  Observing in six frequency bands from 27 to 280 GHz over a large sky area,
the Simons Observatory (SO) is poised to address many questions in Galactic
astrophysics in addition to its principal cosmological goals. In this work, we
provide quantitative forecasts on astrophysical parameters of interest for a
range of Galactic science cases. We find that SO can: constrain the frequency
spectrum of polarized dust emission at a level of $\Delta\beta_d \lesssim 0.01$
and thus test models of dust composition that predict that $\beta_d$ in
polarization differs from that measured in total intensity; measure the
correlation coefficient between polarized dust and synchrotron emission with a
factor of two greater precision than current constraints; exclude the
non-existence of exo-Oort clouds at roughly 2.9$\sigma$ if the true fraction is
similar to the detection rate of giant planets; map more than 850 molecular
clouds with at least 50 independent polarization measurements at 1 pc
resolution; detect or place upper limits on the polarization fractions of
CO(2-1) emission and anomalous microwave emission at the 0.1% level in select
regions; and measure the correlation coefficient between optical starlight
polarization and microwave polarized dust emission in $1^\circ$ patches for all
lines of sight with $N_{\rm H} \gtrsim 2\times10^{20}$ cm$^{-2}$. The goals and
forecasts outlined here provide a roadmap for other microwave polarization
experiments to expand their scientific scope via Milky Way astrophysics.
