NASA exoplanet-hunting spacecraft hears a red giant star ‘singing’ to its partner black hole

Using NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have listened to the cosmic song a red giant star croons to its black hole companion. The song, related to the starquake studied by TESS, revealed the turbulent history of this red giant, hinting that it once collided and merged with another star.

The rapidly spinning red giant star, a phase stellar bodies around the size of the sun enter when they exhaust their fuel for nuclear fusion, dwells in the binary system Gaia BH2. Located around 3,800 light-years away in the constellation Centaurus, this system was discovered by the European Space Agency (ESA) star mapping spacecraft Gaia in 2023.

The team of astronomers used TESS to observe starquakes rippling through the red giant of Gaia BH2. Just as seismic waves can be used to probe the interior of Earth, starquakes can be employed by astrophysicists to investigate below the surface layers of stellar bodies, right down to their cores.

“Just like seismologists use earthquakes to study Earth’s interior, we can use stellar oscillations to understand what’s happening inside distant stars,” team leader Daniel Hey, a scientist at the University of Hawaii Institute for Astronomy (IfA), said in a statement. “These vibrations told us something unexpected about this star’s history.”

Red giant star is younger than it looks

Hey and colleagues were surprised when they took a close look at the composition of this red giant star, finding it to be packed with heavy elements, or “alpha-rich.” This is something common to more ancient stars. However, the starquakes observed by TESS suggest that the star is only 5 billion years old.

For context, our 4.6 billion-year-old star, the sun, will only reach its red giant phase when it runs out of hydrogen in its core in around 5 billion years.

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

This isn’t the only puzzle surrounding this red giant, either.

Observing the red giant using ground-based instruments, scientists found that it is rotating once every 398 Earth days, which is much faster than isolated red giants of similar ages normally spin. That rapid spin hints at the fact that it may have merged with another star in its distant history, or that it once interacted with the massive star that died to birth its black hole companion.

“If this rotation is real, it can’t be explained by the star’s birth spin alone,” team member Joel Ong, a NASA Hubble Fellow at IfA, said in the statement. “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 same team also investigated another black hole system discovered by Gaia, designated Gaia BH3, located just 2,000 light-years from Earth. Gaia BH3 has a companion star that may be even more strange than the red giant of Gaia BH2.

This star is light in elements heavier than hydrogen and helium, which astronomers call “metals.” Usually, metal-poor stars show clear oscillations, but they are absent in the metal-poor star of Gaia BH3.

The stellar motion of both Gaia BH2 and Gaia BH3 could reveal more about binary systems with black holes that are not actively feeding on their stellar companions.

The team will continue to observe Gaia BH2 with Gaia, hoping to catch a better look at its starquakes, which could help confirm a past merger with another star.

The team’s research was published on Nov. 13 in The Astronomical Journal.