Free Astronomy Magazine January-February 2023

40 JANUARY-FEBRUARY 2023 ASTRO PUBLISHING Gaia BH1. This dormant black hole is about 10 times more massive than the Sun and is located about 1600 light-years away in the constellation Ophiuchus, making it three times closer to Earth than the previous record holder, an X-ray binary in the constellation of Monoceros. The new discovery was made possible by making exquisite observations of the motion of the black hole’s com- panion, a Sun-like star that orbits the black hole at about the same dis- tance as the Earth orbits the Sun. “Take the Solar System, put a black hole where the Sun is, and the Sun where the Earth is, and you get this system,” explained Kareem El-Badry, an astrophysicist at the Center for Astrophysics | Harvard & Smithson- ian and the Max Planck Institute for Astronomy, and the lead author of the paper describing this discov- ery. “While there have been many claimed detections of systems like this, almost all these discoveries have subsequently been refuted. This is the first unambiguous detec- tion of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy.” Though there are likely millions of stellar-mass black holes roaming the Milky Way Galaxy, those few that have been detected were uncovered by their energetic interactions with a companion star. As material from a nearby star spirals in toward the black hole, it becomes superheated and generates powerful X-rays and jets of material. If a black hole is not actively feeding (i.e., it is dormant) it simply blends in with its surround- ings. “I’ve been searching for dormant black holes for the last four years using a wide range of datasets and methods,” said El-Badry. “My previ- by NOIRLab − Charles Blue Astronomers discover closest black hole to Earth B lack holes are the most ex- treme objects in the Universe. Supermassive versions of these unimaginably dense objects likely reside at the centers of all large galaxies. Stellar-mass black holes — which weigh approximately five to 100 times the mass of the Sun — are much more common, with an esti- mated 100 million in the Milky Way alone. Only a handful have been confirmed to date, however, and nearly all of these are ‘active’ – meaning they shine brightly in X- rays as they consume material from a nearby stellar companion, unlike dormant black holes which do not. Astronomers using the Gemini North telescope on Hawai’i, one of the twin telescopes of the Interna- tional Gemini Observatory, operated by NSF’s NOIRLab, have discovered the closest black hole to Earth, which the researchers have dubbed