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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

The Open Cluster NGC 7789. II. CCDVIPhotometry