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 due to the entire star expanding and contracting. This produces a change in temperature along with radius, both of which factors cause the variation in luminosity. The pulsation period is a function of the mass and radius of the star. Early models of Mira stars assumed that the star remained spherically symmetric during this process (largely to keep the computer modelling simple, rather than for physical reasons). A recent survey of Mira variable stars found that 75% of the Mira stars which could be resolved using the IOTA telescope are not spherically symmetric, a result which is consistent with previous images of individual Mira stars, so there is now pressure to do realistic three dimensional modelling of Mira stars on supercomputers.
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, such as R Leporis 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 including deviations from spherical symmetry.
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.