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Astronomers discover closest black hole to Earth
Astronomers using the International Gemini Observatory, operated by NSF's NOIRLab, have discovered the closest-known black hole to Earth. This is the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its close proximity to Earth, a mere 1600 light-years away, offers an intriguing target of study to advance our understanding of the evolution of binary systems. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

There are black holes in the universe. These unimaginably dense objects are most likely located at the center of all large galaxies. It is estimated that there are 100 million stellar-mass black holes in the Milky Way.

Only a few have been confirmed to date, and most of them are active, meaning they shine brightly in X-rays as they consume material from a nearby stellar companion.

The closest black hole to Earth has been discovered using a telescope on Hawai'i operated by the NOIR Lab of the National Science Foundation. The black hole is three times closer to Earth than the previous record holder, an X-ray binaries in the constellation Monoceros.

Observations of the motion of the black hole's companion, a sun-like star, allowed for the discovery.

"Take the solar system, put a black hole where the sun is, and the sun where the Earth is, and you get this system," said El- Badry.

Almost all of the discoveries made about systems like this have been found to be false. The first unambiguous detection of a sun-like star is around a stellar-mass black hole.

The few stellar-mass black holes that have been detected were found by interacting with a companion star. Powerful X-rays and jets of material are generated when material from a nearby star spirals towards a black hole. If a black hole isn't feeding, it blends in with its surroundings.

"I've been looking for black holes for the last four years using a wide range of methods and data," said El- Badry. This is the first time that the search has resulted in a system that masquerades as a black hole.

The team used data from the European Space Agency to identify the system as potentially hosting a black hole. The star's motion was recorded by Gaia because of the gravity of the object. To explore the system in more detail, El- Badry and his team used the instrument on the black hole to measure the star's speed as it passed by.

The team was able to identify the central body as a black hole because of the follow up observations.

"Our follow-up observations confirmed the existence of a normal star and a black hole in the binaries," said El- Badry. We couldn't find a plausible astrophysical scenario that could explain the observed path of the system.

The team relied on Gemini's ability to provide data on a tight deadline as they only had a short window in which to do follow-up observations.

We only had a week before the two objects were close to each other. "Measurements at this point are necessary to make accurate mass estimates in a system." The project's success was dependent onGemini's ability to provide fast turn around observations. We would have had to wait another year if we missed that window.

Astronomers can't explain how the strange configuration of the BH1 system came about. The progenitor star that turned into the black hole would have been 20 times the size of the sun.

A few million years is how long it would have lasted. If both stars formed at the same time, this massive star would have quickly become a supergiant and engulfed the other star before it had time to become a main-sequence star.

The observations of a black hole suggest that the solar-mass star could have ended up being a normal star. The theoretical models that allow for survival all agree that the star should have ended up on a tighter path.

It is possible that there are gaps in our understanding of how black holes form and evolve, and that there is an undiscovered population of black holes in binaries.

It is interesting that the system is not easy to accommodate. There are a lot of questions about how this system was formed and how many black holes there are.

"Gemini North has provided strong evidence for the nearest black hole to date but also the first pristine black hole system, which was unaffected by the usual hot gas interacting with the black hole," said Martin Still.

Despite a shared history with its exotic neighbor, why is the companion star in this binaries system so normal?

A sun-like star is circling a black hole. 10.093/mnras/stac3140

Journal information: Monthly Notices of the Royal Astronomical Society

It was provided by the National Optical-Infrared Astronomy Research Lab.