Number 291 (Story #2), October 16, 1996 by Phillip F. Schewe and Ben Stein THE FIRST DIRECT MEASUREMENTS OF THE PROPER MOTION OF STARS at our galaxy's core provides new evidence for the existence there of a black hole. Deducing a star's radial velocity, its speed along our line of sight, is relatively easy to do; just measure the Doppler shift in the star's spectrum. By contrast, measuring a star's proper motion, its movement across our line of sight, is difficult, especially for stars as far away (25,000 light years) as the galactic center. And yet the proper motions of stars are what astronomers need to formulate a full traffic report for the vicinity of Sagittarius A*, the radio source at the very pivot of the Milky Way. Now Reinhard Genzel and Andreas Eckart of the Max Planck Institute in Garching, Germany have used the New Technology Telescope in Chile to make 5-year sightings of 39 stars near Sgr A*. The proper motions they find are as big as the radial velocities for these stars. This swirl of activity in turn suggests the presence of a 2.5-million-solar-mass dark object (perhaps many small or one large black hole) packed within a 0.1-light-year volume at the galaxy's nucleus. (Nature, 3 October 1996.)