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by Spektrum der Wissenschaft
Astrophysics: This star is too young to be old
3.12.2025
Translation: machine translated
Things are pretty violent in the Gaia BH2 star system: it involves a black hole, a red giant and an unknown third party that has given the star astonishing properties.
3800 light years away from us, a red giant orbits the black hole Gaia BH2: it is the third closest binary star system to our Earth that astronomers know of - it was only discovered in 2023. However, a closer examination of the red giant produced astonishing data, as reported by astronomers led by Daniel Hey from the University of Hawaii in Honolulu. This is because the star is actually too young for the amount of heavy elements it contains. Given the detected concentrations, it should be ten billion years old, but it has actually only existed for five billion years.
These so-called alpha elements normally dominate the chemical signature of stars that were formed in the early days of the universe. NASA's Transiting Exoplanet Survey Satellite (TESS) space telescope, however, has recorded data that suggests the star is much younger: The original oscillations recorded by TESS provide signals that allow conclusions to be drawn about the internal structure of the red giant - similar to the way seismic waves do for the interior of the Earth. By modelling these measured variables, the astronomers determined an actual age of five billion years, which actually contradicts its chemical signature.
In addition, the red giant rotates significantly faster than other, isolated red giants of its age: it turns on its axis in 398 days. As stars lose rotational speed over the course of their lives, an external process must have driven the star. Scientists suspect that the red giant either merged with another star or absorbed large amounts of matter when its former stellar companion formed today's black hole. Both events would give it additional mass, which could explain both its unusual chemism and the increased angular momentum.
Gaia BH2 is considered a quiescent black hole, a relatively new class of gravitational centres: it does not suck matter from its companion and therefore does not emit X-rays. As a result, this type of black hole was undetectable for a long time. Only precise motion measurements by the ESA mission «Gaia» made their discovery possible: the companion star reveals the hidden black hole through slight deviations in its orbit.
Hey and his team had also investigated Gaia BH3, another dormant black hole with an even more unusual companion. Theoretically, even more pronounced oscillations should have been detected in this system, but they were completely absent. The researchers conclude that current models of extremely metal-poor stars may therefore need to be revised. They hope that future, larger data sets from TESS will help to clarify or reformulate existing hypotheses on stellar mergers.
Spectrum of Science
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Original article on Spektrum.de
Header image: ESO/L. Calçada/Space Engine (created with AI); press image for Hey, D. et al: Asteroseismology of the Red Giant Companions to Gaia BH2 and BH3. The Astronomical Journal 170, 2025
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