by An astronomical research group has discovered a black hole just 1,924 light-years from our solar system in the constellation Eagle. The object Gaia BH3 is the most massive black hole ever discovered in the Milky Way. It has 33 times the mass of the Sun.
It is the second closest black hole to Earth and would not have been noticed at all if it were not in a binary orbit with a companion star. This is far enough away from the black hole to not be swallowed up by its gravitational pull. However, it is close enough to be gravitationally bound to BH3.
Discovered by the Gaia space telescope
Despite its close proximity to Earth, BH3 poses absolutely no threat. The gravitational field of a black hole is no stronger than that of a star of the same mass. BH3 is the third dormant black hole discovered in Gaia space telescope data. In total, only 20 such objects were found.
Gaia is a space probe of the European space agency Esa that shares the Earth’s orbit around the Sun and has been in operation since 2013. It maps the three-dimensional positions and movements of the stars in the Milky Way with the highest precision to date. The longer she observes the stars, the more precise her measurements become.
Black holes large and small
Black holes can be roughly divided into different mass categories. There are supermassive black holes that can have millions to billions of times the mass of the Sun. They are mostly found in the centers of galaxies. How they arise is still unclear.
The smaller black holes with stellar masses are formed by the collapse of stellar cores when massive stars go supernova. They can be up to 65 times as massive as the Sun, but larger ones can form from mergers. There are said to be up to 100 million black holes with a stellar mass in the Milky Way. They are not easy to detect because they do not emit light.
When such a black hole diverts material from a passing star or a companion star, heat is generated. A flare occurs, allowing astronomers to detect these stellar objects.
What we know about the system
The objects in the discovered binary system are about 16 times as far apart as the Earth is from the Sun. They orbit each other in a cycle of 11.6 years. The black hole has a mass of about 32.7 solar masses.
The star, on the other hand, only has 76 percent of the mass of our sun. It is very poor in heavy elements. This is a sign of its great age, because these elements are only released into the universe when stars explode. Accordingly, the companion star must belong to the earlier generations of stars that were unable to absorb these elements when they were formed because they were hardly present.
The star also shows no signs of pollution from the material that the black hole progenitor must have ejected during its supernova. The two objects probably only became a binary pair after the black hole had already formed.
Stars with cores large enough to form a black hole nearly 33 times the mass of the Sun are difficult to explain. However, models suggest that this is possible if the massive progenitor star also has low metallicity.
To the study
The research results were published on March 30, 2024 in the journal Astronomy & Astrophysics: Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry.
