  Using high precision parallaxes from the Hipparcos catalog, we construct H-R
diagrams for two samples of bright stars. The first is a magnitude-limited
sample that is over 90% complete and uses uniform photometry from the Catalog
of WBVR Magnitudes of Northern Sky Bright Stars (declination above -14 deg).
This sample shows a smooth distribution of stars along the main sequence, with
no detectable gaps. The second contains all of the stars closer than 100
parsecs in the Hipparcos catalog with declination less than -12 deg. Uniform
spectroscopy from the Michigan Spectral Survey shows that some stars which
appear on the main sequence in the H-R diagram, particularly those in the 0.2 <
B-V < 0.3 region that has been labeled the Bohm-Vitense gap, are classified as
giants by the MK system of spectral classification. Other gaps that have been
identified in the main sequence are also affected by such classification
criteria. This analysis casts doubt on the existence of the Bohm-Vitense gap,
which is thought to result from the sudden onset of convection in stars. The
standard identification of main sequence stars with luminosity class V, and
giants with luminosity class III, must be reconsidered for some spectral types.
The true nature of the stars that lie on the main sequence in the H-R diagram,
but which do not have luminosity class V designations, remains to be
investigated.
