Astronomers detect Milky Way's second-largest known black hole

An artist's impression shows the orbits of the most massive stellar black hole in our galaxy, dubbed Gaia BH3, and a companion star, in this handout image obtained by Reuters on April 16, 2024. European Southern Observatory/L. Calcada/Handout via REUTERS

In a significant astronomical breakthrough, scientists have unearthed a black hole boasting a mass approximately 33 times greater than that of our sun. This newly discovered black hole stands as the largest known in the Milky Way, excluding the supermassive black hole ensconced at the heart of our galaxy.

Situated a mere 2,000 light-years from Earth—an astronomically proximate distance—the colossal black hole resides within the constellation Aquila. Notably, it is accompanied by a companion star in orbit, as detailed by researchers on Tuesday. A light-year, measuring the distance light travels in a year, spans a staggering 5.9 trillion miles (9.5 trillion km).

Black holes, characterized by their incredible density and gravitational pull, pose a formidable challenge to detection due to their ability to trap even light. However, the discovery of this particular black hole was made possible through observations conducted as part of the European Space Agency's Gaia mission. Gaia's mission involves compiling a comprehensive stellar census, and the black hole's presence was discerned due to its gravitational effect on its companion star, causing it to exhibit a noticeable wobbling motion.

To confirm the mass of the black hole, data from the European Southern Observatory's advanced Very Large Telescope in Chile, along with observations from other ground-based observatories, were utilized. This multi-pronged approach allowed astronomers to verify the extraordinary mass of the newly identified black hole.

"This black hole is not only incredibly massive, but it also exhibits several peculiar characteristics that defy our expectations," remarked Pasquale Panuzzo, a research engineer at the French research agency CNRS based at the Observatoire de Paris and the lead author of the study published in the journal Astronomy & Astrophysics.

One notable peculiarity is the unique trajectory of the black hole, named Gaia BH3, and its companion within the galaxy. Unlike the typical orbit of stars in the Milky Way, Gaia BH3 and its companion are traversing in the opposite direction—a phenomenon that adds to the enigmatic nature of this celestial discovery.

The researchers suggest that Gaia BH3 likely originated following the demise of a star with a mass exceeding 40 times that of our sun. Black holes stemming from the collapse of a single massive star are classified as stellar black holes. Gaia BH3 stands out as the largest-known stellar black hole, as highlighted by astronomer and study co-author Tsevi Mazeh from Tel Aviv University in Israel.

However, it's worth noting that stellar black holes pale in comparison to their supermassive counterparts residing at the cores of most galaxies. One such colossal entity, known as Sagittarius A* or Sgr A*, occupies the heart of the Milky Way. Sporting a mass 4 million times that of our sun, Sgr A* is situated approximately 26,000 light-years away from Earth.

The progenitor star of Gaia BH3 was predominantly composed of hydrogen and helium, mirroring the chemical composition characteristic of stars in the early universe—a state referred to as low metallicity. This particular star likely formed relatively soon after the universe's inception, possibly around 2 billion years following the Big Bang event.

Upon reaching the end of its life cycle, the star underwent a cataclysmic explosion known as a supernova, dispersing some of its material into the surrounding space while the remaining core violently collapsed, giving rise to a black hole.

Pasquale Panuzzo, the lead author of the study, suggests that the discovery of Gaia BH3 lends support to stellar evolution models. These models indicate that massive stellar black holes can only be produced by low-metallicity stars like the progenitor of Gaia BH3. This finding sheds light on the intricate interplay between stellar composition and the formation of black holes, further enriching our understanding of the cosmos.

Gaia BH3's companion star, sharing a similar age to its predecessor, boasts approximately 76% of the mass of our sun and exhibits slightly cooler temperatures. However, it shines about 10 times brighter, adding to its luminosity. The companion star follows an elliptical orbit around the black hole, ranging from a distance of approximately 4.5 astronomical units (AU)—with one AU being the average distance between Earth and the sun—to 29 AU. To put this into perspective, Jupiter orbits at around five AU from the sun, while Neptune's orbit extends to approximately 30 AU.

Elisabetta Caffau, an astronomer from the Observatoire de Paris and a co-author of the study, expressed surprise at the findings regarding the companion star's chemical composition. Despite its proximity to the supernova explosion that birthed Gaia BH3, the companion star's chemical makeup remains largely unaltered—an unexpected revelation that adds intrigue to our understanding of stellar dynamics and interactions within binary systems.

Scientists are not sure just how big stellar black holes can be.

"The maximum mass for a stellar black hole is a matter of active scientific debate," Panuzzo said.

Source: Reuters