Imagine a normal star, much like our Sun, trapped in a tight orbit around a supermassive black hole millions of times more massive than the Sun. Every orbit brings the star closer to the black hole. As it moves through this extreme environment, it begins to lose material to the black hole—not in one explosive event, but slowly over millions of years.
This unusual process could produce gravitational waves that future space observatories such as the Laser Interferometer Space Antenna (LISA) may be able to detect.
A recent study by Sandomirsky and collaborators examines what happens when a main-sequence star gets extremely close to a supermassive black hole (SMBH) and begins transferring its mass. Their calculations reveal that the star does not simply disappear. Instead, it passes through four distinct stages, changing its size, structure and gravitational-wave signal along the way.
A Star Living Dangerously Close to a Black Hole
Most galaxies are believed to contain a supermassive black hole at their center. These enormous objects can have masses millions or even billions of times greater than the Sun. They are surrounded by dense populations of stars, creating an environment where unusual gravitational interactions can occur.
One well-known phenomenon is a tidal disruption event (TDE). If a star passes too close to a black hole, the black hole's enormous tidal forces can tear the star apart, stretching it into streams of gas.
But not every close encounter has to end so violently.
A star moving on an almost circular orbit around an SMBH can gradually lose material without being completely destroyed. As the star approaches the black hole, it can fill a region called its Roche lobe. Once the star becomes larger than this gravitational boundary, gas can flow toward the black hole through a point between the two objects.
This process is known as Roche-lobe overflow (RLOF).
Instead of a dramatic explosion lasting hours or days, the mass transfer can continue for millions of years.
Gravitational Waves Slowly Change the Orbit
A star orbiting very close to an SMBH emits gravitational waves. These waves carry energy and angular momentum away from the system, causing the orbit to gradually evolve.
For a compact object such as a stellar-mass black hole, this process generally causes the object to spiral inward. As the orbit becomes smaller, the gravitational-wave frequency increases.
A normal star behaves differently once mass transfer begins.
As the star loses mass, the orbital dynamics change. Because the donor star is much lighter than the SMBH, transferring mass can cause the orbit to expand rather than shrink as quickly.
This has an important consequence: the system's gravitational-wave frequency evolves more slowly.
The researchers therefore needed to understand not only how the orbit changes, but also how the star itself changes as it continuously loses mass.
Using a Computer Model to Follow the Star
Sandomirsky and the team used the MESA stellar evolution code to model stars with initial masses between about 0.5 and 10 times the mass of the Sun.
They placed these stars in extremely close orbits around a supermassive black hole with a mass of about one million Suns and studied how gravitational-wave emission and mass transfer affected their evolution.
The researchers identified four major evolutionary phases.
Stage I: The Star Remains a Normal Main-Sequence Star
The first stage occurs for stars that initially have masses above roughly 2 solar masses.
Even though the star is losing material to the black hole, it can still maintain approximate thermal equilibrium. As its mass decreases, the star also gradually becomes smaller.
This stage continues until the star reaches a mass of around 2 Suns.
At this point, the balance between the star's internal thermal evolution and the rapidly changing orbit begins to break down.
Stage II: The Star Rapidly Shrinks
Once the stellar mass falls below approximately 2 solar masses, something dramatic happens.
The gravitational-wave inspiral timescale becomes shorter than the star's thermal adjustment timescale. In simple terms, the star's orbit is changing too quickly for the star to continuously rearrange its internal structure and maintain normal thermal equilibrium.
The star therefore begins evolving adiabatically, meaning its structure changes faster than heat can redistribute through it.
Interestingly, the star initially shrinks rapidly.
Why?
The outer layers of the star have relatively high entropy and occupy a large fraction of its volume. When mass transfer strips away these extended outer layers, much of the star's volume disappears. Meanwhile, its dense central core remains comparatively compact.
The result is a rapidly contracting star.
This phase is particularly important for gravitational-wave detection because the star's rapid structural evolution strongly affects the orbital evolution.
