- Astronomers have located the closest stellar-mass black hole to Earth ever discovered.
- Because this black hole doesn’t feed on matter or emit radiation, making it virtually invisible, scientists had to take a different approach to finding it.
- A star orbiting the black hole exhibited a “wobble,” which helped them locate the black hole-star binary system Gaia BH1.
What would you get if you took the solar system and put a black hole where the sun is and put the sun where the earth is? The answer is something like the newly discovered black hole-star binary system Gaia BH1, which contains the closest stellar-mass black hole to Earth ever discovered and a star orbiting at an incredible 223,000 miles per hour.
“That’s because while the black hole is about ten times the mass of the sun, the star that orbits it is very similar to the sun and has an orbital period of about six months,” said Kareem El-Badry, astrophysicist at the Center. of astrophysics at the Harvard/Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy, tells Popular mechanics. This similarity to the solar system extends to the separation between the black hole and its companion star, which is about one and a half times the separation between Earth and the sun.
New research on this first unambiguous detection of a stellar-mass black hole in the Milky Way – which, notably, does not feed on matter and does not emit radiation – has been published in the journal Royal Astronomical Society Monthly Notices in November. Astronomers describe these non-feeding black holes as dormant, and they’re hard to spot because any light that gets too close to them is trapped behind a light-capturing boundary called the event horizon, rendering them invisible.
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This means that although it is estimated that there are millions of stellar-mass black holes, with masses between three and ten times that of the Sun, the few that have been detected so far have nibbled material from a companion star. When this stellar material falls towards a black hole, it is superheated. This causes it to emit powerful radiation that can be used to detect the feeding black hole. So how did scientists manage to find a black hole with no appetite?
Detect an oscillation
To discover this dormant black hole in the absence of such emissions, El-Badry and his team used the effect the black hole has on the star that orbits it to infer its presence.
Sifting through data from the Gaia Space Telescope, which has been creating a 3D map of the Milky Way since 2013, astronomers saw that Gaia’s star BH1 exhibited a telltale “wobble”.
“Gaia very accurately measures the positions of all the stars in the sky against a fixed background at different times, so one of the things you can do with that data is look for flickering stars,” says El. -Badry. “What we learned about this system is that the star wobbles more than you would expect for any star with a normal companion.”
Concluding that the gravitational influence of a massive compact object caused the wobble, the astronomers made follow-up observations with the Gemini Multi-Object Spectrograph instrument on the Gemini North Telescope.
This allowed them to measure the rapid speed of the companion star as it orbits the black hole, and confirmed the masses of the two objects, cementing the theory that Gaia’s central object BH1 is a black hole of stellar mass.
“When we look with images and spectra, we don’t see anything else that’s bright, so we can pretty much rule out possibilities that aren’t a black hole,” says El-Badry. “We are not detecting the black hole at all. We just see a huge mass source contributing no light.
However, this newly discovered Gaia BH1 black hole will not always be dormant or invisible.
End of the fast
El-Badry says that when the orbiting star reaches the end of its hydrogen burning phase, its core will collapse and its outer layers will swell. This will allow the central black hole to begin pulling this puffy outer material away from its companion, which it will begin to feed on, heat it up, and make its presence known through light emissions.
“Gaia BH1 is very stable at the moment, but it is doomed,” adds El-Badry. “The black hole will end up being something like a million times brighter than the sun when it’s now totally dark.”
Gaia’s BH1 black hole itself experienced a red supergiant phase, which is a mystery to astronomers. Indeed, to leave the black hole detected today, the progenitor star had to have about 20 times the mass of the sun. At this size during the red supergiant phase, the outer layers of the black hole should have bulged out to where the sun-like star orbits it today.
That means it’s hard to explain how the companion star survived so close to the black hole’s birth process without being irrevocably altered or swallowed up entirely. And this means that Gaia BH1 could change the way we think about the evolution of binary systems.
“The Gaia BH1 system is not quite what we expected it to be,” says El-Badry.
The Black Hole Police
For El-Badry, the discovery of the closest black hole to Earth is particularly heartening. Indeed, the astrophysicist inadvertently became part of a team of researchers more accustomed to demystifying local stellar-mass black holes. This has led them to be affectionately referred to as the “black hole police” in astronomy circles.
“I didn’t come into the game planning to ‘demystify’ black holes, I was just looking for them and it just so happened that I found different interpretations for black holes that others had suggested,” says El-Badry. . “It’s really exciting to find one and be able to study it. It feels good to be on the other side of the coin. »
El-Badry expects this discovery to be scrutinized as closely as he and his colleagues have examined other suspected black holes, but he is confident that the nature of Gaia BH1 will only be confirmed by follow-ups. In fact, El-Badry points out that a separate paper by a separate research team came to very similar conclusions regarding Gaia BH1.
However, the system’s central object could still lose its status as the closest stellar-mass black hole to Earth. If the technique used by El-Badry and the team to find it is successfully adapted to search for other similar systems containing black holes in the Milky Way, it could lead to the discovery of some even closer to home.
“The Gaia satellite that provided the data that we used to make this discovery continues to take data, and the more data it takes, the more accurate or useful the data becomes for this type of detection,” says El-Badry. “We estimate that in the Milky Way there are something like 40,000 of these normal star plus black hole binaries and by the end of its mission in five years, Gaia will find dozens.”
“Of course, that means there are many more that Gaia won’t find, but are still there,” he concludes.
Robert Lea is a freelance science journalist specializing in space, astronomy and physics. Rob’s articles have been published in Newsweek, Space, Live Science, Astronomy magazines and new scientist. He lives in the North West of England with too many cats and comics.
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