Parallax data from the Hipparcos mission allow the direct distance to open clusters to be compared with the distance inferred from main sequence (MS) fitting. There are surprising differences between the two distance measurements, which indicate either the need for changes in the cluster compositions or reddening, underlying problems with the technique of main sequence fitting, or systematic errors in the Hipparcos parallaxes at the 1 mas level. We examine the different possibilities, focusing on MS fitting in both metallicity-sensitive B −V and metallicity-insensitive V − I for five well-studied systems (the Hyades, Pleiades, α Per, Praesepe, and Coma Ber). The Hipparcos distances to the Hyades and α Per are within 1 σ of the MS fitting distance in B −V and V − I, while the Hipparcos distances to ComaBer and the Pleiades are in disagreement with the MS fitting distance at more than the 3 σ level. There are two Hipparcos measurements of the distance to Praesepe; one is in good agreement with the MS fitting distance and the other disagrees at the 2 σ level. The distance estimates from the different colors are in conflict with one another for Coma but in agreement for the Pleiades.Changes in the relative cluster metal abundances, age related effects, helium, and reddening are shown to be unlikely to explain the puzzling behavior of the Pleiades. We present evidence for spatially dependent systematic errors at the 1 mas level in the parallaxes of Pleiades stars. The implications of this result are discussed.

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Utilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca ii H and K emission measurements, literature-based abundance estimates, and updated photometry (including recent resolved measurements of close doubles), we revisit the Ursa Major moving group membership status of some 220 stars to produce a final clean list of nearly 60 assured members, based on kinematic and photometric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based membership assignments, indicating the usefulness of criteria based on photometric and chromospheric emission to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, failing to do so only for 15% of objects, perhaps considerably less. Our UMa members demonstrate nonzero vertex deviation in the Bottlinger diagram, behavior seen in older and recent studies of nearby young disk stars and perhaps related to Galactic spiral structure. Comparison of isochrones and our final UMa group members indicates an age of 500 AE 100 Myr, some 200 Myr older than the canonically quoted UMa age. Our UMa kinematic=photometric members' mean chromospheric emission levels, rotational velocities, and scatter therein are indistinguishable from values in the Hyades and smaller than those evinced by members of the younger Pleiades and M34 clusters, suggesting these characteristics decline rapidly with age over 200-500 Myr. None of our UMa members demonstrate inordinately low absolute values of chromospheric emission, but several may show residual fluxes a factor of !2 below a Hyades-defined lower envelope. If one defines a Maunder-like minimum in a relative sense, then the UMa results may suggest that solar-type stars spend 10% of their entire main-sequence lives in periods of precipitously low activity, which is consistent with estimates from older field stars. As related asides, we note six evolved stars (among our UMa nonmembers) with distinctive kinematics that lie along a 2 Gyr isochrone and appear to be late-type counterparts to disk F stars defining intermediate-age star streams in previous studies, identify a small number of potentially very young but isolated field stars, note that active stars (whether UMa members or not) in our sample lie very close to the solar composition zero-age main sequence, unlike Hipparcos-based positions in the H-R diagram of Pleiades dwarfs, and argue that some extant transformations of activity indices are not adequate for cool dwarfs, for which Ca ii infrared triplet emission seems to be a better proxy than H-based values for Ca ii H and K indices.