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Synoptic measurements of activity in Sun-like stars have been performed continuously since 1966, and the largest set comes from the Mount Wilson HK project, in the form of the well-known S index. We have been monitoring the activity and variability of the Sun and a large sample of Sun-like stars, in terms of S and absolute flux, since 1994 with the Solar-Stellar Spectrograph (SSS) at Lowell Observatory. Directly inspired by the similar long-term program at Mount Wilson Observatory, the SSS incorporates both an HK spectrograph and an echelle for visible and far-red observations. This is the first of three papers presenting the results of some 20,000 observations of the Sun and Sunlike stars with the SSS. In this paper we describe our program, review the calibration of solar and stellar fluxes to S and the chromospheric emission fraction R 0 HK , compare our derived stellar activity measures to those from other programs, and discuss the broad characteristics of the activity and variability in our target set, with particular attention to good solar analogs and noncycling stars. In subsequent papers we will discuss the echelle data and present detailed examinations of stars of particular interest.

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We examine patterns of variation of 32 primarily main sequence Sun-like stars (selected at project onset as stars on or near the main sequence and color index 0.42≤(B-V) ≤ 1.4), extending our previous 7-12 year time series to 13-20 years by combining Strömgren b, y photometry from Lowell Observatory with similar data from Fairborn Observatory. Parallel chromospheric Ca II H & K emission data from the Mount Wilson Observatory span the entire interval. The extended data strengthen the relationship between chromospheric and brightness variability at visible wavelengths derived previously. We show that the full range of photometric variation has probably now been observed for a majority of the program stars. Twenty-seven stars are deemed variable according to an objective statistical criterion. On a year-to-year timescale, young active stars become fainter when their Ca II emission increases while older less active stars such as the Sun become brighter when their Ca II emission increases. The Sun's total irradiance variation, scaled to the b and y stellar filter photometry, still appears to be somewhat smaller than stars in our limited sample with similar mean chromospheric activity, but we now regard this discrepancy as probably due mainly to our limited stellar sample.

We present 14 years of contemporaneous photometric and spectroscopic observations of 28 solar analog stars, taken with the Tennessee State University Automatic Photometric Telescopes at Fairborn Observatory and the Solar-Stellar Spectrograph at Lowell Observatory. These are the best observed and most nearly Sun-like of the targets in our magnitude-limited (V 7.5) sample. The correlations between luminosity and activity reveal the expected inverse activity-brightness correlations for active stars. Strong direct correlations between activity and brightness are not prevalent for the less active solar age stars, but are precision limited. The Sun does not appear to have unusually low photometric variability when compared with the most Sun-like inactive solar analogs. We present evidence that the activity index R HK is not a good discriminant of Maunder Minimum candidate stars. On the basis of a star that appears to have transitioned from a low-variability state to a cycling state, we investigate the regime in which stars might switch from faculae-dominated to spot-dominated variations.