Under the assumption that magnetic activity is due to the action
of a magnetic field in a rotating star with a convective zone, it is
possible to draw the following picture of a sequence of events in the
Hertzsprung-Russell diagram.
The H-R diagram is divided into two regions by an almost vertical
line. Stars of later type have a hydrogen convective zone of great
extension (region C); stars of earlier type have no hydrogen convective
zone or a convective zone of small extension (region D).
Stars in region C show stellar activity of electrodynamic origin
like solar activity; stars in region D show little or no stellar activity.
When matter is ejected by a star, the magnetic field compels the
matter to rotate with the star even at very large distances where it
carries away large angular momentum per unit mass. Stars which reach the
main sequence in region C may lose a large amount of angular momentum,
while stars reaching the main sequence in region D lose only a little
