We’ve all seen the drama in cosmic cinema—the black hole, a celestial predator, lurking silently until a star wanders too close. In a violent embrace, the star is torn apart in a spectacular tidal disruption event (TDE), its luminous death throes lighting up the galaxy. But what if the star doesn’t venture quite close enough to be completely destroyed? What if the encounter is more subtle—a slow and quiet stripping, rather than a dramatic feast?
In a recent and fascinating study submitted to the arXiv preprint server, astronomers have explored just such a scenario. The focus isn’t on a head-on collision with doom, but rather a long, intimate dance between a dying star and the monster at the heart of the Milky Way. The star, a subgiant just past its prime, becomes caught in the gravitational net of Sagittarius A* (Sag A*), the supermassive black hole anchoring our galaxy. What unfolds is not a quick death, but a cosmic slow burn that may become a powerful new tool for understanding black holes—and even the universe itself.
A Dying Star with One Last Story
The star in question is a subgiant, a transitional object in stellar evolution. Subgiants are stars that have used up the hydrogen fuel in their cores, leaving behind a dense ball of helium. Though not yet red giants, these stars have begun to swell in size, their outer envelopes bloating as the core contracts. Our Sun, too, will one day become a subgiant on its slow march toward becoming a white dwarf.
In the paper’s scenario, this subgiant is part of a binary system—two stars gravitationally bound in orbit around one another. The binary, through a cosmic stroke of bad luck, drifts too close to the galactic center where Sag A* lies hidden behind clouds of dust and gas. The immense gravitational field of the black hole disrupts the binary. One star is flung out of the system, possibly becoming a hypervelocity star, while the other—our subgiant—is ensnared into a tight, eccentric orbit around the black hole.
But the story doesn’t end with capture. It begins there.
The Black Hole’s Delicate Touch
You might expect a star this close to a black hole to be immediately torn apart in a TDE. But the reality is far more nuanced. Instead of being wholly devoured, the outer hydrogen-rich layers of the subgiant are gently stripped away over time, like layers of an onion being peeled back. This slow gravitational shredding removes the star’s envelope, leaving behind a dense, exposed helium core.
Unlike traditional TDEs, which emit a brilliant burst of electromagnetic radiation as the star is torn apart, this process is quiet—no fireworks, no blaze of glory. The gradual stripping doesn’t release a bright flare that can be seen across the cosmos. Instead, the core of the star simply keeps orbiting Sag A*, inching ever closer to its final doom in near-silence.
This process creates what astrophysicists call an “inspiral”—a slow, spiraling motion of the stellar core toward the black hole. And while it may be invisible in the traditional sense, it does create something else: gravitational waves.
Echoes in the Fabric of Spacetime
Gravitational waves are ripples in spacetime caused by the acceleration of massive objects. LIGO and Virgo, ground-based gravitational wave observatories, have detected such waves from colliding black holes and neutron stars. But these observatories are tuned for short-duration, high-frequency events. The slow inspiral of a helium core around a supermassive black hole creates long-duration, low-frequency gravitational waves—just the kind that a new space-based observatory called LISA (Laser Interferometer Space Antenna) will be able to detect.
Set to launch in the 2030s, LISA will orbit the Sun in formation with three spacecraft connected by laser beams, designed to detect faint whispers in the gravitational fabric of the universe. The stripped helium cores orbiting black holes—formerly quiet and invisible—may suddenly have a voice.
According to the study, LISA could detect dozens of such inspiraling helium stars, potentially even in galaxies a billion light-years away. These stars, now ghost-like remnants of their former selves, would announce their presence through spacetime itself. Incredibly, the researchers estimate that even during its initial 4-year mission, LISA could detect at least a few such inspirals. There’s even a 1% chance that one might be in our own Milky Way.
Helium Flashes and Multi-Messenger Fireworks
While gravitational waves are the primary expected signal, the study also opens the door to something even more tantalizing—multi-messenger astronomy. If the stripped helium cores undergo helium flashes—brief but intense nuclear burning episodes—they may emit X-rays as they orbit Sag A*. This opens the possibility of combining gravitational wave data with electromagnetic observations, offering a richer, more complete picture of the phenomenon.
Such a discovery would be revolutionary. Multi-messenger astronomy has already transformed our understanding of the universe, as seen in the famous neutron star merger of 2017 that produced both gravitational waves and gamma rays. These quiet, inspiraling helium cores could be the next chapter in that unfolding story.
Peering into the Shadows of Black Holes
The significance of this work goes beyond just detecting a few exotic stellar remnants. It points to a new regime of black hole astrophysics—a way to study the quiet, long-term effects of gravity in its most extreme form. Most black hole observations involve dramatic events: explosive collisions, energetic jets, luminous accretion disks. But this is a study of subtlety, of long-term gravitational influence. It’s a reminder that the universe often whispers as much as it shouts.
Moreover, such detections could inform our understanding of the population of stars near supermassive black holes, the nature of stellar evolution in extreme environments, and the mechanisms of stellar disruption. These stripped stars may even play a role in the growth of black holes themselves—drip-feeding matter over millennia, quietly fueling the monsters at galactic centers.
The Elegant Dance of Death
This research reframes our understanding of stellar encounters with black holes. It’s not always about violent endings and cosmic mayhem. Sometimes, it’s about celestial erosion—a gentle but inevitable dance toward death, choreographed by gravity. A star that avoids immediate destruction can still meet a slow, gravitationally orchestrated end, whispering its fate in gravitational waves across the cosmos.
With LISA on the horizon, we’re on the cusp of hearing these whispers for the first time. The stripped stars spiraling toward black holes may become one of the next great frontiers in astronomy—telling us not just how stars die, but how black holes grow, how galaxies evolve, and how the universe itself sings in waves we are only just beginning to hear.
Reference: Aleksandra Olejak et al, Supermassive black holes stripping a subgiant star down to its helium core: a new type of multi-messenger source for LISA, arXiv (2025). DOI: 10.48550/arxiv.2503.21995