by The Milky Way has a newly discovered super black hole, and it lies close to Earth! This sleeping giant was discovered by the European space telescope Gaia, which tracks the movement of billions of stars in our galaxy.
Stellar mass black holes are formed when a massive star runs out of fuel and collapses. The new discovery is a landmark, representing the first time a supermassive black hole with such a close-to-Earth origin has been found.
The stellar mass black hole, named Gaia-BH3, is 33 times more massive than our sun. The previous largest 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 more massive than 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 nearest black hole to Earth is Gaia-BH1 (also discovered by Gaia), which is 1,560 light-years away. The mass of Gaia-BH1 is about 9.6 times that 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,” George Seabrook, a scientist at the Mullard Space Science Laboratory at University College London and a member of the Gaia Black Hole Task Force, said in a statement sent to Space.com. “Our case, our galaxy’s population ‘matrix’ of dormant stellar black holes, was hidden from us before Gaia detected them.”
Seabroke added that Gaia BH3 is important for this population, as it is the largest stellar black hole found in our galaxy.
Of course, Gaia-BH3 is small fry compared to the supermassive black hole that dominates the Milky Way’s core, 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 death of massive stars, but by mergers of gradually larger and larger black holes.
Sleeping giant black hole caused companion star to throw wobbly
Every black hole is marked by an outer boundary called the exit 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 from 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 matter that they are gradually consuming. 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 massive gravitational influence of black holes creates intense tidal forces in the surrounding matter, causing it to glow brightly with material being destroyed and consumed, and also emit X-rays. In addition, the material not feasted on by the black hole can be directed to the poles and exploded as near-light speed jets, accompanied by light emission.
All of these light emissions can allow astronomers to see black holes. The question is, how can “dormant” black holes that are not feeding gas and dust around them be detected? For example, what if a black hole has a stellar-mass companion star, but both are too far apart for the black hole to remove stellar matter from its binary partner?
In such cases, 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.
When the center of mass is placed in orbit, the star’s motion is in orbit, which can be seen by astronomers. Because Gaia is adept at measuring the motion of stars precisely, it is the ideal instrument to observe this glow.
Gaia’s Black Hole Task Force looked for odd bits that could not be accounted for by the presence of another star or planet and that indicated a heavier companion, possibly a black hole.
Approaching an ancient giant star in the constellation Aquila, located 1,926 light-years from Earth, the team discovered a hint of a star’s path. That wobble suggests that the star is locked in orbital motion with a supermassive dormant black hole. The two are separated by a distance that is the distance between the sun and Neptune at its widest and our star and Jupiter at its 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 a lifetime of research. So far, black holes of this size have only been found 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 able to place constraints on the mass of Gaia-BH3, finding it to be 33 solar masses.
“Gaia-BH3 is the first black hole whose mass we could measure so accurately,” said Tsevi Mazeh, a scientist and member of the Gaia collaboration at Tel Aviv University. “At 30 times the mass of our sun, the object’s mass is typical of our estimates of the mass of distant black holes observed in gravitational wave experiments. Gaia measurements provide the first unequivocal proof [stellar-mass] there are black holes of this gravity.”
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However, the Gaia-BH3 system will certainly be of great interest to scientists due to its proximity to Earth and the mass of its black hole.
The star in this system is a subgiant star about five times the size of the sun and 15 times its brightness, although it is cooler and no less dense than our star. The companion star Gaia-BH3 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 form Gaia-BH3 lacked heavier elements. Metal-poor stars are expected to lose more mass than their metal-rich counterparts during their lifetime, so scientists have questioned whether they can retain enough mass to give birth to black holes. Gaia-BH3 shows the first hint that metal-poor stars can do so.
“The next Gaia data release is expected to contain much more, helping us to ‘see’ more of the ‘matrix’ and understand how dormant stellar black holes form,” Seabroke said.
The team’s research was published today (April 16) in the journal Astronomy & Astrophysics.
