Gravitational Lensing and Number of Images
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.
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Update: June 2012