I have a homework problem that asks for us to solve for the center of mass of an open cluster - for the single stars in the cluster. We have been giving the RA and DEC and the V and B-V values of the star members. I know the equation to find the center of mass between binary stars is M1+M2= Kepler's 3rd law and that using the virial theorem we can solve for the radius but then density rho is involved. I also know that for given B-V values, you can determine the stellar classification of the stars. So my question is, how am I suppose to solve for the center of mass for the cluster?