Astronomers at the University of Hawaiʻi at Mānoa’s Institute for Astronomy (IfA) have pieced together a cosmic puzzle, revealing the dramatic past of a distant red giant star by analyzing the faint rhythmic pulses hidden in its light.
Every star has a celestial “song” whose rhythm is evidenced by changes in its brightness. Astronomers believe that long ago, the star that orbits a dormant black hole in the Gaia BH2 system once collided and merged with another star, an event that left it spinning far faster than expected.
The Gaia BH2 system was first discovered by the European Space Agency’s Gaia mission in 2023 through measurements of stellar motion. Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers found evidence of faint “starquakes” within Gaia BH2’s companion star. Similar to seismic waves, oscillations ripple through a star, revealing details about its internal density, rotation, and structure to researchers.
“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, in a statement. “These vibrations told us something unexpected about this star’s history.”
A Star and Its Past
One thing that surprised the team in their observations was the star’s chemical makeup. The red giant is “alpha-rich,” meaning it has higher levels of heavier elements typically produced by earlier generations of stars. Based on this, the star’s chemistry would seem to suggest an older age—yet the star’s oscillations reveal it is only about 5 billion years old, making it far too young to have such an ancient chemical signature under normal conditions.
“Young, alpha-rich stars are quite rare and puzzling,” said Hey in a statement. “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.”
Ground-based observations added another twist to the story of the star’s intriguing origins. The red giant rotates once every 398 days, a rate that is much faster than an isolated star of its age and type.
“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. “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 decided to take a look at Gaia BH3, another dormant black hole system with an even stranger partner star. When reviewing the models, they predicted they would find strong oscillations, but TESS data revealed none. The result was baffling to the researchers, and leaves open questions about extremely metal-poor stars that they hope to revisit in future studies.
Both Gaia BH2 and BH3 house dormant black holes and are not actively feeding their companion stars, nor do they emit any X-rays. Through the subtle wobbles of their orbiting stars, astronomers continue to unravel new details about the quiet black holes lurking throughout the Milky Way.
The team’s research was published in The Astrophysical Journal.
Chrissy Newton is a PR professional and the founder of VOCAB Communications. She currently appears on The Discovery Channel and Max and hosts the Rebelliously Curious podcast, which can be found on YouTube and on all audio podcast streaming platforms. Follow her on X: @ChrissyNewton, Instagram: @BeingChrissyNewton, and chrissynewton.com.