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 (CMD) for 73000 stars brighter than M_I,814_=-5 in the Whirlpool Galaxy (M51). Our Cycle 24 Hubble Space Telescope (HST) program acquired V606- and I814-band images over 34 epochs spanning 1 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,814_=-7 show strong evidence for variability within our Cycle 24 data; among stars with V_606_-I_814_>2 the variability fraction rises to ~100%. Large amplitude variability (>0.3mag) on decade timescales is restricted to red supergiants (RSGs) 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 CMD broadly agrees with theoretical sources of variability, including the instability strip, RSG 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.

Cone search capability for table J/ApJ/864/111/catalog (Catalog of stars and derived parameters)

Cone search capability for table J/ApJ/864/111/lcs (*HST cycle 24 V- and I-band measurements for the 72623 stars; table added by CDS)