater clarity.
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