Astronomers Discover Heaviest Stellar Black Hole in the Milky Way

Depiction of the binary system, in which a star orbits the unusually large stellar black hole. - Image: ESO
Depiction of the binary system, in which a star orbits the unusually large stellar black hole. - Image: ESO

The supermassive black hole at the center of our galaxy is the undisputed heavyweight champion of the Milky Way, but a newly spotted object takes the crown for the most massive stellar black hole known in our galaxy, weighing in at an impressive 33 times the mass of our Sun.

A team led by Pasquale Panuzzo, an astronomer at the Observatoire de Paris, has uncovered the most massive stellar black hole ever detected in the Milky Way. Gaia BH3 dwarfs the previous record holder, Cygnus X-1, which weighs just 21 solar masses. The findings are detailed in a paper released today in the journal Astronomy and Astrophysics.

BH3 is now the heaviest of the three largest known black holes in the Milky Way. - Image: ESO
BH3 is now the heaviest of the three largest known black holes in the Milky Way. - Image: ESO

Gaia BH3 is in the constellation Aquila, roughly 2,000 light-years from Earth. The team discovered it during a review of data from the European Space Agency’s Gaia mission, a space-based observatory that has been operational since 2013. Gaia’s ongoing mission is to construct the most detailed three-dimensional map of our galaxy. The star orbiting BH3 was already known to astronomers, but its status as the companion of a black hole came as a complete surprise, and the resulting weight even more so.

“When I saw the results for the first time, I was convinced there was a problem in the data. I could not believe it,” Panuzzo told Gizmodo. “Now, I feel I’ve really done the discovery of my life!”

The discovery was backed by a suite of ground-based observatories and sophisticated instruments, including the Ultraviolet and Visual Echelle Spectrograph (UVES) on the European Southern Observatory’s Very Large Telescope in Chile, the HERMES spectrograph at the Mercator Telescope in Spain, and the SOPHIE high-precision spectrograph in France.

The astronomers used Gaia’s precise measurements to determine the size of the orbit and the time it takes for the star to circle around the black hole. They then applied Kepler’s laws, which are principles that describe the motions of planets and stars, to calculate the black hole’s mass from the orbit’s size and period. They employed two methods: astrometric measurements, which track the slight wobbling movements of the companion star as it appears to shift positions in the sky, and spectroscopy, which uses the Doppler effect to measure the speed at which the star is moving toward or away from us.

Stellar black holes are remnants of massive stars that collapsed under their own gravity, typically forming black holes about 10 times the mass of our Sun. Gaia BH3's significant mass suggests it originated from a metal-poor star, which retained more mass over its lifetime and could thus form a larger black hole upon its death, according to the new research.

By contrast, supermassive black hole Sagittarius A*, parked at the galactic core, is vastly larger, with about 4 million times the mass of the Sun. These behemoths do not form from the collapse of a single star but likely grow from the merger of smaller black holes and the accumulation of gas and stellar material over millions of years.

The stellar black hole “formed by the gravitational collapse of a massive star—a star probably 40 to 50 times more massive than our Sun—at the end of its life,” Panuzzo explained. “These kinds of stars have a short life, a few million years, compared to the 10 billion years of the Sun, and they end their life with a supernova, leaving behind a black hole. This is why we call them ‘stellar’ black holes, to not confuse them with the supermassive black holes at the center of the galaxies.”

Panuzzo said it’s “quite probable” that even larger stellar black holes exist in our galaxy. Previously, the LIGO-Virgo-KAGRA gravitational telescopes detected the merging of black holes of more than 80 solar masses in distant galaxies. Indeed, heavy stellar black holes have been detected before, but in other galaxies and using alternative methods of detection. These faraway black holes are identified through gravitational wave astronomy, which observes the ripples in spacetime caused by the mergers of stellar black holes. I asked Panuzzo why we’ve been able to find huge stellar black holes in galaxies far, far away, but only recently spotted one in our own galaxy.

“There are two reasons,” he said. “The first is that the LIGO-Virgo-KAGRA gravitational telescopes are able to detect black hole mergers very far away, probing billions of galaxies. The second one is that these black holes are produced by massive stars that have a low metallicity,” that is, stars composed almost exclusively of hydrogen and helium, with only traces of the other elements. “These stars were present in our galaxy only in its infancy, so we cannot see the formation of new massive black holes in our galaxy anymore,” according to Panuzzo.

The data used in the study were initially intended for the next Gaia data release, expected by the end of 2025. Due to the significance of the discovery, however, the team opted to publish the findings early. “This discovery has a lot of implications for the stellar evolution models and the gravitational waves field,” Panuzzo explained. “It was considered that this exceptional discovery could not be kept hidden to the community for two years waiting for the next release.” What’s more, by disclosing it now, the scientific community can perform follow-up observations earlier, he added.

To that end, future observations with the GRAVITY instrument on the ESO’s Very Large Telescope Interferometer will aim to determine if this black hole is pulling in matter from its surroundings, offering deeper insights into its nature and behavior.

More: Ripples in Spacetime Reveal Mystery Object Colliding With a Star’s Corpse.

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