  We analyze the effect of solar features on the variability of the solar
irradiance in three different spectral ranges. Our study is based on two
solar-cycles' worth of full-disk photometric images from the San Fernando
Observatory, obtained with red, blue and Ca II K-line filters. For each image
we measure the photometric sum, Sigma, which is the relative contribution of
solar features to the disk-integrated intensity of the image. The photometric
sums in the red and blue continuum, Sigma_r and Sigma_b, exhibit similar
temporal patterns: they are negatively correlated with solar activity, with
strong short-term variability and weak solar-cycle variability. However, the Ca
II K-line photometric sum, Sigma_K, is positively correlated with solar
activity and has strong variations on solar-cycle timescales. We show that we
can model the variability of the Sun's bolometric flux as a linear combination
of Sigma_r and Sigma_K. We infer that, over solar-cycle timescales, the
variability of the Sun's bolometric irradiance is directly correlated with
spectral line variability, but inversely correlated with continuum variability.
Our blue and red continuum filters are quite similar to the Str\"omgren b and y
filters used to measure stellar photometric variability. We conclude that
active stars whose visible continuum brightness varies inversely with activity,
as measured by the Ca HK index, are displaying a pattern that is similar to
that of the Sun, i.e. radiative variability in the visible continuum that is
spot-dominated.
