  NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days
of time on the Hubble Space Telescope, we have constructed the deepest ever
color-magnitude diagram for this cluster. We see a clear truncation in each of
its two major stellar sequences. Faint red main sequence stars run out well
above our observational limit and near to the theoretical prediction for the
lowest mass stars capable of stable hydrogen-burning in their cores. We also
see a truncation in the number counts of faint blue stars, namely white dwarfs.
This reflects the limit to which the bulk of the white dwarfs can cool over the
lifetime of the cluster. There is also a turn towards bluer colors in the least
luminous of these objects. This was predicted for the very coolest white dwarfs
with hydrogen-rich atmospheres as the formation of H2 causes their atmospheres
to become largely opaque to infrared radiation due to collision-induced
absorption.
