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Why does "Einstein's Cross" (found in the constellation Pegasus) have four outer points? I understand the concept of gravitational lensing but why 4 points? what not 3 or 5? Or is this just a chance occurrence? Would it have anything to do with the amount of arms the galaxy has that is doing the 'lensing'?

It is largely chance how they will appear to us. However, you are correct that how matter in the lens is distributed will determine what we see. For a purely symmetrical lens, then the image is a ring. As you add more matter and more variation to the lens, then the ring becomes smeared arcs, and with enough variation, individual spots as what you see in "Einstein's Cross."

Look for "Einstein Rings" on the Internet and you will see several examples. Some even have 3 spots.

While the lensing that we see does depend upon variations within extremely massive objects, the effect is probably coming from the core of the galaxy and not due to the arms themselves. However, the fact that arms are present in the galaxy could lead to some asymmetries in the distribution of extremely massive stars at the core of the galaxy. However at this point we are well beyond my area of expertise, so that is speculation.

Michael Pierce

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