When two galaxies lie along one line of sight, the nearer galaxy's mass bends light from the more distant one, as predicted by the general theory of relativity. The phenomenon, called gravitational lensing, usually creates an arc of light tight against the closer galaxy. Only twice before had astronomers witnessed a complete circle, a so-called Einstein ring, formed by two perfectly aligned galaxies.
Last week, the Hubble Space Telescope snapped pictures of 19 gravitationally lensing elliptical galaxies, eight of which boast Einstein rings. Pictured here is a ring in the constellation Serpens. Astronomers will use the radius of the ring to calculate the closer galaxy's mass, says Adam Bolton of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.