# How many stars in our galaxy if they had the mass of our Sun.

balletgirl

## Homework Statement

The Sun rotates about the center of the Milky Way Galaxy at a distance of about 3.00x10^4 light years from the center (1ly= 9.50x10^5 m). If it takes about 200 million years to make one rotation, estimate the mass of our galaxy. Assume the mass distribution of our galaxy is concentrated mostly in a central uniform sphere. If all the stars had about the same mass as our Sun, how many stars would there be in our galaxy?

## Homework Equations

I have no clue, the most common equation I use is g=GM/r^2 and a=v^2/r.

## The Attempt at a Solution

I do not know where to start.

## Answers and Replies

shallgren
Treat the galaxy as if it was a planet that the sun was orbiting.

Gold Member
Well, g is just gravitational acceleration, and is the a in a=v^2/r... but there's an equation missing.

$$a=\frac{4 \pi^2 r}{T^2}$$

balletgirl
Okay, so I did T= 200,000,000 yrs = 6.31x10^15 s
& R= 2.85x10^20

a=4pi^2*r/T^2
a= 4(3.14)^2*(2.85x10^20)/(6.31x10^15s)
a=39.4(2.85x10^20)/3.98x10^31
a= 2.81x10^-10

I doubt this is right since it is negative

Gold Member
No it's not... the exponent is negative, but the number is positive, just very small. And it should be small...