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Oct - Nov 97
Astronomical
Feature of the Month
GRAVITATIONAL LENSING
Gravitational lensing is a rare but beautiful and scientifically useful phenomena. The principle behind it is that a massive foreground object, like a galaxy or cluster of galaxies, generates a strong gravitational field that can actually bend or refract light from distant background objects. If a gravitational lens was at work, an observer on Earth would see not only the "real" distant object, but also an illusionary double or perhaps multiple false images, including ring or arc-like structures. Even though lenses were predicted by Albert Einstein early in the 20th century, the first example of a grav lens was not found until 1979. That turned out to be a visible galaxy that was distorting the image of a quasar along the same sight line. In April of 1995, the Hubble Space Telescope discovered a lens in Abell 2218 (see picture below), showing the characteristic multiple "arcs" of a lensed quasar. An interesting note, if the gravity source was perfectly aligned between Earth and the distant object, the lensed view that we’d see would be a complete circle around the object. (Would that make for an interesting painting or what?) Recently, the HST has discovered a galaxy that is estimated to be over 13 billion light years from Earth, currently the farthest away known object. Too dim to be seen normally, the galaxy happens to lie directly on the sight line of a cluster of galaxies 5 billion light years away. This cluster lensed the light of the distant galaxy making it 5 to 10 times brighter than normal and thus visible to astronomers via the Hubble on Earth.
International Association of Astronomical Artists