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.
Multiple
stars.
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.