Milky Way’s core black hole may have devoured another black hole.

Astronomers suggest that the giant black hole at the heart of the Milky Way may have experienced a collision with another black hole within the last 10 million years. This supermassive black hole, known as Sagittarius A*, is located around 26,000 light-years away in space and is approximately 4 million times more massive than the sun. The idea is that Sgr A* may have integrated a smaller black hole, potentially unlocking the long-held mystery surrounding a peculiar group of stars close to the galaxy’s center.

These intriguing S-stars orbit in wild, tilted, and elongated paths, starkly differing from other nearby stars that move in a neat, orderly fashion. The origins of the S-stars and the reasons for their chaotic orbits remain uncertain, especially since they are relatively young and likely experienced rapid dynamical events.

A team of researchers utilized computer simulations to retrace the lineage of past cosmic events. Their findings indicate that if a smaller black hole indeed merged with Sgr A*, the resultant momentum could have caused a recoil similar to that of a firing gun. This recoil might have disrupted the orbits of nearby stars without displacing the black hole from its central position. The results of this research have been shared in The Astrophysical Journal Letters.

An image of Sagittarius A*, a supermassive black hole at the Milky Way’s galactic center. Credit: Event Horizon Telescope (EHT) collaboration

Black holes remain some of the most enigmatic phenomena in the cosmos. About 50 years ago, they existed primarily as theoretical constructs — an imaginative solution to complex physics problems. Even leading astronomers were not entirely convinced of their existence.

Now, black holes are recognized as a profound aspect of science, with images captured by a network of massive, synchronized radio dishes on Earth. Humanity witnessed a clear view of Sgr A* for the first time in 2022. Unlike a planet or star, black holes lack surfaces; instead, they possess a boundary known as an “event horizon,” a point of no return. Should anything approach too closely, it would succumb to the hole’s gravitational pull. The most common type, a stellar black hole, typically forms from the remnants of a massive star that has died in a supernova explosion, collapsing into an incredibly dense region. The formation of supermassive black holes, however, remains more elusive. Astrophysicists theorize that these giants inhabit the centers of nearly all galaxies. NASA’s Hubble Space Telescope observations support the notion that they may form in the dusty cores of starburst galaxies, where new stars are created at a rapid pace, though this remains an area of ongoing research.

Scientists theorize that a massive black hole could have gravitated toward Sgr A*, bringing with it gas and stars, eventually leading to a merger. The resulting remnant would have experienced a recoil kick, potentially causing the unique tilted orbits of the S-star cluster over millions of years. Credit: Tatsuya Akiba et al. / doi:10.3847/2041-8213/addc5d diagram

The Milky Way is ancient, over 13 billion years old, and throughout its existence, it has likely absorbed many smaller galaxies, each possibly containing its own black holes. If one of these smaller black holes spiraled into Sgr A*, it would have released a wave of gravitational energy that shifted Sgr A* slightly off-center. In time, the uneven gravitational field could have influenced nearby stars, distorting their orbits and forming the S-star cluster we observe today.

To explore this hypothesis, researchers conducted simulations mimicking the interactions between stars and black holes. Their model depicted a smaller black hole—approximately 200,000 times the mass of the sun—falling into Sgr A*. After a simulated passage of 2 million years, it produced a group of stars with orbits remarkably resembling those of the S-stars. Though the model is not without its imperfections—the stars’ orbits were somewhat less extreme than the observed ones—future studies may refine it further. Researchers express that this model not only provides fresh perspectives on the history of the Milky Way’s central black hole but also illuminates the broader processes of galaxy evolution.