# Homework Help: Mass of a Cluster of galaxies

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1. Aug 10, 2016

### rohanlol7

1. The problem statement, all variables and given/known data
An estimate of the mass of the Coma cluster can be made from the velocity of the galaxies relative to the centre of mass of the cluster; the velocities are around 2000 km s-1. The stars that form the cluster have been together for a long time. Show that a simple model will predict a mass of the cluster of the order of 2*10^43kg. The radius of the Coma cluster is about 10^21m.

2. Relevant equations
GMm/R^2 =Mv^2/R

3. The attempt at a solution
Tried a few things. First i tried to consider that as a spherical hollow shell. I calculated by using the above equation ( just plugin in values ) and i was off by a factor of 3. Next i tried to consider 2000 as being the AVERAGE speed. This time i was wrong by a factor of about 10.
I tried to model it as a disk but that seemed to complex

2. Aug 10, 2016

### Staff: Mentor

Wikipedia says the Coma cluster has est. $10^{14} - 10^{15}$ sun masses, i.e. $\; 2\cdot 10^{44}-2\cdot 10^{45}\,kg$.
Isn't this your factor $10$?

3. Aug 11, 2016

### rohanlol7

Maybe but the problem wants me to obtain 2*10^43