Almost but not quite in time for Halloween, astronomers announced on Friday that they had discovered the closest known black hole. It is a biggie, a shell of yawning emptiness 10 times as massive as the sun, orbiting as far from its own star as the Earth is from ours.
Not to worry, however: This black hole is 1,600 light-years away, in the constellation Ophiuchus; the next nearest known black hole is about 3,000 light-years away in the constellation Monoceros. What sets this new black hole apart from the thousands of others already identified in our Milky Way galaxy, besides its proximity, is that it isn’t doing anything — not drawing the nearby star to its doom, not gravitationally consuming everything nearby. Rather, the black hole is dormant, a silent killer waiting for the currents of space to feed it.
Black holes are objects so dense that, according to Einstein’s theory of general relativity, not even light can escape them. This makes them the most intriguing and violent phenomena in nature; when they feed, they can become the most brilliant objects in the universe, as gas, dust and even smaller stars are ripped and heated to incandescence, spewing energy as they approach the gates of eternity.
Most every galaxy has a supermassive black hole millions of billions times more massive than the sun; scientists aren’t sure where they come from. Smaller black holes are thought to form from massive stars that have reached the ends of their thermonuclear lives and collapsed. There are probably millions of black holes in the Milky Way. They typically make themselves known by the X-rays they spit out as they strip gas from their companions in double-star systems.
But what about dormant holes, those that are not currently coughing fire? Kareem el-Badry, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, has been searching for such hidden demons for four years. He found this black hole by scrutinizing data from the European Space Agency’s GAIA spacecraft, which has been tracking with exquisite precision the positions, motions and other properties of millions of stars in the Milky Way.
Dr. el-Badry and his team detected a star, virtually identical to our sun, that was jittering strangely, as if under the gravitational influence of an invisible companion. To investigate further, the researchers commandeered the Gemini North telescope atop Mauna Kea in Hawaii, which could measure the speed and period of this wobble and thus determine the relative masses of the objects involved. The technique is identical to the process by which astronomers analyze the wobbles of stars to detect the presence of orbiting exoplanets — except this time the quarry was far bigger.
Their results and subsequent calculations were consistent with a black hole of 10 solar masses being circled by a star similar to our own. They named it Gaia BH1.
“Take the solar system, put a black hole where the sun is and the sun where the Earth is, and you get this system,” Dr. el-Badry said in a news release from the National Optical and InfraRed Laboratory, which runs the Gemini North Telescope.
“This is the nearest known black hole by a factor of three, and its discovery suggests the existence of a sizable population of dormant black holes in binaries,” he and his co-authors wrote in a paper published on Wednesday in the Monthly Notices of the Royal Astronomical Society. Astronomers said that the new discovery raised questions about their presumed knowledge of how such binary star systems evolved. The progenitor of this black hole must have been a star of about 20 solar masses. According to the leading theories, the star’s death and the subsequent black hole formation would have involved a supernova explosion and other processes that would have severely disrupted the other, smaller star in the system. So why does the other star appear so normal?
“It poses many questions about how this binary system was formed,” Dr. el-Badry said in the news release, “as well as how many of these dormant black holes there are out there.”
