This is a time series of magnification patterns for one of the 
images of the 4-image gravitational lens system called the 
Einstein Cross, Q2237+0305.  The four images, separated by about
an arcsecond, are seen around the center of a nearby barred 
spiral, z=0.0394. The source QSO is at z=1.69.

The image that you see is the magnification pattern on the 
observer plane---if there were a screen ~2,000 by ~2,000 AU 
placed at the location of the Solar System then this would be
the pattern of illumnation displayed on it. The changing 
pattern of caustics (note: darker areas correspond to brighter 
illumination) is due to the random velocities of individual 
stars in the lensing galaxy. In this particular movie the stellar 
parameters are as follows (note: these are not necessarily very
realistic) star masses are distributed according to Salpeter MF
with lower and upper limits of 0.01 and 0.5 Solar Masses 
respectively. Stars have a 2D velocity dispersion of 100 km/sec. 
400 of the 10000 stars used in this simulation are locked up in 
binaries with periods between 0.1 days and 173 years. The total 
lensing optical depth is 0.4 and all of it is due to individual 
stars; there is also external shear---due to the rest of the 
galaxy---of 0.4. The whole movie is equivalent to about 200 years
of Earth's time.

This work is done in collaboration with Prof. Neal Katz.