(Note: this exercise is a continuation of Exercise 10.)

(a) You measure the apparent width of Abell 2218, and find that
the cluster subtends an angle of 9.0 arcminutes on the sky.  What
is the diameter of the cluster in kiloparsecs?  How does that size
compare with the diameter of the Milky Way Galaxy?

(b) You take a picture of this area of sky and find that there are
approximately 120 galaxies in this cluster, which are similar in size 
and mass to our own Milky Way Galaxy.  What, then, is the
density of visible matter in Abell 2218?  Make reasonable estimates
about the mass of visible matter in these galaxies, and of the volume
of Abell 2218, based on lecture discussions and your calculations.