Our Sun follows an 11-year cycle of low-to-high activity. During high activity, there are much more sunspots and much higher chance of flares and other disruptive behavior.
Can we observe the activity of other stars? I'm particularly interested in other G-types like the Sun. Do they also follow an 11-year cycle?
The answer is yes. Early work showed that just over half of sun-like stars exhibit magnetic activity cycles similar to the Sun. These cycles can be monitored by looking at chromospheric activity, measured in the emission line cores of their Ca II H and K lines; by looking at spot modulation of their light curves; or for a few stars, looking at their coronal X-ray emission over time.
Subsequent work has found cycles in most stars where there is good data (Olah et al. 2016). The cycles vary in length from star-to-star, where they have been found. Some stars show multiple cycle periods (e.g. Olah et al. 2009). There is obviously an observational bias towards finding shorter periods, but the Sun's cycle is towards the longer period end of what has been found. There seems to be a rough correlation between cycle period and rotational period - the cycle period is 100-300 times as long as the rotation period.
An excellent short review is given by Korhonen (2015).
