In 1966, Olin Wilson began making monthly measurements at Mount Wilson Observatory of the relative fluxes in the Ca II H (396.8 nm) and K (393.3 nm) emission cores of approximately 100 stars on or near the lower main sequence. In the late 1970's the Ca II fluxes of nearly 1,000 lower main-sequence stars were sampled, and by 1980 the program had expanded to include near-nightly observations of the stars in Wilson's sample. In 1984 the project was again extended to include the measurement of post-main sequence stars. Today, the project monitors the Ca II fluxes of 400 dwarf and giant stars, with great emphasis on stars close in mass and age to the Sun. The relative Ca II fluxes are thought to closely correspond to the strength and coverage of surface magnetism on such stars. Three general classes of long-term variations of surface magnetism have been seen in lower main sequence and post main sequence stars: 1. substantial fluctuations on time scales of a few years, with little observed repitition of periodicity; 2. nearly-periodic variations with time scales of a decade or more, with some variability in the amplitude, length and shape of each successive cycle; 3. either low-amplitude modulation on time scales of several decades or more, or essentially no long-term variability. In the lower main-sequence stars both the class of long-term variability and the time-averaged level of Ca II fluxes are influenced primarily by a star's angular momentum. In a related matter, most of the detected extra-solar planets (with orbital periods ranging from 3 to 1200 days) orbit sun-like stars with long-term Ca II flux records that are virtually flat (Class 3, above). The lack of variability is an observational bias that enhances detection of extra-solar planets orbiting sun-like stars.
Cool Stars, Stellar Systems, and the Sun
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