Special & General Relativity Questions and Answers
Why are multiple gravitational lenses more common than rings?
Because ring gravitational lenses only happen if the lensing galaxy is EXACTLY along the line of sight between you and the distant galaxy/quasar. If it is off by even a few arc seconds, the ring degenerates into crescents and multiple images depending on the exact distribution of mass within the 'lumpy' lensing galaxy. To get a ring, you need a lensing galaxy like an elliptical galaxy with a smooth, symmetric mass distribution. A spiral galaxy is so internally lumpy with arms and dense nucleus, that I believe it gives multiple images and no rings. Also, entire clusters of galaxies have a roughly smooth gravitational field so they are also observed to give arcs and partial rings.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.