Why does the Einstein Cross looks like a cross? Shouldn’t the gravitation of the galaxy project a circle of the quasar instead of a cross?

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Why does the Einstein Cross looks like a cross? Shouldn’t the gravitation of the galaxy project a circle of the quasar instead of a cross?

In: Physics

Basically, it’s because the [lensing galaxy](http://simbad.u-strasbg.fr/simbad/sim-id?Ident=%5BY88%5D%20G) is elliptical. Light from the source needs to pass through a region with the right amount of gravity pulling in the right direction to hit Earth. This basically means that it needs to pass through the galaxy at where the gravitational pull is at at the exact right amount. Since the galaxy is an ellipse, the band where gravity is perfect is also an ellipse.

The light from the quasar forms a cone, and the cross-section that’s passing through the galaxy is a circle (technically the galaxy and cross-section are 3d shapes, but the thicknesses are so small compared to all the other distances involved that they are basically flat for our purposes). The points where the circle and ellipse overlap are the points where light is moving in the right direction and gets bent in the right direction to hit us.

http://mathworld.wolfram.com/images/eps-gif/CircleEllipseIntersection_1000.gif

It depends on how the shapes overlap (the Einstein Cross is off-center) but you can see how you get four small spots where the image can reach us.