Long-Period Red Variables

Another important class of pulsating variables consists of the long-period red variables. These are red supergiants, the largest stars of all. They also are called irregular red variables, because they typically do not pulse with a completely regular period.
Long Pulsation Periods
Since these variables are red supergiant stars, by our previous arguments they should have low densities and thus long periods. Indeed, the periods for stars in this class are even longer than for Cepheid variables; they typically pulsate with periods of hundreds of days. The reason for the pulsation is not well known, but may be associated with internal ionization zones that alter the opacity and thus the energy flow in the star.

Mira the Wonderful
Some long-period variables exhibit dramatic light changes, since their brightness can change by as much as 7 magnitudes over a cycle. Indeed, the most famous red variable, Omicron Ceti, which was the first variable star discovered (in 1596 by German clergyman Johann Goldsmit, 14 years before the telescope was invented), changes from visible to invisible for a naked-eye observer over its period because it varies from 9.3 to 3.5 apparent visual magnitude. For this reason, it is sometimes called Mira ("wonderful" in Latin). The lightcurve for Mira is shown in the top right figure; its period is around 11 months. Red variables with characteristics similar to Omicron Ceti are sometimes called Mira variables. Their periods can range from several months up to about 2 years.