  Polaris, the nearest and brightest classical Cepheid, is a single-lined
spectroscopic binary with an orbital period of 30 years. Using the High
Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space
Telescope (HST) at a wavelength of ~2255\AA, we have directly detected the
faint companion at a separation of 0\farcs17. A second HST observation 1.04 yr
later confirms orbital motion in a retrograde direction. By combining our two
measures with the spectroscopic orbit of Kamper and an analysis of the
Hipparcos and FK5 proper motions by Wielen et al., we find a mass for Polaris
Aa of 4.5^{+2.2}_{-1.4} M_\odot--the first purely dynamical mass determined for
any Cepheid. For the faint companion Polaris Ab we find a dynamical mass of
1.26^{+0.14}_{-0.07} M_\odot, consistent with an inferred spectral type of F6 V
and with the flux difference of 5.4 mag observed at 2255\AA. The magnitude
difference at the V band is estimated to be 7.2 mag. Continued HST observations
will significantly reduce the mass errors, which are presently still too large
to provide critical constraints on the roles of convective overshoot, mass
loss, rotation, and opacities in the evolution of intermediate-mass stars.
  Our astrometry, combined with two centuries of archival measurements, also
confirms that the well-known, more distant (18") visual companion, Polaris B,
has a nearly common proper motion with that of the Aa,Ab pair. This is
consistent with orbital motion in a long-period bound system. The ultraviolet
brightness of Polaris B is in accordance with its known F3 V spectral type if
it has the same distance as Polaris Aa,Ab.
