Luminosity Variations of Stars Similar to the Sun

Using precision differential photoelectric photometry at 472 and 551 nm, the variability characteristics of 36 solar-type stars spectral types F, G, and K and their 65 comparison stars were determined from nearly 2000 measurements made on more than 350 nights between March 1984 and December 1987. Each observation consists of four cycles two at each wavelength of intercomparison of the brightnesses of a trio or quartet of stars, thus producing 12 or 24 pairwise differential magnitudes. Program stars and comparison stars within each trio or quartet group are measured identically. The precision of nightly measurement cycles is typically 0.2 rms, and the median range of annual mean brightness over four seasons is 0.2 for stars whose output appears to be intrinsically constant. The principal conclusions are 1 about one-half the program stars and one-fourth the comparison stars are demonstrably variable on short-time scales at levels typically 0.3 and greater, 2 variability is about twice as common among K stars as among F- and G-type stars, 3 rotational modulation was measurable in nine stars, 4 sixteen program stars and eight comparison stars varied by more than 0.5 over four seasons, 5 twenty-six pairs of exceptionally stable stars varied by less than 0.3 over four years, 6 variability of the program stars is strongly correlated with independently determined chromospheric activity levels, and 7 inter-season variability is more consistent with long-term cyclic variation than with stochastic fluctuations.