physics
posted by Sam .
A star of mass 2.0 1031 kg that is 4.8 1020 m from the center of a galaxy revolves around that center once every 4.2 108 years. Assuming that this star is essentially at the edge of the galaxy, each of the stars in the galaxy has a mass equal to that of this star, and the stars are distributed uniformly in a sphere about the galactic center, estimate the number of stars in the galaxy. (Do not round your answer to an order of magnitude.)
stars

Use ^ in front of exponents please.
Express the galactic mass in terms of the number of equalsize stars, N.
Set the centripetal force equal to the gravitational frce on the edge star, and sziolve for N. 
So using Keplers third law,
T^2=(4pi^2/GM)r^3
T=4.2 x 10^8
r=4.8 x 10^20
G=6.67 x10^11
solve for M, then divide by 2.0x10^31?
which would give 1.855 x 10^25 stars