Gravitational Lensing: White Dwarf Passes In Front of Distant Background Star

This animation shows the motion of a white dwarf star passing in front of a distant background star. During the passage, the faraway star appears to change its position slightly, because the light from it has been deflected by the white dwarf’s gravity. Employing this trick of nature, astronomers using the NASA/ESA Hubble Space Telescope have for the first time directly measured the mass of a single, isolated star other than our Sun.

This effect, called gravitational lensing, was predicted as a consequence of Einstein’s general theory of relativity from a century ago. Observations of a solar eclipse in 1919 provided the first direct evidence for general relativity. But Einstein didn’t think the same experiment could be done for stars beyond our Sun because of the precision required.