Achi news desk-
The Milky Way has a new super black hole, and it’s lurking 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 milestone, representing the first time a supermassive 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 an X-ray binary black hole in the constellation Cygnus (Cyg X-1), estimated to be about 20 times the mass of the sun. The average stellar mass 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 1,560 light years away. Gaia-BH1 has a mass of about 9.6 times that of the sun, making it much smaller than this newly discovered black hole.
Related: New view of supermassive black hole at center of Milky Way hints at exciting hidden feature (image)
“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 the Black Hole Task Force Gaia, He 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 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 galaxy.
Of course, Gaia-BH3 is 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 such as Sgr A* are not created by the deaths of supermassive stars but rather by mergers of increasingly larger black holes.
A large black hole’s sleep caused a stellar companion to throw a wobbly motion
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. That means that the 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 that they can only be “seen” when they are surrounded by material that they are steadily feeding on. Sometimes, this involves a black hole in a binary system removing material from a companion star, which forms a disk of gas and dust around it.
The immense gravitational influence of black holes produces intense tidal forces in this surrounding matter, causing it to glow brightly with material being destroyed and consumed, also emitting X-rays. In addition, the material that the black hole does not feast on can be channeled to its poles and blown out as near-light speed jets, which are accompanied by light emission.
All these light emissions can allow astronomers to see black holes. The question is, how can “dormant” black holes that do not feed on gas and dust around them be detected? 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 capture stellar material 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.
The rotation of the center of mass results in a wobble in the star’s motion, which is visible to astronomers. As Gaia is adept at measuring the motion of stars precisely, it is the ideal instrument to observe this wobble.
The Gaia Black Hole Task Force set out to search for odd wobbles that could not be accounted for by the presence of another star or planet and that indicated a heavier companion, possibly a black hole.
Stopping 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 extremely 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. “This is the kind of discovery you make once in your research life. To date, black holes this large have only ever been detected in distant galaxies by the LIGO-Virgo-KAGRA collaboration, thanks to observations of gravitational waves.”
Related: What are gravitational waves?
Thanks to Gaia’s sensitivity, the Black Hole Task Force was also able to constrain the mass of Gaia-BH3, finding it to be 33 solar masses.
“Gaia-BH3 is the very first black hole for which we could measure the mass so accurately,” said Tsevi Mazeh, scientist and Gaia collaborator at Tel Aviv University. “At 30 times the mass of our sun, the object’s mass is typical of the estimates we have for the masses of very distant black holes observed by gravitational wave experiments. The Gaia measurements provide that first indisputable proof [stellar-mass] black holes this massive exist.”
However, the Gaia-BH3 system is sure 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 which is about five times the size of the sun and 15 times brighter, although it is cooler and less dense than our star. Gaia-BH3’s companion star is composed mostly of hydrogen and helium, the two lightest elements in the universe, with no 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 was also lacking in heavier elements. Metal-poor stars are expected to eject more mass than their more metal-rich counterparts during their lives, so scientists have questioned whether they can sustain enough mass to give birth to black holes. Gaia-BH3 represents the first hint that metal-poor stars can do so.
“Gaia’s next data release is expected to contain much more, which should help us ‘see’ more of the ‘matrix’ and to understand how dormant stellar black holes form,” concluded Seabroke.
The team’s research was published today (April 16) in the journal Astronomy & Astrophysics.
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