"A star, with mass 5.40*10^30 kg, revolves about the center of the Milky Way galaxy, which is 3.93*10^20 m away, once every 3.41*10^8 years. Assuming that each of the stars in the galaxy has a mass equal to that of our star, that the stars are distributed uniformly in a sphere about the galactic center, and that our star is essentially at the edge of that sphere, estimate roughly the number of stars in the galaxy." I don't really understand how I am supposed to answer this question. Using simple mathematics I can calculate the distance travelled of this star, and the speed with which it travels, but I do not know what else I need to do. I don't see where the mass comes into it, is volume and/or density relevant? I suspect this might be something to do with the Maxwell-Boltzmann distribution, am I in the right direction? Thanks.