TESS Hears a Red Giant 'Sing' to Its Black Hole — Starquakes Reveal a Past Stellar Merger

Using NASA's Transiting Exoplanet Survey Satellite (TESS), astronomers have detected rhythmic stellar oscillations — "starquakes" — in a rapidly rotating red giant that orbits a black hole in the binary system Gaia BH2. Located about 3,800 light-years away in the constellation Centaurus and first identified by ESA's Gaia mission in 2023, Gaia BH2 gives scientists a rare window into a quiet black-hole binary.

TESS measured tiny pulsations that travel through the star's interior; by analysing their frequencies, researchers probed the giant's inner structure much like seismologists use earthquakes to image Earth's interior. The oscillation data show the red giant is "alpha-rich" — enriched in elements heavier than helium, a chemical signature typically associated with older stars — yet seismic analysis indicates an age of roughly 5 billion years. That chemical-versus-seismic mismatch is unexpected.

"Just like seismologists use earthquakes to study Earth's interior, we can use stellar oscillations to understand what's happening inside distant stars," said Daniel Hey of the University of Hawaii Institute for Astronomy (IfA). "These vibrations told us something unexpected about this star's history."

Ground-based spectroscopy provides an additional clue: the red giant completes one rotation every 398 Earth days, considerably faster than typical isolated red giants of similar seismic age. The combination of unusual chemistry and faster-than-expected spin suggests the star likely gained mass in its past — either by merging with another star or by accreting material when its companion exploded and left behind the black hole.

"If this rotation is real, it can't be explained by the star's birth spin alone," said Joel Ong, a NASA Hubble Fellow at IfA. "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 team also examined a second Gaia-discovered system, Gaia BH3, about 2,000 light-years away. Its companion star appears metal-poor — low in elements heavier than hydrogen and helium — and, unusually, shows no clear oscillations despite expectations that metal-poor stars should display them. Ongoing observations with Gaia, TESS and ground-based instruments will refine the seismic measurements and test the merger and mass-transfer scenarios.

Studying these quiet, non-accreting black-hole binaries helps astronomers understand how black holes and companion stars interact when the black hole is not actively feeding. The team's findings were published on Nov. 13 in The Astronomical Journal.