Special & General Relativity Questions and Answers
Why aren't all gravitational lenses perfect rings?
As I have mentioned before, to get a perfect ring you need to be located exactly along the line connecting the center of the distant quasar and the center of the lensing galaxy. Also, the distribution of mass inside the lensing galaxy has to be symmetric around its center. Elliptical galaxies have rather symmetric mass distributions, but spiral galaxies are pretty lumpy with spiral arms, star clusters and the like. About 2 or three 'Einstein Rings' have now been detected, but the vast majority of lens candidates show complex, multiple images of varying brightness. In distant galaxy clusters, pieces of rings called 'arcs' are also seen.
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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.