Record breaker! Milky Way's most monstrous stellar-mass black hole is sleeping giant lurking close to Earth

The Milky Way has a big newfound black hole, and it lurks close to Earth! This sleeping giant was discovered with the European space telescope Gaia, which tracks the motion of billions of stars in our galaxy.

Stellar-mass black holes are created when a large star runs out of fuel and collapses. The new discovery is a landmark, representing the first time that a big black hole with such an origin has been found close to Earth.

The stellar-mass black hole, designated Gaia-BH3, is 33 times more massive than our  sun. The previous most massive black hole of this class found in the Milky Way was a black hole in an X-ray binary in the Cygnus constellation (Cyg X-1), whose mass is estimated to be around 20 times that of the sun. The average stellar-mass black hole in the Milky Way is about 10 times heftier than the sun.

Gaia-BH3 is located just 2,000 light years from Earth, making it the second-closest black hole to our planet ever discovered. The closest black hole to Earth is Gaia-BH1 (also discovered by Gaia), which is 1,560 light-years away. Gaia-BH1 has a mass around 9.6 times that of the sun, making it considerably smaller than this newly discovered black hole.

“Finding Gaia BH3 is like the moment in the film ‘The Matrix’ where Neo starts to ‘see’ the matrix,” George Seabrook, a scientist at Mullard Space Science Laboratory at University College London and a member of Gaia’s Black Hole Task Force, said in a statement sent to Space.com. “In our case, ‘the matrix’ is our galaxy’s population of dormant stellar black holes, which were hidden from us before Gaia detected them.”

Seabroke added that Gaia BH3 is an important clue to this population, because it is the most massive stellar black hole found in our galaxy.

Of course, Gaia-BH3 is a small fry compared to the supermassive black hole that dominates the heart of the Milky Way, Sagittarius A* (Sgr A*), which has a mass 4.2 million times that of the sun. Supermassive black holes like Sgr A* aren’t created by the deaths of massive stars but rather by mergers of progressively larger and larger black holes.

A diagram showing the location of the three black holes discovered by Gaia (Image credit: ESA/Gaia Collaboration)

Sleeping giant black hole caused stellar companion to throw a wobbly

All black holes are marked by an outer boundary called an event horizon, at which point the black hole’s escape velocity exceeds the speed of light. That means an event horizon is a one-way light-trapping surface beyond which no information can escape.

As a result, black holes don’t emit or reflect light, meaning they can only be “seen” when they are surrounded by material that they gradually feed on. Sometimes, this means a black hole in a binary system pulling material from a companion star, which forms a disk of gas and dust around it.

The tremendous gravitational influence of black holes generates intense tidal forces in this surrounding matter, causing it to glow brightly with material that is destroyed and consumed, also emitting X-rays. Additionally, the material the black hole doesn’t feast on can be channeled to its poles and blasted out as near-light speed jets, which are accompanied by the emission of light.

All of these light emissions can allow astronomers to spot black holes. The question is, how can “dormant” black holes that aren’t feeding on gas and dust around them be detected? For instance, what if a stellar-mass black hole has a companion star, but the two are too widely separated for the black hole to snatch stellar matter from its binary partner?

In cases like this, the black hole and its companion star orbit a point that represents the system’s center of mass. This is also the case when a star is orbited by a light companion, such as another star or even a planet.

Orbiting the center of mass results in a wobble in the motion of the star, which is visible to astronomers. Because Gaia is adept at precisely measuring the motion of stars, it is the ideal instrument to see this wobble.

Gaia’s Black Hole Task Force set about looking for odd wobbles that couldn’t be accounted for by the presence of another star or a planet and that indicated a heavier companion, possibly a black hole.

The region around the black hole Gaia-BH3. (Image credit: ESO/Digitized Sky Survey 2. Acknowledgement: D. De Martin.)

Homing in on an old giant star in the constellation Aquila, located 1,926 light-years from Earth, the team found a wobble in the star’s path. That wobble suggests that the star is locked in orbital motion with a dormant black hole of exceptionally high mass. The two are separated by a distance that ranges from the distance between the sun and Neptune at their widest and our star and Jupiter at their closest.

“It’s a real unicorn,” lead researcher Pasquale Panuzzo of CNRS, Observatoire de Paris in France, said in a statement. “This is the kind of discovery you make once in your research life. So far, black holes this big have only ever been detected in distant galaxies by the LIGO-Virgo-KAGRA collaboration, thanks to observations of gravitational waves.”

Three stellar-mass black holes in our galaxy: (left) Gaia BH1, (middle) Cygnus X-1, and (right) Gaia BH3, whose masses are 10, 21, and 33 times that of the sun, respectively. Gaia BH3 is the most massive stellar black hole found to date in the Milky Way. (Image credit: ESO/M. Kornmesser)

Thanks to the sensitivity of Gaia, the Black Hole Task Force was also able to put constraints on the mass of Gaia-BH3, finding it to possess 33 solar masses.

“Gaia-BH3 is the very first black hole for which we could measure the mass so accurately,” said Tsevi Mazeh, a scientist and Gaia collaboration member at Tel Aviv University. “At 30 times that of our sun, the object’s mass is typical of the estimates we have for the masses of the very distant black holes observed by gravitational wave experiments. Gaia’s measurements provide the first undisputable proof that [stellar-mass] black holes this heavy do exist.”

However, the Gaia-BH3 system is bound to be of great interest to scientists for more than just its proximity to Earth and the mass of its black hole.

The star in this system is a sub-giant star that is around five times as large as the sun with 15 times its brightness, though it is cooler and less dense than our star. The Gaia-BH3 companion star is mainly composed of hydrogen and helium, the universe’s two lightest elements, lacking heavier elements, which astronomers (somewhat confusingly) call “metals.”

The fact that this star is “metal-poor” suggests that the star that collapsed and died to create Gaia-BH3 also lacked heavier elements. Metal-poor stars are expected to shed more mass than their more metal-rich counterparts during their lives, so scientists have questioned if they can maintain enough mass to birth black holes. Gaia-BH3 represents the first hint that metal-poor stars can indeed do so.

“Gaia’s next data release is expected to contain many more, which should help us to ‘see’ more of ‘the matrix’ and to understand how dormant stellar black holes form,” Seabroke concluded.

The team’s research was published today (April 16) in the journal Astronomy & Astrophysics.