UH astronomers decode a star’s secret past

Astronomers from the University of Hawaiʻi Institute for Astronomy (IfA) have uncovered the turbulent past of a distant red giant by listening to its celestial “song.” Subtle variations in the star’s brightness suggest that it potentially 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.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), IfA astronomers detected faint “starquakes” rippling through the companion star of Gaia BH2, a black hole system first identified by the European Space Agency’s Gaia mission in 2023. 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 remarkable precision. The team’s findings were recently published in Astronomical Journal. 

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

Age-defying star 

The biggest surprise came from the star’s makeup. It’s considered “alpha-rich”, which means 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’s 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,” explained Hey. “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.”

Faster than expected 

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 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 Gaia BH3, another black hole system with an even more unusual companion star. Although models predicted that 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.

Peering deeper ahead

Future TESS observations 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.

---

---