As we all understand, black holes have the ability to swallow stars, tearing them apart and assimilating their remains. However, this occurs solely when a star comes dangerously near to a black hole. Now consider what might happen if a star approaches closely enough to another celestial body to undergo intense gravitational stretching without being instantly consumed. This situation has been examined further. recent paper on the arXiv preprint server.
The research focused on a dying, 2-solar-mass star referred to as a subgiant. Such stars are nearing the conclusion of their existence. The majority of the hydrogen within their cores has converted into helium; however, they have not yet expanded into red giants. Before our Sun transforms into a red giant and ultimately becomes a white dwarf, it too will evolve into a subgiant star.
In their scenario, the star begins as part of a binary system located at the Milky Way’s core. As this binary pair approaches near enough to the supermassive black hole known as Sagittarius A* (Sag A*), one star—the subgiant—is ensnared into a tight orbit around the black hole, whereas its partner manages to break free. Gradually, the subgiant's orbital path shrinks until it finds itself perilously close to Sag A*. It is from here that events become particularly intriguing.
Since the external parts of the subgiant expand slightly, these regions become susceptible to capture by the black hole initially. In essence, the black hole has the ability to strip away the exterior sections of the star, revealing a compact helium center. The stripped-down stellar remnant keeps spiraling nearer to the black hole until eventually getting absorbed.
The researchers find this model intriguing due to the significant observational puzzle it presents. In contrast to a tidal disruption event where a black hole tears apart a star, the disintegration of a star would not result in a noticeable flare that we could observe. This certainly holds true for Sagittarius A* (Sag A*), which is enveloped by gas and dust. However, with the compact helium core remaining near the black hole, its final orbits might be discernible via gravitational waves. These waves are currently too faint for detectors like LIGO and Virgo. Nonetheless, a future space-based gravitational wave detector such as LISA may have the capability to pick up these signals.
The researchers crunched the data to determine what can realistically be achieved. Can LISA indeed identify a merging helium core star, and how probable is such detection? Their analysis revealed that these stars may fall well within LISA’s observational capabilities and could emit gravitational waves strong enough for us to potentially spot them circling supermassive black holes outside our Milky Way—even from distances up to one billion light-years away.
Experts believe that during its first four years of operation, LISA could identify gravitational waves from at least a few such stars, with approximately a 1% probability of spotting one inside our Milky Way Galaxy. Should the star exhibit high activity levels and undergo helium flashes, the resulting X-rays may also be detectable, facilitating combined electromagnetic and gravitational wave studies.
More information: Aleksandra Olejak et al., Supermassive Black Holes Stripping a Subgiant Star Down to Its Helium Core: A New Kind of Multi-Messenger Source for LISA arXiv (2025). DOI: 10.48550/arxiv.2503.21995
Provided by Universe Today
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