Homework Help: Dark Matter, calculating missing mass

1. Dec 8, 2013

cherioslover

1. The problem statement, all variables and given/known data
The andromeda galaxy has a velocity of 250 km/s as it rotates. The observed mass is 1 x 10^42 kg. If the galaxy is roughly a circle with a diameter of 66,00 light years how much mass is not accounted for visibly?

2. Relevant equations

Fc=mv^2/r and Fg=Gm1m2/r^2

3. The attempt at a solution

There was no answer, i know im supposed to calculate the the velocity and its mass, then but to get the percent of mass missing i think you divide the difference between the the calculated mass and given mass by the calculated mass or something like that. But i am still rather confused at what to do. Im not looking for someone to complete my homework, just to help me understand how to do this question.

2. Dec 9, 2013

Staff: Mentor

Start with the part you described - calculate the mass with the formulas. It will be larger than 1042 kg, and this number is (visible mass)+(invisible mass). Can you calculate the invisible mass?