Programs designed to observe gravitational microlensing are poised to provide a great deal of information about binary populations far from Earth, including those in the Galactic Bulge, in the Magellanic Clouds, in M31, and perhaps in other external galaxies. Because many millions of stars are monitored, microlensing observations allow us to study binaries in three ways: (1) when they are “involved” in a microlensing event (as either a lens or lensed source), (2) when variability due to binarity produces significant variations in the light curve, and (3) when light from a more distant star is attenuated or refracted by matter associated with the binary system (e.g., in a disk).
Microlensing observations will discover large numbers of binaries and planetary systems in a variety of galactic environments. Thus, comparative statistical studies of binary properties (distributions of mass ratios and orbital separations) are possible.
An intriguing sign that we have already begun to learn about binaries through microlensing observations comes from work indicating that all of the lenses detected to date may in fact be binaries. For observations along the direction of the Magellanic Clouds this would imply that, if the lenses are primarily located in the Halo, then MACHOs tend to be binaries. If, on the other hand, most of the lenses are located in the Magellanic Clouds, microlensing observations are giving us a unique way to explore a distant stellar population of binaries.