(Note: this exercise is a continuation of Exercise 11.)

It turns out that, not only is the average position of the 
hydrogen-alpha line at 789.0 nm, but the half-width at half maximum 
of your distribution is 21.0 nm.  Your random and systematic measurement
errors total 1.0 nm.  (You may find a review of statistical analysis
useful - it's posted on last semester's webpage.)

(a) How do you explain the large spread of velocities you've measured?
Also, what does this tell you about the motion of galaxies in clusters
such as Abell 2218?  A short paragraph or two will suffice.

(b) You find that one of the galaxies in the cluster, at a distance
of 1000 kpc from the cluster's center, is moving at a velocity of
950 km/sec relative to the cluster's center.  Remember that the
escape velocity of an object is given by v = sqrt(2 * G * M / r).
Given the mass of visible matter you calculated in Question 2(b),
should this galaxy stay gravitationally bound to the cluster, or
should it escape?