Introduction

Many stars in the universe vary in brightness, the most common are the so called RR Lyrae-stars, that go through a complete brightness cycle in just one day. There are more than 2,500 of these identified so far in our galaxy alone. They all have more or less the same intrinsic brightness.

Another type of variable star is called a Cepheid variable. A Cepheid star cycle may take between 1 to 100 days. Yet other stars have brightness cycles that last 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 irregularly vary in brightness because of a physical change in their size and/or temperature. Some cepheid stars can even double in diameter during their cycle, but usually the change in size is only minor. During the expansion phase the star gets brighter and slowly becomes less intense during contraction. Stars are not cepheid variables all their lifetime, instead this occurs in normal stars that are going through a temporary and unstable period.

Star Clusters

The change in the brightness of stars is not always caused by the activity of the stars themselves. Often this variation in brightness is caused by a cluster of stars. About 50% of all stars of the universe belong to a binary star system, where two stars orbit each other due to their mutual gravity. The stars may come so close that they almost collide. Some multiple star systems are made up of more that just two stars.

From Earth we can observe the changes in brightness of a visual binary star if the plane of its orbit lies in our line of sight. The variations in brightness in this type of system can be explained by the fact that the stars overlap each other from time to time. Binary stars are best observed when the brighter star passes in front of the weaker star. At that moment, when the weaker star overlaps the brighter star, most of the light from the bright star is taken away and appears to shine less bright.