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Stellar photometric variability offers a novel probe of the interiorstructure and evolutionary state of stars. Here we present a census of stellarvariability on day to decade timescales across the color-magnitude diagram for73,000 stars brighter than $M_I$=-5 in the Whirlpool Galaxy (M51). Our Cycle 24HST program acquired V and I-band images over 34 epochs spanning one year withpseudo-random cadences enabling sensitivity to periods from days to months. Wesupplement these data with archival V and I-band HST data obtained in 1995 and2005 providing sensitivity to variability on decade timescales. At least 50% ofstars brighter than $M_I$=-7 show strong evidence for variability within ourCycle 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 tored supergiants and very luminous blue stars. Both populations display fairlysmooth variability on month-year timescales. The Cepheid instability strip isclearly visible in our data, although the variability fraction within thisregion never exceeds ~10%. The location of variable stars across the colormagnitude 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 inmassive stars. Our data can be used to place stringent constraints on theprecise onset of these various instabilities and their lifetimes and growthrates.

A Complete Census of Luminous Stellar Variability on Day to Decade Timescales