Originally Posted by

**cherioslover**
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?

Equations needed i think are Fc=mv^2/r and Fg=Gm1m2/r^2

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.