Mira variables are believed to be stars with less than two solar masses, but can be thousands of times more luminous than the Sun due to their very large, distended envelopes. They are believed to be pulsating in radial modes, in which the entire star expands and contracts in spherical symmetry. This results in a change in both radius and temperature, causing the change in luminosity. The pulsation period is a function of the mass and radius of the star.
Though most Mira variables share many similarities in behavior and structure, they are a heterogeneous class of variables due to differences in age, mass, pulsation mode, and chemical composition. For example, many have spectra dominated by carbon, suggesting that material from the core of the star has been transported to the surface. This material often forms dust shrouds around the star, which also contribute to periodic dimming and brightening. A few Mira variables are also known to be natural maser sources.
A small subset of Miras appear to change their period over time -- the period increases or decreases by a substantial amount (up to a factor of three) over the course of several decades to a few centuries. This is believed to be caused by thermal pulses, where a shell of hydrogen near the core of the star becomes hot and dense enough to undergo nuclear fusion. This changes the structure of the star, which manifests itself as a change in period. This process is predicted to happen to all Mira variables, but the relatively short duration of thermal pulses (a few thousand years) over the asymptotic giant branch lifetime of the star (a few million years), means we only see it in a few of the several thousand Mira stars known. However, most Mira variables exhibit slight cycle-to-cycle changes in period, probably caused by nonlinear behavior in the stellar envelope.
Mira variables are popular targets for amateur astronomers interested in variable star observations, because of their dramatic changes in brightness. Some Mira variables (including Mira itself) have reliable observations stretching back well over a century.