Microlensing Black Hole

The star-filled sky in this Hubble Space Telescope photo is located in the direction of the Galactic center. The brightness of stars are monitored to see if any change in apparent brightness is made by a foreground object drifting in front of them. The warping of space by the interloper would momentarily brighten the appearance of a background star, due to an effect called gravitational lensing. One such event is shown along the four close-up frames at the bottom. The arrow points to a star that momentarily brightened, as first captured by Hubble beginning in August, 2011. This was caused by a foreground black hole drifting in front of the star, along our line-of-sight. The star brightened and then subsequently faded back to its normal brightness as the black hole passed by. Because a black hole doesn't emit or reflect light, it cannot be directly observed. But its unique thumbprint on the fabric of space can be measured through these so-called microlensing events. Though an estimated 100 million isolated black holes roam our galaxy, finding the telltale signature of one is a needle-in-haystack search for Hubble astronomers.