Many stars vary in brightness, there are many types of these variabilities. Most common are the so called RR Lyrae-stars, that change in brightness within one day. More than 2,500 of this type of stars are known in our galaxy, they all have more or less the same intrinsic brightness.
With other types, the Cepheid variables, the cycle may take between one to one hundred days. Yet other stars have brightness cycles of more than 2 years, these are called the Myra-stars. Mostly these are red giants or supergiant stars.
Cepheid variable stars.
Cepheid variables are supergiant stars that periodically or irragulary vary in brightness because of physical change in size and temperature.
Some cepheids even double in diameter during their pulsating cycle, but usually the change in size is only minor. During the expansion phase the star gets brighter, while the radiation intensity decreases during contraction.
Stars are not cepheid variable all their lifetimes, actually these are normal stars in a temporary unstable period.
The change in the brightness of stars is not always caused by the activity of the stars themselves. Often the variety is caused by a cluster of stars. About 50% of all stars of the Universe belong to a binary star system, where two suns revolve about their common center of mass. The pair is held together by their mutual gravitational attraction. Both stars may come so close to each other that they almost come into collision, others may be millions of kilometers away from each other.
Some multiple starssystem comprise more than two suns, such as the triple stars with three suns or the quadruple stars with four.
From Earth we can observe the changes in brightness of a visual binary star if the plane of its orbit lays in the line of sight. The variations in brightness are then caused by the fact that the stars overlap each other from time to time. The binary stars are observed best when the brightest star passes in front of the weaker star. At the moment the weaker star overlaps the brightest, most light of the brightest is taken away and so the 'star' shines less bright.