Long-term solar brightness changes estimated from a survey of Sun-like stars

THE brightness of the Sun varies during the 11-year solar cycle, typically by less than 0.1% (refs 1, 2), and a larger brightness variation is thought to have occurred during the Maunder minimum, from AD 1645 to 1715 (refs 3-5). But because individual solar cycles are different in form, amplitude and length, and because accurate solar data have been available only for the most recent two or three cycles, there is no direct way of understanding long-term solar variability. Here we present a compilation of eight years of observations of 33 Sun-like stars and report year-to-year brightness changes that substantially exceed the analogous solar fluctuations. We have also measured chromospheric magnetic activity in these stars and find that it correlates with the brightness variations. During 1980-88, solar chromospheric variability was comparable to that observed in the stellar survey, but solar brightness variations were only one-quarter as large. This suggests that the Sun is in an unusually steady phase compared to similar stars, which means that reconstructing the past historical brightness record, for example from sunspot records, may be more risky than has been generally thought.