Stellar vibrations offer rare glimpse into red giant’s core, history

Astronomers from the University of Hawaiʻi Institute for Astronomy have uncovered the turbulent past of a distant red giant by listening to its celestial “song.”

Subtle variations in the star’s brightness suggest it once collided and merged with another star, an explosive event that left it spinning rapidly. It now orbits a quiet black hole in the Gaia BH2 system, identified by the European Space Agency’s Gaia mission in 2023.

Using data from NASA’s Transiting Exoplanet Survey Satellite, astronomers detected faint “starquakes” rippling through the companion star of Gaia BH2. Much like seismic waves reveal Earth’s inner layers, these stellar vibrations gave scientists a rare glimpse beneath the star’s surface, allowing them to measure its core properties with precision.

“Just like seismologists use earthquakes to study Earth’s interior, we can use stellar oscillations to understand what’s happening inside distant stars,” said research scientist Daniel Hey, lead author of the study. “These vibrations told us something unexpected about this star’s history.”

The most surprising discovery came from the star’s makeup. It’s considered “alpha-rich,” meaning it is packed with heavier elements usually found in much older stars, suggesting it should be ancient. However, when scientists studied its vibrations, they discovered it is actually only about 5 billion years old, too young to have formed with those chemical traits.

“Young, alpha-rich stars are quite rare and puzzling,” Hey said. “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.”

The mystery deepens with long-term observations from ground-based telescopes showing the star rotates once every 398 days, much faster than expected for an isolated red giant of its age.

“If this rotation is real, it can’t be explained by the star’s birth spin alone,” said co-author Joel Ong, a NASA Hubble Fellow at the Institute for Astronomy. “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 Gaia BH3, another black hole system with an even more unusual companion star. Although models predicted this star should show clear oscillations, none were detected, hinting that current theories about extremely metal-poor stars may need updating.

Both Gaia BH2 and BH3 are dormant black hole systems, meaning they aren’t feeding on their companion stars and therefore emit no X-rays. Their discovery through precise measurements of stellar motion is reshaping how astronomers understand black holes in our galaxy.

Future observations by the Transiting Exoplanet Survey Satellite of Gaia BH2 will give scientists a closer look at its stellar vibrations and may confirm whether it formed through a past merger, helping unravel how these quiet black hole pairs came to be.

Listen to audio of the star’s vibrations along with sound bites from scientists here.