Repeating Stellar Destruction
Supermassive black holes at the centers of galaxies occasionally capture stars in a deadly orbital dance. When a star passes close enough for gravity to strip away its outer layers but not close enough for complete destruction, astronomers observe a repeating partial tidal disruption event. These events trigger distinct flares of light as the stolen stellar material falls toward the black hole.
Astronomers have long noted that these flares often dim over time. New research from Syracuse University identifies the spin of the star as the primary driver behind this fading light. By modeling the physics of these stellar encounters, researchers now believe that the internal structure of the star determines how much material is lost during each pass.
The Role of Stellar Structure and Spin
Low-mass stars possess a loose, meringue-like structure that makes them highly susceptible to tidal forces. In these cases, the black hole strips material from deep within the star. Conversely, high-mass stars exhibit a denser, onion-like composition. Their cores resist the black hole's influence, meaning only the outer layers get removed. As these outer shells are whittled away, the star offers less fuel for the black hole, leading to fainter flares.
But structural density provides only a partial explanation. During these encounters, the black hole exerts torque on the star, forcing it to spin faster. This increased rotation usually shortens the orbital period, which keeps the flares bright by delivering fresh material more frequently. The team discovered that if a star is already spinning rapidly before its first encounter, this torque has little effect. Without that speed boost to shorten the orbit, the star simply runs out of surface material, causing the flares to die out.
Binary Origins and Future Implications
The mystery of why a star would arrive at a black hole already spinning at high speeds points to the chaotic history of galactic centers. Most stars do not exist in isolation. They typically live in binary pairs. Researchers suspect that when a binary system nears a supermassive black hole, gravity rips the pair apart. One star is ejected, while the other is captured into a tight, rapid orbit.
This process, known as Hills capture, results in a star that is both tightly bound and rotating at high speed. Eric Coughlin, a team member from Syracuse University, notes that this theory explains multiple peculiarities observed in these systems. The findings were published on August 18 in The Astrophysical Journal.
While the Milky Way’s central black hole, Sagittarius A*, is not currently undergoing such a flare, it holds many stars in rapid orbits. Understanding the mechanism behind these captured stars provides a clearer picture of how galactic centers evolve. The work marks a shift in how theorists interpret the life cycles of stars caught in the gravity of cosmic giants.