The Milky Way has a large newly discovered black hole, and it’s lurking close to Earth! This sleeping giant was discovered with the European space telescope Gaia, which tracks the movement of billions of stars in our Milky Way.
Stellar-mass black holes form when a large star runs out of fuel and collapses. The new discovery is a milestone and represents the first time a large black hole with such an origin has been found close to Earth.
The stellar-mass black hole, called 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 star in the constellation Cygnus (Cyg X-1), estimated to have a mass of 20 times that of the Sun. The average stellar-mass black hole in the Milky Way is about ten times more massive than the Sun.
Gaia-BH3 is located just 2,000 light-years from Earth, making it the second-closest black hole 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 about 9.6 times that of the Sun, making it significantly smaller than this newly discovered black hole.
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“Finding Gaia BH3 is like the moment in the movie ‘The Matrix’ when Neo begins to ‘see’ the matrix,” said George Seabrook, a scientist at University College London’s Mullard Space Science Laboratory and member of Gaia’s Black Hole Task Force, said in a statement sent to Space.com. “In our case, ‘the matrix’ is the population of dormant stellar black holes in our galaxy, which were hidden from us before Gaia discovered 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 Milky Way.
Of course, Gaia-BH3 is tiny 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* are not created by the death of massive stars, but rather by the merger of increasingly larger black holes.
The dormant giant black hole caused the stellar companion to make a wobbly sound
All black holes are marked by an outer boundary called the event horizon, at which point the black hole’s escape velocity exceeds the speed of light. This means that an event horizon is a one-way light-trapping surface beyond which no information can escape.
As a result, black holes do not emit or reflect light, meaning they can only be ‘seen’ when surrounded by material on which they gradually feed. Sometimes this means that a black hole in a binary star system attracts material from a companion star, which forms a disk of gas and dust around it.
The enormous gravitational influence of black holes generates intense tidal forces in the surrounding matter, causing it to glow brightly with material being destroyed and consumed, also emitting X-rays. Furthermore, the material that the black hole doesn’t feast on can be channeled toward the poles and shot out as near-light-speed jets, accompanied by the emission of light.
All these light emissions could allow astronomers to discover black holes. The question is: how can ‘dormant’ black holes that are not feeding on gas and dust around them be detected? For example, what happens if a stellar-mass black hole has a companion star, but the two are so far apart that the black hole cannot steal the stellar matter from its binary star partner?
In such cases, the black hole and its companion star orbit around a point that represents the center of mass of the system. This is also the case when a star is orbited by a light companion, such as another star or even a planet.
Rotating around the center of mass results in a fluctuation in the star’s motion, which is visible to astronomers. Because Gaia is adept at accurately measuring the movement of stars, it is the ideal instrument to see this fluctuation.
Gaia’s Black Hole Task Force looked for strange fluctuations that could not be explained by the presence of another star or a planet and that indicated a more massive companion, possibly a black hole.
While focusing on an ancient giant star in the constellation Aquila, located 1,926 light-years from Earth, the team discovered a wobble in the star’s path. This fluctuation suggests that the star is in orbital motion with a dormant black hole of exceptionally high mass. The two are separated by a distance that varies 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. “You only make discoveries like this once in your research life. So far, such large black holes have only been discovered in distant galaxies by the LIGO-Virgo-KAGRA collaboration, thanks to observations of gravitational waves.”
Related: What are gravitational waves?
Gaia’s sensitivity also allowed the Black Hole Task Force to place constraints on the mass of Gaia-BH3, determining it to have a mass of 33 solar masses.
“Gaia-BH3 is the very first black hole whose mass we were able to measure so accurately,” said Tsevi Mazeh, a scientist and member of the Gaia collaboration at Tel Aviv University. ‘With a mass 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 indisputable evidence that [stellar-mass] such massive black holes do exist.”
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However, the Gaia-BH3 system will undoubtedly be of great interest to scientists because of more than just its proximity to Earth and the black hole’s mass.
The star in this system is a subgiant star that is about five times the size of the Sun and fifteen times as bright, although it is cooler and less dense than our star. The companion star Gaia-BH3 is composed mainly of hydrogen and helium, the universe’s two lightest elements, but lacks 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 lose more mass over their lifetimes than their more metal-rich counterparts, so scientists have wondered whether they can retain enough mass to allow black holes to form. Gaia-BH3 represents the first indication that metal-poor stars can indeed do this.
“Gaia’s next data release is expected to contain much more, which should help us ‘see’ more of “the matrix” and understand how dormant stellar black holes form,” Seabroke concluded.
The team’s research was published today (April 16) in the journal Astronomy & Astrophysics.