This illustration shows how the Sun's gravity bends the path of light from a distant star, changing its apparent location in the sky. The effect is highly exaggerated here. From Earth, the apparent deflection would appear to be no more than the width of a dime seen at 1.25 miles away.
Einstein’s general theory of relativity explains gravitational fields as bends and curves in space-time that affect the motions of objects including stars, planets — and even light. One prediction of the general theory of relativity is that light passing by an object feels a slight “tug,” causing the light’s path to bend slightly. This predicted effect sets up one of Einsteins's suggested tests of general relativity — measuring how starlight bends around the Sun, the strongest source of gravity in our neighborhood. Starlight that passes near the edge of the Sun on its way to Earth is deflected, altering by a small amount where those stars appear to be. The amount of deflection is very small — about the width of a dime if you saw it at a mile and a quarter away.
Such a measurement could only be made during a total solar eclipse, when the Moon blocks out the bright light of the Sun. By comparing two sets of photographs of the same patch of sky – one set taken during the eclipse and another set a few months before or after, when the Sun was out of the way — researchers could see if the apparent star positions changed as predicted by Einstein.