Cartoon of the system's key evolutionary stages. Top left: a binary enters the supermassive black hole's Hill sphere and is disrupted. One star is captured on an eccentric orbit, the other ejected as a hyper-velocity star. Top right: the captured star's orbit shrinks and circularizes via gravitational wave emission. Bottom left: the sub-giant star begins stable mass transfer onto the supermassive black hole. Bottom right: after losing its hydrogen envelope, the compact core continues inspiraling via gravitational wave emission, eventually becoming a loud LISA-band source. Adopted from Olejak et al. 2025.
Imagine a star not crashing into a supermassive black hole in a fiery explosion, but instead slowly spiraling in, circling closer and closer to its horizon. This is the story of a sub-giant star that is stripped of its hydrogen layer by a black hole companion with a few million solar masses. The left-over helium core is gently drawn in due to strong gravitational wave emission and can be placed so close to the supermassive black hole that it becomes a promising gravitational wave source for the future detector LISA (Laser Interferometer Space Antenna). This scenario has been recently investigated by a team at MPA.
The story begins with two stars in a binary system that drift too close to a supermassive black hole. The black hole’s powerful gravity tears them apart through the so-called Hills mechanism (see Fig. 1): One star is flung out at incredible speed (a so-called hyper-velocity star), while the other star is captured to orbit the black hole on a highly eccentric orbit. If the separation of the captured star is in a certain regime, gravitational waves will lead to gradual circularization and decay of the orbit (see Fig.1). As a consequence, the star will finally start to transfer mass onto the supermassive black hole on a relatively circular orbit.
Imagine a star not crashing into a supermassive black hole in a fiery explosion, but instead slowly spiraling in, circling closer and closer to its horizon. This is the story of a sub-giant star that is stripped of its hydrogen layer by a black hole companion with a few million solar masses. The left-over helium core is gently drawn in due to strong gravitational wave emission and can be placed so close to the supermassive black hole that it becomes a promising gravitational wave source for the future detector LISA (Laser Interferometer Space Antenna). This scenario has been recently investigated by a team at MPA.
The story begins with two stars in a binary system that drift too close to a supermassive black hole. The black hole’s powerful gravity tears them apart through the so-called Hills mechanism (see Fig. 1): One star is flung out at incredible speed (a so-called hyper-velocity star), while the other star is captured to orbit the black hole on a highly eccentric orbit. If the separation of the captured star is in a certain regime, gravitational waves will lead to gradual circularization and decay of the orbit (see Fig.1). As a consequence, the star will finally start to transfer mass onto the supermassive black hole on a relatively circular orbit.
If the captured star is a so called sub-giant, relatively soon after its main sequence phase (i.e. the end of its core hydrogen burning), it has already developed a helium core. Such a star may lose its outer layers to the supermassive black hole companion and be stripped – slowly but steadily – down to its helium-rich core (Fig.1).
A Slow, Steady Spiral Inward
Unlike in the dramatic tidal disruption events often observed in galactic centers, where a star on a highly eccentric orbit might be ripped apart in one go, the mass transfer process investigated in this study happens over hundreds of thousands or millions of years. The star doesn’t disappear right away. Instead, it gradually loses mass, becoming a stripped helium core, and spirals inward.
Such a stripped core is compact enough that it can get very close to the supermassive black hole, at a separation comparable to the size of the black hole’s Schwarzschild radius. As the helium-core star slowly spirals in, it sends out a gravitational wave signal with gradually increasing frequency that space-based detectors like LISA are designed to pick up.
Moreover, every now and then, the core might light up again due to hydrogen reignition on the residual hydrogen-rich surface. Accompanying brief bursts of X-rays might be the visible sign of what’s happening – and a counterpart to the gravitational wave signal. If the spin of the supermassive black hole is sufficiently high, the final disruption of the helium core will happen near the so-called ‘innermost stable orbit’. This could be observable via both electromagnetic and gravitational wave emission, making it a very exciting multi-messenger transient.
These objects could be among the brightest gravitational wave sources in the Milky Way. Due to their loudness, they might also be detectable from large distances in the local Universe (see Fig. 2). In its several-year mission, LISA could detect dozens of them; hopefully even one right at the center of our own galaxy (with a chance of about 1%).
Illustration of a black hole stripping a star.
Credit: NASA/JPL-Caltech
A New Window into the Heart of Galaxies
Author:
Image of Dr. Aleksandra Olejak
Olejak, Aleksandra
Postdoc
tel:2231
aolejak@mpa-garching.mpg.de
Original publication
Aleksandra Olejak et al.
Supermassive Black Holes Stripping a Subgiant Star Down to Its Helium Core: A New Type of Multimessenger Source for LISA
2025 ApJL 987 L11
DOI
More Information
LISA
Website of the Laser Interferometer Space Antenna
Such a stripped core is compact enough that it can get very close to the supermassive black hole, at a separation comparable to the size of the black hole’s Schwarzschild radius. As the helium-core star slowly spirals in, it sends out a gravitational wave signal with gradually increasing frequency that space-based detectors like LISA are designed to pick up.
Moreover, every now and then, the core might light up again due to hydrogen reignition on the residual hydrogen-rich surface. Accompanying brief bursts of X-rays might be the visible sign of what’s happening – and a counterpart to the gravitational wave signal. If the spin of the supermassive black hole is sufficiently high, the final disruption of the helium core will happen near the so-called ‘innermost stable orbit’. This could be observable via both electromagnetic and gravitational wave emission, making it a very exciting multi-messenger transient.
These objects could be among the brightest gravitational wave sources in the Milky Way. Due to their loudness, they might also be detectable from large distances in the local Universe (see Fig. 2). In its several-year mission, LISA could detect dozens of them; hopefully even one right at the center of our own galaxy (with a chance of about 1%).
Credit: NASA/JPL-Caltech
A New Window into the Heart of Galaxies
The system described here is an example of a so-called ‘extreme mass ratio inspiral’ (due to the huge mass asymmetry between the star and the supermassive black hole). Such systems offer a unique opportunity to study the surroundings of supermassive black holes. Detecting one would not only shed light on how stars evolve in these exotic environments, but also on how they can feed black holes over extended timescales. Unlike typical interactions involving stellar-mass black holes, these systems may also produce short X-ray bursts from hydrogen flashes and end in a final tidal disruption.
This makes them promising candidates for multi-messenger astronomy, potentially linking gravitational wave signals with electromagnetic observations and offering a richer, more complete view of our universe.
This makes them promising candidates for multi-messenger astronomy, potentially linking gravitational wave signals with electromagnetic observations and offering a richer, more complete view of our universe.
Author:
Image of Dr. Aleksandra Olejak
Olejak, Aleksandra
Postdoc
tel:2231
aolejak@mpa-garching.mpg.de
Original publication
Aleksandra Olejak et al.
Supermassive Black Holes Stripping a Subgiant Star Down to Its Helium Core: A New Type of Multimessenger Source for LISA
2025 ApJL 987 L11
DOI
More Information
LISA
Website of the Laser Interferometer Space Antenna
