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You don't have to guesstimate to come up with the answer. What you do is look at the dynamics of stars with respect to the Galactic plane - in particular, the velocity dispersions of stars with known distances from the plane, combined with a reasonable assessment of where the Sun is with respect to the plane (close), yields an almost model-independent ...


3

Very roughly: $3.5 \times 10^{33}kg$, or 1800 solar masses. Here's how I came by that number, it is a very rough approximation. The major mass components of the galaxy are stars, the interstellar medium, and dark matter. According to the HYG Database there are approximately 1000 stars within 50 light years of the Earth. The average mass of a star is ...


1

Your claim that sphericity is assumed is incorrect when referred to hard scientific evidence. Even if that was assumed, the resulting error in the rotation speed is small for spheroidal galaxies and smallish for flat galaxies, provided they are centrally concentrated (which their stellar distribution is). Scientists are, of course, well aware of the ...


3

We can't. That is an over simplification only used in elementary treatments simply to provide the the flavour of the argument. If you see it done somewhere in the refereed literature, it is probably incorrect. Of course it may be true that the mass is almost spherically symmetric, especially if it is dominated by a spherically symmetric dark matter ...



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