  We investigate the variability properties of main sequence stars in the first
month of Kepler data, using a new astrophysically robust systematics
correction, and find that 60% of stars are more variable then the active Sun.
We define low and high variability samples, with a cut corresponding to twice
the variability level of the active Sun, and compare the properties of the
stars belonging to each sample. We show tentative evidence that the more active
stars have lower proper motions and may be located closer to the galactic
plane. We also investigate the frequency content of the variability, finding
clear evidence for periodic or quasi-periodic behaviour in 16% of stars, and
showing that there exist significant differences in the nature of variability
between spectral types. Of the periodic objects, most A and F stars have short
periods (< 2 days) and highly sinusoidal variability, suggestive of pulsations,
whilst G, K and M stars tend to have longer periods (> 5 days, with a trend
towards longer periods at later spectral types) and show a mixture of periodic
and stochastic variability, indicative of activity. Finally, we use
auto-regressive models to characterise the stochastic component of the
variability, and show that its typical amplitude and time-scale both increase
towards later spectral types, which we interpret as a corresponding increase in
the characteristic size and life-time of active regions.
