Variable or Ceiphid Stars

When a Red Giant or Supergiant has burned up most of its fuel it becomes unstable. It shrinks and grows and shrinks again in a regular manner. The brightness of the star also changes, and this can be observed and plotted on a graph.  
It turns out that the time (or period) between minimum brightness depends on the size of the star. The bigger the star the longer the time. 
The size and absolute magnitude of a far pulsating star can be obtained by comparing its period with known stars. 
The distance to the star can then be calculated by comparing its absolute and apparent magnitudes. 

Right click on the image and choose reload to see it again.

ceiphid star animation
Animation by S.Mack & Gif Movie Gear
 Links