The Dark Affect

ESA/Gaia/DPAC Image of the Milky Way showing the locations of three black holes. Active black holes are pretty easy to detect. Their superheated accretion disks emit bright x-rays, and the jets streaming from their poles are easily seen at visible and radio wavelengths. But most stellar-mass black holes are inactive. They either drift through the galaxy alone or orbit a companion star without consuming its material. We can’t observe inactive black holes directly. We can only observe their effect on nearby objects. We currently know of three such black holes thanks to the Gaia spacecraft.1 The main mission of Gaia is to map the positions and motions of more than a billion stars in our region of the Milky Way. To achieve its goal, the motion measurements must be very precise. Precise enough that we can detect small wobbles in a star’s motion. Usually this means the star has a planet, and the gravitational tug of the planet causes the star to wobble. But in at least three cases the…