Gravitational Lenses - Nature's Telescope
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NASA / A. Fruchter / STScI
The possibility that the path of light could be bent by the gravity of a large object was predicted by Einstein's Theory of General Relativity, and this effect was observed soon after the theory was published. Because people normally think of glass or plastic lenses as bending light, we call any massive object that bends light rays a "gravitational lens." By measuring the amount of bending, we can then determine the mass of whatever is doing the bending.
In the cluster Abell 2218 (pictured here), distant blue galaxies behind the large
cluster of galaxies are "squished" into a circular shape around the
middle of the foreground cluster. By measuring the amount of
distortion in the more distant blue galaxies, we can determine the mass of the cluster. In fact, we can even
measure how much mass there is that we can't see -- this galaxy
cluster happens to have nearly 400 trillion times the sun's mass in
"dark" matter.
