  A (V,V - I)--diagram for the intermediate-age open cluster NGC 7789 has been
derived from CCD observations of more than 15,000 stars within ~ 18 arcmin of
the cluster center. From the brightest giants and blue stragglers at V ~ 11 to
the faintest lower main-sequence stars that were observed (at V ~ 21, M_V ~ 9),
the C-M diagram is well defined. A prominent clump of core helium-burning stars
is evident at V = 13.0 and the upper end of the main sequence shows a fairly
pronounced curvature to the red, which is indicative of significant convective
core overshooting. Indeed, comparisons with up-to-date stellar models show that
it is not possible to explain the observed morphology in the vicinity of the
turnoff unless the overshooting is quite extensive. Interestingly, if
sufficient overshooting is assumed in order to match the main-sequence data, it
is not possible to reproduce the cluster's extended giant branch unless the
cluster age is at least 1.6 Gyr (assuming a metallicity in the range -0.2 <=
[Fe/H] <= 0.0). This, in turn, requires that the cluster have an apparent
distance modulus (m-M)_V <= 12.2. Thus, sometime within the past few hundred
million years, the ignition of helium burning in NGC 7789 has switched from a
quiescent to an explosive (``flash'') phenomenon, and the length of the
cluster's red-giant branch has been steadily increasing with the passage of
time since then. From main-sequence fits to models that have been carefully
normalized to the Sun, we infer a reddening 0.35 <= E(V-I) <= 0.38.
