  We have identified the H-alpha absorption feature as a new spectroscopic
diagnostic of luminosity class in K- and M-type stars. From high-resolution
spectra of 19 stars with well-determined physical properties (including
effective temperatures and stellar radii), we measured equivalent widths for
H-alpha and the Ca II triplet and examined their dependence on both luminosity
class and stellar radius. H-alpha shows a strong relation with both luminosity
class and radius that extends down to late M spectral types. This behavior in
H-alpha has been predicted as a result of the density-dependent overpopulation
of the metastable 2S level in hydrogen, an effect that should become dominant
for Balmer line formation in non-LTE conditions. We conclude that this new
metallicity-insensitive diagnostic of luminosity class in cool stars could
serve as an effective means of discerning between populations such as Milky Way
giants and supergiant members of background galaxies.
