2.2 Solar-type stars

Stars on the lower main-sequence are known to show chromospheric activity similar to that on the Sun which is detected, e.g., in the $Ca II H & K$ emission (Wilson, 1978 ). Shortly after the variability of the Sun in the visual continuum (total irradiance) was established during the Solar Maximum Mission (Willson et al., 1981 ), evidence for continuum variability among single solar-type stars was also revealed (Radick et al., 1982

, 1983a ,b ). The solar variations, which never exceed a few tenths of a percent, are clearly associated with the disk passage of sunspots and result from a blockage of radiant flux (e.g., Fröhlich

, 2002 ). The amplitude of the stellar variability can be as large as several percent in some cases. It appears to be analogous to the solar phenomenon and is caused by starspots. The survey by Radick et al.

revealed that with a precision of about $0.003 mag$ none of the variable stars had a spectral type earlier than F7 or later than K2, although the list of stars included A0 to K8 candidates. Thus, the starspot phenomenon in solar-type stars peaks seemingly at the effective temperature range from $6400 K$ to $4900 K$ .

Evidence linking the photometric variability of solar-type stars to the sunspot phenomenon is provided by the fact that continuum variability seems to occur in anti-phase with variations in $Ha$ and $Ca II H & K$ emission variations (Dorren and Guinan, 1982 ). The anticorrelation implies that the surface activity of such stars is confined to localised activity centres that include both emission plages and dark spots, similar to active regions observed on the Sun. Moreover, short-term stellar irradiance variations may be largely explained as rotational modulation by active regions which can persist for several rotation periods (Lockwood et al., 1984 ).

The onset of solar-type activity at F7 stars with subsurface convection zones was firmly established by the survey of Radick et al., while the reality of the limit at K2 seemed to be less certain. For instance, a higher precision of $0.001 mag$ achieved aboard the Hubble Space Telescope allowed for the detection of periodic brightness variations of the red dwarf Proxima Cen (V645 Cen, M5Ve) with an amplitude of $0.01 mag$ and a period of $41.6 d$ , which were interpreted as rotational modulation of starspots in the stellar photosphere (Benedict et al., 1993 ).

It was firmly established that magnetic activity in solar-type stars declines with age and that it is closely related to a loss of angular momentum throughout the main-sequence lifetime (Skumanich, 1972 ; Noyes et al., 1984 ; Baliunas et al., 1995 ; Güdel et al., 1997 ). Thus, young stars exhibit high average levels of activity and rapid rotation, while stars as old as the Sun and older have slower rotation rates and lower activity levels.