Because light rays are bent when they pass close to a massive object, light from a distant source may be focussed by a closer object to producing a sudden brightening.
When a small nearby star passes directly between the Earth and a more distant star, the smaller object's gravitational field acts as a lens to bend and focus the light. If the smaller objects path takes it precisely in front of the other one, the image formed by the "gravitational lens" is a circular ring, referred to as an "Einstein Ring".
The focusing of the light passing around the edges of the object all towards the Earth causes a sudden and temporary increase in the brightness of the distant star. Although the likelihood that any individual star will be thus lensed, it has been hypothesized that events might be detected through careful observation of many, many stars.
Gravitational microlensing is the technique being used to search for MAssive Compact Halo Objects MACHOs.
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