🌟 Enigmatic: This Star, Paired with a Black Hole, is Both Young and Old

A peculiar celestial couple has been identified about 3,800 light-years away, in the constellation Centaurus. Designated Gaia BH2, this system pairs a rapidly spinning red giant star with an invisible companion: a black hole. The pair was first detected in 2023 by the European Space Agency's Gaia satellite, which specializes in stellar mapping. To delve deeper into this discovery, a team turned to NASA's TESS satellite, originally designed for hunting exoplanets.

The primary tool of this investigation was the study of starquakes, oscillations that agitate the surface of stars. Much like seismologists probing Earth's interior using seismic waves, astrophysicists use these vibrations to probe the internal layers of stars (see the explanation at the end of the article). The team leader, Daniel Hey of the Institute for Astronomy at the University of Hawaii, indicated that these stellar oscillations allow us to understand the internal structure of distant stars.


The analysis of the red giant's composition yielded a first surprise. The star appears enriched in heavy elements, a typical characteristic of very ancient stars. However, its age deduced from its pulsations is only about 5 billion years. For comparison, our Sun, aged 4.6 billion years, will not enter its red giant phase for another 5 billion years. This combination of youth and ancient chemistry is unusual and poses an enigma (more details below).

Another particularity puzzles researchers: the star's rotation speed. Ground-based observations showed that it completes a full rotation on its axis in 398 Earth days, a rate much faster than that of solitary red giants of the same age. This rapid motion cannot be explained by the star's initial rotation speed at its birth. It could be the result of past interactions, such as a merger with another star or a period of matter exchange with the massive star that spawned the black hole.

The study also examined another system, Gaia BH3, located only 2,000 light-years away. The companion star of this black hole is poor in heavy elements, but contrary to expectations, it does not exhibit the stellar oscillations usually observed in this type of star. The joint observation of motions in Gaia BH2 and Gaia BH3 could provide new insights into the dynamics of binary systems hosting silent black holes.

Scientists plan to continue monitoring Gaia BH2 with the Gaia satellite. The goal is to capture its starquakes more precisely, which could provide definitive confirmation of a past stellar merger. This work opens a new window into the tumultuous history of some stars paired with compact objects.

Stellar Seismology, a Window into the Heart of Stars

Starquakes, or stellar oscillations, are slight periodic vibrations that agitate the surface of stars. They are caused by pressure and gravity waves propagating inside the star, much like sound travels through air. By measuring the frequency and amplitude of these oscillations, astronomers can deduce internal properties such as density, temperature, and chemical composition.

This technique, called asteroseismology, works on a principle similar to terrestrial seismology. Different vibration modes react to structural variations inside the star. By analyzing these signatures, it becomes possible to determine a star's age, size, and even its evolutionary stage—information impossible to obtain by simply observing its light.

For studying systems like Gaia BH2, asteroseismology is particularly valuable. It allows probing the red giant despite the disruptive presence of a black hole companion. Data collected by TESS offer a direct glimpse into processes that have affected the star during its lifetime, including violent events like mergers.

This method has profoundly transformed our understanding of stellar structure and finds applications for many types of stars, from red dwarfs to giants. It perfectly complements other observation techniques and remains an important tool for illuminating the evolution of exotic binary systems.

Young Stars Rich in Heavy Elements, a Cosmic Puzzle

In the standard model of stellar evolution, the amount of heavy elements, which astronomers call 'metals', in a star is generally linked to its age.

A young star but rich in metals, like the red giant in Gaia BH2, therefore constitutes an anomaly. Its chemistry suggests an ancient star, while its pulsations indicate relative youth. This discrepancy strongly suggests that the star did not follow a solitary and calm evolution.

The most likely solution involves mass transfer or a merger with another star. The star may have absorbed a large amount of matter rich in heavy elements from a companion, mixing their compositions.