Mira variables //, named after the prototype star Mira, are a class of pulsating variable stars characterized by very red colors, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths. They are red giant stars in the very late stages of stellar evolution, on the asymptotic giant branch, that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.
Mira variables are stars massive enough that they have undergone helium fusion in their core but are less than two solar masses, stars that have already lost about half their initial mass. However, they can be thousands of times more luminous than the Sun due to their very large distended envelopes. They are 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 depends on the mass and radius of the star and there is a well-defined relationship between period and luminosity (and colour).  The very large visual amplitudes are not due to large luminosity changes, but due to a shifting of energy output between infra-red and visual wavelengths as the stars change temperature during their pulsations.
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 helium near the core of the star temporarily 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 at most) over the asymptotic giant branch lifetime of the star (less than a million years), means we only see it in a few of the several thousand Mira stars known, possibly in R Hydrae. Most Mira variables do 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.
- Glass, I.S.; Lloyd Evans, T. (1981). "A period-luminosity relation for Mira variables in the Large Magellanic Cloud". Nature (Macmillan) 291: 303–4. Bibcode:1981Natur.291..303G. doi:10.1038/291303a0.
- Bedding, T. R.; Zijlstra, A. A. (1998). "\ITAL]Hipparcos\/ITAL] Period-Luminosity Relations for Mira and Semiregular variables". The Astrophysical Journal 506: L47. arXiv:astro-ph/9808173. Bibcode:1998ApJ...506L..47B. doi:10.1086/311632.
- First Surface-resolved Results with the IOTA Imaging Interferometer: Detection of Asymmetries in AGB stars, 2006
- Optical aperture synthetic images of the photosphere and molecular atmosphere of Mira, 1992
- Asymmetries in the atmosphere of Mira, 1991
- Surface imaging of long-period variable stars, 1999
- The evolution of the Mira variable R Hydrae