How Supermassive Black Holes Were Discovered
Radio images made from VLBI observations soon revealed that the sources at the centers of radio galaxies are “microscopic” by galaxy standards, even smaller than the distance between the sun and our nearest star. By matching the positions of these stars at the two wavebands, the radio position of Sgr A* can be transferred to infrared images with an accuracy of 0.001 seconds of arc. Finding that the compact radio source Sgr A* is at the precise location of the unseen mass and is motionless provides even more compelling evidence for a supermassive black hole.
Source: nautil.us