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

## 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.

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

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

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

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

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