The Sun in Time: Starspots and Luminosity Variations of the Solar-type Stars of Different Ages

We report on progress with the "Sun in Time" project, a coordinated multi-wavelength study of a sample of nearby, single, solar-type stars selected as proxies for the Sun at different stages in its life, from ZAMS (Zero Age Main Sequence) to the TAMS (Terminal Age Main Sequence). The stars observed have spectral types of G0V to G5V and measured rotation periods. The ages of many of the stars are determined from memberships in star clusters or moving groups. Other stars have ages determined from isochronal fits using evolutionary models and some have ages inferred from activity-age and rotation-age relationships. Observations of these stars have been carried out starting in 1988 from X-ray to Radio wavelengths. The X-ray and EUV and radio observations are used to study the coronal properties of the stars, the transition regions and chromospheres are studied primarily from the FUV-NUV observations while the stellar photospheres and starspot activity are investigated from the ground-based UBVRI or Str\H{o}mgren uvby photometry. Here we primarily focus on the results of the photometric studies that have been carried out since 1988. The photometry is being used to determine the rotational periods and the presence of differential rotation of the more active stars. Modeling of the light curves yields estimates of the fractional coverage of the stars' surfaces with starspots, the temperatures of the starspots, and distribution of starspots over the surfaces of the stars. The youngest stars in the sample have relatively large rotationally induced brightness variations (0.03-0.09 mag) while stars with ages greater than about 2 Gyr have light variations typically much less than 0.01 mag. Several of the stars show long-term changes in brightness arising from probable starspot cycles. The results of modeling and implications for activity of the young Sun will be discussed. This research is supported by grants from NSF/RUI and NASA which we gratefully appreciate.