VARIATIONS IN SURFACE-ACTIVITY OF THE SUN AND SOLAR-TYPE STARS

Twenty-five-year records of relative Ca II H and K emission fluxes of lower Main-Sequence stars have been measured at Mount Wilson Observato ry and reveal surface activity in most of the older G- and K-type dwar f stars that is similar to the aperiodical activity cycle of the conte mporary Sun (i.e., the cyclic and the occasional episode of reduced ac tivity in the past few centuries). We find an inverse relationship bet ween the amplitude of the activity cycle and the length of the cycle f or the ensemble of those solar-type stars. We also find a similar rela tionship using the 250-year sunspot record (Cycles 1 to 21). The simil arity between the two inverse relationships for the solar-type stars o bserved for 25 years and the Sun for a longer interval of time may sug gest one common underlying physical mechanism that is responsible for the variations in surface activity ranging from decades to centuries.