Colloquium: Dr. Kailash Sahu, Space Telescope Science Inst.

In a reprise of the famous 1919 solar eclipse experiment that confirmed

Einstein's general relativity, the nearby white dwarf Stein 2051B passed very

close to a background star in March 2014. As Stein 2051B passed by, the

background star's position was relativistically deflected. Measurement of this

deflection with the Hubble Space Telescope (HST) allowed us to determine the

mass of Stein 2051B, which confirms the physics of degenerate matter and

provides a new tool for mass determinations of isolated stars. Another consequence of the relativistic bending of light is the temporary

brightening of the background star, commonly known as gravitational microlensing. The combination of brightening and deflection provides a powerful technique to detect isolated black holes, and measure their masses. I will discuss the technique, and our HST programs aimed at the first detections of stellar-mass black holes through this technique.