Analyzing the spectrum of stars can usually determine their age, composition and what stage of life they are in. But the companion of the recently discovered black hole Gaia BH2 was an exception: its characteristics turned out to be very contradictory.

The chemical composition of the red giant is rich in so-called alpha elements – a sign of very old stars formed in the early stages of the history of the Universe. Based on this parameter alone, its age should reach 10 billion years. However, data from NASA’s TESS satellite, which recorded vibrations inside the star using asteroseismology, showed otherwise: the giant is about 5 billion years old. This is reported by Universe Today.

“The combination of ‘young’ age and ancient chemistry is highly unusual and indicates that the star did not evolve alone,” said lead author Daniel Hay of the study, published in Astrophysical Journal.

Asteroseismology is a method similar to terrestrial seismology: vibrations that cause the faint flickering of a star make it possible to determine the parameters of its core.

The measurements also showed another deviation: the giant rotates around its axis in 398 days. This is much faster than a single red giant of this age should rotate. Typically, such stars slow down significantly, losing angular momentum.

The best explanation is a merger of two stars or an active exchange of mass with a star, from which a black hole later formed. In both cases, the giant would have received additional matter and “promotion,” which explains its young core and “old chemistry.”

In the Gaia BH2 system, the so-called “sleeping” black hole is an object that does not accrete matter and therefore does not emit X-rays. These began to be discovered only recently thanks to data from the Gaia mission: companion stars reveal their presence by small “swings” in their motion.

Scientists also studied the Gaia BH3 system, where there is another dormant black hole with an even stranger companion. The theory predicted obvious oscillations, but none were detected, suggesting that current models of very old stars need to be revised.

Future observations will help determine whether similar “young old men” occur among other quiet black holes. These systems, scattered across the Galaxy, may contain traces of ancient stellar collisions that active black holes would have long since erased.