1. The problem statement, all variables and given/known A star at the edge of the Andromeda galaxy appears to be orbiting the center of that galaxy at the speed of about 200km/s. The star is about g*10^9AU from the center of the galaxy. Calculate a rough estimate of the mass of the Andromeda galaxy. Earth orbital radius is 1.40*10^8 2. Relevant equations Fg=gm1m2/r^2 3. The attempt at a solution 1 a.u. = 1.40*10^8 km = 1.40*10^11 m For the mass of the entire galaxy, M, you will have to assume a spherical distribution of mass, with the star in question at the outside, at distance R. ( what does that mean by assume spherical) In that case, the centripetal acceleration of the star is V^2/R = G M/R2 Solve for M. M = R V^2/G I got this solution from a web page however I'm trying to understand how it works. Isn't fg=Gm1m2/r^2 And then that will be equal to centripetal force Why in the above solution did they omit the one of the mass??