  Hipparcos parallaxes and proper motions have made it possible to construct
Hertzsprung-Russell (HR) diagrams of nearby clusters with unprecedented
accuracy. The standard deviation of high fidelity non-binary non-variable stars
about a model stellar evolution isochrone in the Hyades cluster is about 0.04
magnitudes. We use this deviation to estimate an upper limit on the scatter in
metallicities in stars in this cluster. From the gradient of the isochrones
evolution in the HR diagram we estimate an upper limit for the scatter of
metallicities Delta [Fe/H] <~ 0.03 dex, a smaller limit than has been measured
previously spectroscopically. This suggests that stars in open clusters are
formed from gas that is nearly homogeneous in its metallicity. We consider the
hypothesis that processes associated with planet formation can pollute the
convection zone of stars. If the position on the HR diagram is insensitive to
the metallicity of the convection zone and atmosphere, then stars which have
very polluted convection zones can be identified from a comparison between
their metallicity and position on the HR diagram. Alternatively if the
pollution of the star by metals results in a large change in the position of
the star on the HR diagram in a direction perpendicular to the isochrone, then
the low scatter of stars in the Hyades can be used to place constraints on
quantity of high-Z material that could have polluted the stars.
