ESO / L. Calçada / Space Engine
Giant red star orbiting an inactive black hole in the Gaia BH2 system (artist’s concept)
Gaia BH2’s giant red companion tells a contradictory story that only makes sense when we consider stellar violence.
Study the starlight allows astronomers and astrophysicists to know, often in detail, its temperature, composition, age and evolutionary state.
However, the giant red companion of the Gaia BH2a black hole system, using data from ESA’s Gaia mission, tells a contradictory story that only makes sense when considering the stellar violence!
The star is full of heavy elements, called alpha elementschemical signatures typically found in ancient starsformed when the Universe was still young. Just based on this chemical composition, should be around ten billion years old.
However, when astronomers at the University of Hawaii measured the vibrations inside the star using NASA’s TESS satellite, they discovered that it has only about five billion years.
The discovery was presented in a recently published in The Astrophysical Journal.
“Young stars rich in alpha elements are quite rare and intriguing. The combination of youth and ancient chemistry suggests that this star did not evolve in isolation,” he says Daniel Heyastronomer at the University of Hawaii and lead author of the study.
The technique used is called asteroseismology and works similarly to seismology on Earth. Just as seismic waves reveal the internal structure of our planet, “stellar earthquakes” — oscillations that make the star’s light subtly wobble — show what’s going on below the surface.
These vibrations allowed the team measure core properties of the star with remarkable precision, explains .
A Star rotation offers another clue: Ground-based telescopes show that it completes a rotation every 398 days, much faster than would be expected for an isolated red giant of this age.
The stars slow down as they agelosing angular momentum. The study demonstrated that something caused this star to accelerate. The most plausible explanation is that this star merged with another or absorbed large amounts of matter when the black hole formed from its previous companion.
Any of these events would have injected extra mass — explaining the unusual composition — and increased angular momentum.
Gaia BH2 is what astronomers usually refer to as a dormant black hole. The black hole does not actively feed on its companion and, therefore, does not emit x-rays.
These systems were only recently identifiedthanks to precise measurements of the motion of stars carried out by the European Space Agency’s Gaia mission, which detect how the companion star wobbles slightly as it orbits the invisible black hole, revealing the presence of the massive object.
The team also analyzed the Gaia BH3another dormant black hole with an even stranger companion. Theory predicted clear oscillations, but none were detected, suggesting that current models for extremely metal-poor stars need to be revised.
Future observations with TESS will provide longer data sets that could confirm the merger hypothesis and reveal whether other companions of dormant black holes hide similarly violent pasts.
These silent systems, dispersed throughout the galaxy, may preserve traces of stellar collisionssignals that more active black holes would have erased long ago.
