Researchers at Syracuse University have identified the cause behind the fading brightness of stars that are repeatedly partially consumed by supermassive black holes. The study suggests that a star's initial rotation speed and its previous existence in a binary system determine the intensity of these cosmic flares.
Why "Meringue" and "Onion" Star Structures Dictate Flare Intensity
When a star wanders too close to a supermassive black hole, it undergoes "spaghettification," where tidal forces stretch the star into a stream of plasma often called "stellar pasta." While some stars are destroyed instantly, others experience repeating partial tidal disruption events (rpTDEs), where the black hole takes successive "bites" of the star's outer layers over multiple orbits.
According to the Syracuse University report, the amount of material stripped during these encounters depends heavily on the star's internal density. Researcher Bandopadhyay describes low-mass stars as being like "fluffy meringues" that are easily deformed, whereas high-mass stars resemble "tightly layered onions." In the case of the "onion" stars, the dense core resists disruption even as the outer envelope is stripped away, which naturally leads to a decrease in the amount of material available for the black hole to consume in subsequent passes.
How Bandopadhyay’s Initial Spin Theory Explains Dimming Flares
A persistent mystery in astronomy has been why these rpTDE flares grow steadily dimmer over time. While the black hole exerts a torque that typically causes a passing star to spin faster and return more frequently—which would normally maintain a consistent flare level—some events do not follow this pattern.. Bandopadhyay and the Syracuse team found that the missing variable is the star's spin prior to its first encounter with the black hole .
If a star is already spinning rapidly before it ever meets the supermassive black hole, the additional torque provided by the black hole has a negligible effect. As reported by the research team, this lack of orbital shortening means the star does not return as frequently or with the same dynamics, resulting in flares that grow progressively dimmer as the star's mass is depleted.
The Hills Capture Process and the Loss of Binary Partners
The research also addresses how a star becomes trapped in such a tight, repeating orbit around a galactic center. Most stars exist in binary systems, and the Syracuse team points to a mechanism called "Hills capture" to explain these orbits. In this scenario, a binary pair passes near a supermassive black hole; the black hole's gravity rips the pair apart, ejecting one star at high velocity while capturing the other.
This violent separation is critical because if the original binary pair was orbiting each other closely, the captured star is left spinning rapidly. This high initial spin, combined with a tight orbit that may take only a few months to complete, creates the exact conditions necessary for the dimming rpTDEs observed by astronomers.
The Challenge of Achieving Month-Long Orbits Without Binary Ejection
The findings highlight a specific orbital anomaly: the extreme difficulty of binding a star so tightly to a supermassive black hole through standard gravitational capture. Team member Eric Coughlin noted that the loss of a binary partner via Hills capture is a compelling explanation because other known mechanisms struggle to produce orbits with such short periods.
However,some details remain unverified, as the study relies on theoretical modeling of stellar structures and orbital dynamics. It remains to be seen if observational data from a wider variety of galaxies will consistently show the same dimming patterns, or if different types of supermassive black holes produce varying results based on their own spin and mass.
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