Stage III: The Hidden Core Emerges
Eventually, the star becomes much lighter—typically around 0.5 to 1 solar mass, depending on its original mass.
By this point, most of the extended envelope has been removed.
What remains is a much more uniform, low-entropy core.
Now the star's behavior reverses.
Instead of continuing to shrink, the exposed core begins to expand adiabatically as more mass is removed.
This transition is one of the most interesting parts of the evolution. A star that had been rapidly contracting suddenly begins expanding because its internal structure has fundamentally changed.
The researchers found that the gravitational-wave signal is strongest around the transition between the rapid contraction and expansion phases.
Stage IV: Cooling and Shrinking Together
The final stage occurs when the star's mass falls to roughly 0.15–0.4 solar masses.
At this point, two important timescales—the thermal timescale and gravitational-wave timescale—become comparable.
The star can now adjust thermally while continuing to lose mass.
It begins to cool and contract, maintaining a balance between its thermal evolution and the changing orbit.
This represents a distinct evolutionary regime that previous simplified models did not fully capture.
The four stages therefore show that stellar mass transfer is far more complicated than simply removing material from a star. The star's internal structure changes dramatically as its envelope disappears.
How Does This Affect Gravitational Waves?
The most important result is that mass transfer slows the orbital evolution.
For a compact object spiraling into a supermassive black hole, the orbit can shrink steadily, causing the gravitational-wave frequency and characteristic strain to rise.
A mass-transferring normal star behaves differently.
Because the orbit tends to expand as the star loses mass, the inward evolution caused by gravitational-wave emission is partly counteracted. The system therefore evolves more slowly and produces gravitational waves at relatively lower frequencies.
This makes the signal more difficult to detect.
However, the slow evolution also means that these systems can spend a very long time near the frequency range where LISA could potentially observe them.
For a system involving a black hole similar to Sagittarius A*—the supermassive black hole at the center of the Milky Way—the researchers found that the strongest gravitational-wave signal can occur during the transition between the rapid contraction and expansion stages, with a maximum signal-to-noise ratio potentially reaching around 600 under the modeled conditions.
Despite this high peak, the overall detectability of such systems in the Milky Way is expected to be poor because the number of stars in such extremely short-period orbits is probably very small.
Why LISA Could Still Find These Strange Systems
The Laser Interferometer Space Antenna (LISA) is designed to detect low-frequency gravitational waves that cannot easily be observed by current ground-based detectors.
Normal stars near SMBHs could become an unusual class of LISA sources. They are also potentially more numerous than stellar-mass black holes, which are the traditional objects considered in extreme mass-ratio inspirals (EMRIs).
The study suggests that although mass-transferring stars are difficult targets in our own Galactic center, similar systems in other galaxies could be more promising. LISA may potentially detect such sources from distances approaching about 1 billion parsecs (1 Gpc) under favorable conditions.
Their gravitational waves could provide a new way to study both stars and the environments surrounding supermassive black holes.
A Slow Death That Becomes a Cosmic Signal
The remarkable part of this scenario is its timescale. A traditional tidal disruption event can destroy a star relatively quickly, while a star undergoing stable mass transfer can feed a black hole for millions of years.
During that long process, the star changes from a normal main-sequence object into a stripped, evolving remnant. Its radius can first decrease, then increase, and finally decrease again as its internal structure transforms.
At the same time, its mass loss changes the orbit and slows the evolution of its gravitational-wave signal.
The study shows that these systems are not simply miniature versions of black-hole extreme mass-ratio inspirals. Their evolving stellar structure is an essential part of the physics.
If LISA detects such a signal in the future, scientists could potentially use it as a window into an extraordinary environment: a normal star slowly being stripped apart while orbiting one of the most powerful gravitational objects in the Universe.
Reference: Andrey Sandomirsky, Re'em Sari, Aleksandra Olejak, Selma E. de Mink, "Evolution of Main Sequence Stars Transferring Mass to a Supermassive Black Hole", Arxiv, 2026. https://arxiv.org/abs/2609.09447
Comments
Post a Comment