Astronomers have analyzed vibrations in a red giant star near a dormant black hole, uncovering evidence of a past stellar merger that defies expectations. The star, part of the Gaia BH2 system, appears ancient in chemistry but young in structure, spinning unusually fast. This discovery challenges models of stellar evolution in quiet black hole binaries.
Astronomers at the University of Hawaiʻi Institute for Astronomy have examined a red giant star orbiting a dormant black hole in the Gaia BH2 system, first identified in 2023 by the European Space Agency's Gaia mission. Using data from NASA's Transiting Exoplanet Survey Satellite (TESS), the team detected subtle "starquakes"—stellar oscillations—that revealed the star's internal properties, much like earthquakes probe Earth's core.
These vibrations indicate the star is about 5 billion years old, yet its alpha-rich chemical composition, rich in heavy elements, suggests it should be far older. "Young, alpha-rich stars are quite rare and puzzling," said lead author Daniel Hey, a research scientist at the institute. "The combination of youth and ancient chemistry suggests this star didn't evolve in isolation. It likely acquired extra mass from a companion, either through a merger or by absorbing material when the black hole formed."
Further observations from ground-based telescopes show the star rotates once every 398 days, faster than expected for a solitary red giant of its age. Co-author Joel Ong, a NASA Hubble Fellow at the institute, noted: "If this rotation is real, it can't be explained by the star's birth spin alone. The star must have been spun up through tidal interactions with its companion, which further supports the idea that this system has a complex history."
The study also looked at Gaia BH3, another dormant black hole system, where the companion star showed no expected oscillations despite low metal content, prompting revisions to current theories. Dormant black holes like these do not emit X-rays, making them hard to detect; astronomers rely on tracking stellar motions. The findings, published in the Astronomical Journal, highlight how such systems reshape understanding of black holes in the Milky Way. Future TESS observations aim to confirm the merger hypothesis and detail these binaries' development.