UNAB Astrophysics Institute Scientist Participates in World Astronomical Discovery

The Gaia Collaboration, a group of scientists from the European Space Agency’s Gaia mission, recently discovered the most massive black hole in the Milky Way, equivalent to about 33 solar masses.

Until now, black holes of this size had only been detected in distant galaxies by the LIGO-Virgo-KAGRA collaboration, thanks to the observation of gravitational waves. This is the first time that a black hole of stellar origin and of this size has been detected in our galaxy, and by observation, not by gravitational waves. The incredible discovery, whose lead author is Pasquale Panuzzo of the National Center for Scientific Research, Paris Observatory, France, was published in the Astronomy & Astrophysics journal.

The Gaia mission brings together hundreds of scientists studying the motions of more than 1.5 billion stars to perform a multidimensional mapping of our Milky Way galaxy. For observation and data collection, they have been using a satellite orbiting in space since 2013, following the Earth’s trajectory around the Sun. Since then, the satellite has generated an enormous amount of data and discoveries, which are regularly made available to the scientific community in «open science» mode.

Among the more than 450 scientists that make up the Gaia Collaboration, the only representative from Chile is Laurent Chemin, an academic and researcher at the Astrophysics Institute of the Universidad Andrés Bello, who explains how the discovery was made.

«The Gaia Data Processing and Analysis Consortium (Gaia DPAC) is the collaboration that does Gaia data processing. During the validation phase, in the framework of releasing new data for the world astronomical community for the year 2026, DPAC observed an old giant star in the constellation of Aquila, almost 2,000 light-years away. It had a very particular motion and velocity, which do not correspond to those of a single star», assures the academic of the School of Exact Sciences of the UNAB, and adds:

In fact, they realized that this star orbits around an invisible object, i.e., an object not seen in images that does not emit light. With a mass almost 33 times the mass of the Sun, this invisible object can only be an inactive stellar-type black hole, i.e., a black hole originating from a star.

The average mass of the black holes of stellar origin known so far is about ten times the mass of the Sun. The record was held by a black hole in an X-ray binary system in the Cygnus (Cyg X-1) constellation, whose mass is about 20 times that of the Sun.

Named BH3, this is the third inactive black hole detected by Gaia. «The first two, BH1 and BH2, correspond to black holes with a mass about nine times that of the Sun, so they are much smaller than BH3, which is why the discovery with Gaia data is exceptional. It opens the door to many more such discoveries in our galaxy, because the Gaia sample is 1.5 billion Milky Way stars,» he adds.

Deciphering a Galactic Mystery

The high quality of the Gaia data allowed scientists to determine the mass of BH3 with unprecedented precision. «Gaia has unparalleled angular precision: we could see a 100-peso coin from Earth on the surface of the Moon from 385,000 km away,» explained Laurent Chemin.

The data also provided the most direct evidence to date that black holes with this mass exist in our galaxy.

«Some black holes with such a high mass had already been observed, but in distant galaxies and only with telescopes that measure gravitational waves, very different from Gaia, which observes the movements of stars and electromagnetic waves,» says the academic from the UNAB Astrophysics Institute.

Astronomers now face the task of explaining the origin of a black hole the size of Gaia BH3, with a mass 33 times the mass of the Sun. Current knowledge of how massive stars evolve and die does not explain how these black holes originated, but the companion star orbiting BH3 may provide a clue.

It is an old giant star that formed in the first two billion years after the Big Bang, when our galaxy began to form. Our galaxy orbits BH3 at a distance about 16 times that between the Earth and the Sun.

«A fascinating point is that we managed to measure the chemical composition of the companion star. It has approximately 160 times less metals than the Sun. That implies that the stellar progenitor of the black hole must have had very few metals as well,» Chemin explains, adding that metals are the elements heavier than hydrogen and helium.

«This result is in perfect agreement with some models developed after the discovery of stellar black holes with gravitational waves and which indicate that this type of black hole has as its origin the gravitational collapse of very massive stars with heavy metal deficiency.»

According to Laurent Chemin, the discovery of Gaia BH3 opens up a whole new field of research. It reaffirms that the work of the Gaia collaboration has much to contribute to deciphering the mysteries of our Milky Way.