Hubble Space Telescope looks for the small, dim inhabitants of a clobular cluster by studying their effect on the light of distant bakground stars in the galaxy's bulge. The gravity of the intervening objects in the cluster briefly amplifies the background stars' image by bending the light waves, as an optical lens does. The mass of the objects can be inferred by the duration of the lensing event: the longer the brightness lasts, the more massive the objects.
