The coming and going of a star in the constellation of the Eagle has revealed the existence of the most massive stellar black hole ever found in the Milky Way. It is also the second closest to Earth. The body, called Gaia BH3, has 33 times the mass of the Sun, and is located just 2,000 light years away, a breeze in astronomical terms, although it does not represent any danger to the solar system.
The discovery, published this Tuesday by Astronomy magazine
“No one expected to find a high-mass black hole lurking nearby, undetected until now,” explains in a press release Pasquale Panuzzo, an astronomer at the Paris Observatory, who has led the research. “It's the kind of discovery you make once in your research life,” he celebrates.
Stellar black holes are the corpses of very massive stars. They are formed when the star collapses, once it runs out of fuel, and normally have a mass around 10 times that of the Sun. The heaviest detected so far in the Milky Way, Cygnus X-1, barely weighs twice as much, Therefore, the 33 solar masses of Gaia BH3 make it a totally exceptional object.
Still, the newly discovered black hole is not the most massive in our galaxy. This title is held by Sagittarius A*, located in the center of the Milky Way, with a mass of four million suns. However, Gaia BH3 is the most massive of all those that originated after the collapse of a star. That is, he is the greatest among his equals.
The discovery of the astronomical object occurred thanks to the wobbly movements of a neighboring star. Black holes are invisible to our eyes, so we can only detect them indirectly if they are accompanied by a star. By analyzing its movements, astrophysicists can calculate the characteristics of the object that causes them, and deduce that there is a black hole around it.
The slight swing caught the attention of the researchers while they were validating a series of data that they will present in 2025 in the fourth installment of the Gaia project catalog. A closer look, and the help of ground-based telescopes located in the Atacama Desert (Chile), La Palma (in the Canary Islands) and French Haute Provence, revealed the existence and exorbitant characteristics of Gaia BH3.
“We took the exceptional step of publishing this article based on preliminary data before the next Gaia delivery given the unique nature of the discovery,” notes fellow Paris Observatory astronomer Elisabetta Caffau. The idea is that this early announcement opens the door for other research groups to study this exotic object (black holes larger than 30 solar masses are rare and little known) and surprisingly close to us.
It is precisely its proximity to the Earth that makes it especially relevant to the field. “The star that accompanies it is easily observable from most telescopes in ground observatories,” Panuzzo explains to La Vanguardia. This will make it easier for many groups to analyze it and we can better understand how these massive objects are formed.
Astrophysicists theorize that these heavy stellar black holes are the remains of enormous stars made almost exclusively of hydrogen and helium, which are extremely metal-poor. Demonstrating it beyond the theoretical framework has been impossible until now, due to the remoteness of the bodies that had been discovered and the little information we had about them.
The new discovery confirms the hypothesis. The star that accompanies the black hole is very poor in metals, and this suggests that the one that originated it was also poor, because two stars that travel together usually have a similar composition. That is, looking at the star is almost like opening a window to the black hole's past.
The discovery “could indicate that there are many other objects around us that we are not able to detect in these preliminary Gaia data,” says the researcher, “but it could also be that we have been very lucky.” After all, “we can only detect a very small fraction of stellar black holes: those that have a star orbiting around them,” he concludes.
