  One of the great challenges in understanding stars is measuring their masses.
The best methods for measuring stellar masses include binary interaction,
asteroseismology and stellar evolution models, but these methods are not ideal
for red giant and supergiant stars. In this work, we propose a novel method for
inferring stellar masses of evolved red giant and supergiant stars using
interferometric and spectrophotometric observations combined with spherical
model stellar atmospheres to measure what we call the stellar mass index,
defined as the ratio between the stellar radius and mass. The method is based
on the correlation between different measurements of angular diameter, used as
a proxy for atmospheric extension, and fundamental stellar parameters. For a
given star, spectrophotometry measures the Rosseland angular diameter while
interferometric observations generally probe a larger limb-darkened angular
diameter. The ratio of these two angular diameters is proportional to the
relative extension of the stellar atmosphere, which is strongly correlated to
the star's effective temperature, radius and mass. We show that these
correlations are strong and can lead to precise measurements of stellar masses.
