Estimating Mass of the Milky Way Galaxy

[myoplex11]

Homework Statement

The sun rotates around the center of the milky way galaxy at a distance of about 30,000 light years (1ly= 9.5*10^15m). if it takes about 200 million years to make one rotation, estimate the mass of our galaxy. Assume that the mass distribution of galaxy is concentrated mostly in a central uniform sphere.

Homework Equations

G Msun Mgalamy / r^2 = Mgalaxy v^2/ r

The Attempt at a Solution

r= (30000 * (9.5 * 10^5) ) m
v= 2 * pi * r / 200 million years
i know i have to convert the years into seconds is my setup right?

 

[mgb_phys]

You have the equations slighty wrong. Your's would cancel the mass of the galaxy leaving only the mass of the sun.

And yes - you should work in m/kg/s

 

[myoplex11]

i am not sure what equation to use

 

[mgb_phys]

You have it almost correct - you just have the mass of galaxy instead of sun in the centrifugal part.
http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion#Third_law