The Milky Way Galaxy contains a newly discovered massive black hole lurking near Earth! This sleeping giant was discovered by the European Gaia space telescope, which tracks the movement of billions of stars in our galaxy.
Stellar-mass black holes are created when a massive star runs out of fuel and collapses. The new discovery is a milestone, as it represents the first time that a large black hole of this origin has been found near 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 the X-ray binary in the constellation Cygnus (Cyg X-1), which has an estimated mass of about 20 times that of the Sun. The average stellar black hole in the Milky Way is about 10 times heavier 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 located 1,560 light-years away. Gaia-BH1 has a mass of about 9.6 times the mass of the Sun, making it much smaller than this newly discovered black hole.
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“Finding Gaia BH3 is like the moment in the movie ‘The Matrix’ where Neo starts to ‘see’ the matrix,” said George Seabrook, a scientist at the Mullard Space Science Laboratory at University College London and a member of Gaia’s Black Hole Task Force. he said in a statement sent to Space.com. “In our case, the ‘matrix’ is the collection of dormant stellar black holes in our galaxy, which were hidden from us before Gaia discovered them.”
Seabrook added that Gaia BH3 is an important clue to this group, because it is the most massive stellar black hole found in our galaxy.
Of course, Gaia-BH3 is small compared to the supermassive black hole that dominates the heart of the Milky Way, Sagittarius A* (Sgr A*), which has a mass of 4.2 million times that of the Sun. Supermassive black holes like Sgr A* are created not by the death of massive stars, but by the merger of progressively larger and larger black holes.
A sleeping giant black hole has caused a stellar companion to throw up a wobble
All black holes have an outer boundary called an event horizon, at which point the escape velocity of a black hole exceeds the speed of light. This means that the event horizon is a unidirectional 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 that gradually feeds on them. Sometimes, this means a black hole in a binary system pulling material from a companion star, forming a disk of gas and dust around it.
The massive gravitational influence of black holes generates intense tidal forces in this surrounding matter, causing it to glow brightly along with the material being destroyed and consumed, and to emit X-rays. In addition, matter that is not fed by the black hole can be directed to its poles and then released in the form of jets with a speed close to that of light, which are accompanied by the emission of light.
All these light emissions could allow astronomers to observe black holes. The question is: How can “dormant” black holes that do not feed on the gas and dust surrounding them be discovered? For example, what if a stellar-mass black hole has a companion star, but the two are too widely separated for the black hole to grab stellar matter from its binary partner?
In such cases, the black hole and its companion star orbit around a point representing the center of mass of the system. This is also the case when it orbits a light companion star, such as another star or even a planet.
The rotation around the center of mass results in an oscillation in the movement of the star, which is what astronomers see. Because Gaia is skilled at accurately measuring the movement of stars, it is the perfect instrument to see this wobble.
Gaia's Black Hole Task Force set out to search for strange oscillations that cannot be explained by the presence of another star or planet, and which point to a heavier companion, perhaps a black hole.
By focusing on an ancient giant star in the constellation Aquila, located 1,926 light-years from Earth, the team found a wobble in the star's path. This oscillation indicates that the star is locked into an orbital motion with an inert black hole of exceptionally high mass. They are separated by a distance ranging between the distance between the Sun and Neptune at their widest and between our star and Jupiter at their closest.
“It is a real rhinoceros,” lead researcher Pascual Panozzo of the National Center for Scientific Research at the Paris Observatory in France said in a statement. “This is the kind of discovery you make once in your research life. Until now, black holes of this size have only been discovered in distant galaxies through the LIGO-Virgo-KAGRA collaboration, thanks to gravitational wave observations.”
Related: What are gravitational waves?
Thanks to Gaia's sensitivity, the Black Hole Task Force was also able to put constraints on the mass of Gaia-BH3, finding that it has 33 solar masses.
“Gaia-BH3 is the first black hole whose mass we can measure with great precision,” said Tzivi Mazi, a scientist and member of the Gaia collaboration at Tel Aviv University. “The object's mass is 30 times greater than the mass of our Sun, which is typical of the estimates we have of the mass of very distant black holes observed through gravitational wave experiments. The Gaia measurements provide the first indisputable evidence of this [stellar-mass] Black holes this massive do exist.”
However, the Gaia-BH3 system should 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 about five times the size of the Sun and 15 times as bright, although it is cooler and less dense than our star. Companion star Gaia-BH3 is composed primarily of hydrogen and helium, the two lightest elements in the universe, and 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 form Gaia-BH3 also lacked heavier elements. Metal-poor stars are expected to lose more mass than their metal-rich counterparts during their lives, so scientists wonder whether they can maintain enough mass to give birth to black holes. Gaia-BH3 represents the first indication that metal-poor stars can actually do this.
“The next Gaia data release is expected to contain even more data, which will help us see more of the array and understand how dormant stellar black holes form,” Seabrook concluded.
The team's research was published today (April 16) in the journal Astronomy and astrophysics.
