  Stellar photometric variability offers a novel probe of the interior
structure and evolutionary state of stars. Here we present a census of stellar
variability on day to decade timescales across the color-magnitude diagram for
73,000 stars brighter than $M_I$=-5 in the Whirlpool Galaxy (M51). Our Cycle 24
HST program acquired V and I-band images over 34 epochs spanning one year with
pseudo-random cadences enabling sensitivity to periods from days to months. We
supplement these data with archival V and I-band HST data obtained in 1995 and
2005 providing sensitivity to variability on decade timescales. At least 50% of
stars brighter than $M_I$=-7 show strong evidence for variability within our
Cycle 24 data; among stars with V-I>2 the variability fraction rises to ~100%.
Large amplitude variability (>0.3 mag) on decade timescales is restricted to
red supergiants and very luminous blue stars. Both populations display fairly
smooth variability on month-year timescales. The Cepheid instability strip is
clearly visible in our data, although the variability fraction within this
region never exceeds ~10%. The location of variable stars across the color
magnitude diagram broadly agrees with theoretical sources of variability,
including the instability strip, red supergiant pulsational instabilities,
long-period fundamental mode pulsations, and radiation-dominated envelopes in
massive stars. Our data can be used to place stringent constraints on the
precise onset of these various instabilities and their lifetimes and growth
rates.
