Surface Differential Rotation & Butterfly Diagram
The above gif image shows how the star like the Sun show a butterfly pattern, and how it is helpful to study Surface Differential Rotation of the star. The visibility of photospheric star-spots is modulated by stellar rotation which causes quasi-periodic brightness variations on time-scales of the order of the rotational period. The modulation period indicates the angular velocity of the latitude at which starspot activity is predominantly centred. Since the circumpolar spots will not affect the rotational modulation, with an inclination angle (i), any modulation observed on stellar surface would be only due to the spot groups present within a latitude of ± i degrees from the stellar equator. Similar to the solar case, the year to year variations of the rotational period can be described as the migration of stellar activity centres towards latitudes possessing different angular velocity. This migration is caused by the internal radial shear, which is assumed to be coupled with observed latitudinal shear (e.g. in α − omega dynamo model).