The nearest black hole is in our "cosmic backyard"? Gaia BH1 is 1,600 light-years away from Earth!

Normal companion star and mutant black hole in a binary system

The nearest black hole is right in our cosmic backyard

(Artist's impression of the black hole Gaia BH1, near its companion star. This is the closest black hole to Earth ever discovered, and was discovered using the Gemini North telescope in Hawaii. Image via International Gemini Observatory/ NOIRLab/ NSF/ AURA/ J. da Silva/ Spaceengine/ M. Zamani.)

Black holes with stellar masses are both strange and fascinating. They are also mostly very far away, probably distributed in our galaxy and other galaxies. On November 2, 2022, astronomers using the Gemini North telescope in Hawaii discovered the closest black hole to Earth, called Gaia BH1. This black hole is about 1,600 light-years away from us, in the direction of the constellation Ophiuchus. That's a long way away. But it is the closest black hole so far. And, from the perspective of the Milky Way, it is right in the backyard of the universe.

Gaia BH1: The closest black hole discovered so far

Previously, data from the European Space Agency's Gaia spacecraft led researchers to suggest that there might be a black hole in this star system - a companion to the black hole. Now, the Gemini North telescope operated by NOIRLab has confirmed this. The companion to the black hole is a Sun-like star, orbiting the black hole at about the same distance as Earth is from the Sun.

Lead author Kareem El Badry, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy, said:

Take the Solar System, put a black hole where the Sun is, put the Sun where the Earth is, and you get this system. While there have been many discoveries claiming to detect such a system, almost all of them have subsequently been disproven. This is the first unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass black hole in the Milky Way.

The black hole's gravity causes tiny irregularities in the star's motion, which Gaia had detected before. With this in mind, Gemini North measured the speed of the companion star as it orbited the black hole. In doing so, it provided a precise measurement of its orbital period. As El Badry explains, the Gemini North telescope also had a narrow window of opportunity for follow-up observations:

When we first had an indication that the system contained a black hole, we had only a week to go, with the two objects at their closest approach in their orbits. Measurements at this point are critical to making precise mass estimates in binary systems. The ability of the Gemini North telescope to provide observations in a short period of time was critical to the success of the project. If we had missed that narrow window, we would have had to wait another year.

A dormant stellar-mass black hole

Gaia BH1 has a mass about 10 times that of our sun, making it a stellar-mass black hole. Other black holes, like those at the centers of galaxies, including our own, can be much more massive than this. The newly discovered black hole is also dormant. That means it no longer emits X-rays like an active black hole would.

Scientists say there could be millions of stellar-mass black holes in our galaxy. Yet they are hard to find, with only a handful confirmed. They are easiest to find when they have a companion star, like Gaia BH1. High-energy interactions between black holes and stars make them easier to detect. This happens when matter from a star spirals into the black hole. As the black hole "feeds" on this matter, it produces X-rays and jets of matter shoot out from near the black hole.

As El Badry noted: "For the past four years, I have been searching for dormant black holes using a variety of datasets and methods. My previous attempts—and those of others—had discovered a large population of binary systems masquerading as black holes, but this is the first time the search has borne fruit."

"Our follow-up observations with the Gemini North telescope have confirmed beyond a reasonable doubt that the binary contains a normal star and at least one dormant black hole. We cannot find any plausible astrophysical scenario to explain the observed orbits of the system that does not involve at least one black hole."

As the paper states, the researchers found no alternative explanation for these observations: We do not find any plausible astrophysical scenario that could explain its orbit that does not involve a black hole.

A special system

The discovery itself is exciting, of course, but astronomers still have many questions. The original star — which later became a black hole — would have been 20 times more massive than our sun, scientists say. If that was the case, it should have only lived for a few million years. That's a very short lifespan for a star. But if this star and its companion formed at the same time, the original star should have become a supergiant. It would have then devoured the companion in the process. However, that clearly didn't happen, as the companion is still there. Could the companion have somehow survived?

The fact that the companion star is still around is a mystery. It means there may be a gap in our theoretical models of black holes. As El Badry comments: "Interestingly, this system is not easily fit by the standard binary evolution model. It raises many questions about how this binary system was formed, and how many of these dormant black holes there are."

Martin Still, Gemini program officer at the National Science Foundation, added: While this could herald future discoveries of a dormant black hole population in our own galaxy, these observations also leave a mystery to be solved: Why is the companion star in this binary system so normal, despite sharing a shared history with its bizarre neighbor?

BY:Paul Scott Anderson

FY: Raywannabethelight

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