Hubble Measures Deflection of Starlight by a Foreground Object

This illustration shows how the gravity of a foreground white dwarf star warps space and bends the light from a distant star behind it. 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 — thanks to this optical trick of nature. The target was a white dwarf — the surviving core of a burned-out Sun-like star. The greater the temporary, infinitesimal deflection of the background star’s image, the more massive the foreground star is. Researchers found that the dwarf is 56 percent the mass of 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 experimental proof for general relativity. But Einstein didn’t think the same experiment could be done for stars beyond our Sun because of the extraordinary precision required.