Order of Magnitude Question...?

Question

The distance from the Sun to the nearest star is about 4 x 10^16 m. The Milky Way galaxy is roughly a disk of diameter ~ 10^21 m and thickness ~ 10^19 m. Find the order of magnitude of the number of stars in the Milky Way. Assume the distance between the Sun and our nearest neighbor is typical.

Answer 1

if the separation between stars is 4e16 m then assume each star occupies a sphere of radius half that, or 2e16. Then the volume occupied by each star is about 4r³ = 30e48 m³ = 3e49

Volume of galaxy, cylinder is about 3r²h or 3(0.5e21)²(e19) = .6e61 = 1e61

divide the two 1e61/3e49 = 1/3 e12 or 3e11 stars.

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