Scientists Discover Closest Known Black Hole to Earth

Black holes are incredibly massive and numerous — scientists believe there may be as many as 100 million in our galaxy alone — but they’re surprisingly hard to spot. Scientists from the Harvard & Smithsonian Center for Astrophysics believe they’ve just identified a dormant black hole just 1,600 light-years away in the constellation Ophiuchus, taking the crown as the closest one currently known. This unusual solar system could also point to problems with our understanding of binary stars.

Many of the black holes we hear about are actively devouring stars and other objects. The spiral of superheated material in the accretion disk of these black holes makes them bright in X-rays, aiding in detection. However, scientists suspect there are many more that aren’t easy to see. This newly discovered black hole, dubbed Gaia BH1, is the first confirmed dormant black hole in a binary system, something astronomers have long hunted.

The first sign of something unusual in this binary system came from the European Space Agency’s Gaia spacecraft, which has been working to produce the most detailed 3D map of star locations and motion in the Milky Way. Gaia identified a sun-like star that appeared to wobble as it orbited an unseen partner. This is similar to how astronomers use radial velocity measurements to spot some exoplanets. Upon further investigation with the Magellan Baade telescope in Chile and Gemini North Telescope in Hawaii, the team confirmed the star is orbiting a stellar-mass black hole. This discovery has been published in the Monthly Notices of the Royal Astronomical Society.

Analysis shows the invisible monster is about 10 times the mass of our sun. Its companion appears to be in a stable orbit far enough away that it does not lose mass to the black hole. “Take [our] Solar System, put a black hole where the Sun is, and the Sun where the Earth is, and you get this system,” says lead author Kareem El-Badry.

The previous record-holder for the closest black hole was about 3,000 light years distant. That object was an active X-ray source in the constellation of Monoceros, making it easier to spot. Gaia BH1 will be easier to study at just 1,600 light-years away, and it’s going to get a lot of attention. In addition to confirming the Center for Astrophysics’ findings, astronomers will have to puzzle over how BH1 and its companion ended up where they are.

According to the team, the progenitor star that formed Gaia BH1 would have been at least 20 times the mass of the sun. Therefore, it would have survived only a few million years before burning out. Assuming it formed at the same time as its companion, BH1’s supergiant phase would have engulfed the sun-like star. All the theoretical models that predict a surviving companion star result in a tighter orbit than seen here. On the other hand, El-Badry says there is no plausible model in which the unseen companion is anything other than a black hole. Quite a cosmic mystery.

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