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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, such as R Leporis have
spectra dominated by carbon, suggesting that material fromthe 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 periodA period is an arbitrary interval of time. The word is applied to many different concepts: generally, in science, the time taken for one complete cycle of an alternating quantity. The period of oscillation of a wave is the time taken for the wave to compl 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 astronomersAmateur astronomy often called back yard astronomy is a hobby whose participants enjoy observing celestial objects. It is usually associated with viewing the night sky when most celestial objects and events are visible, but sometimes amateur astronomers a interested in variable starobservations, because of their dramatic changes in brightness. Some Mira variables (including MiraMira Omicron Ceti, is an oscillating variable star. The first non- supernova variable star discovered, excepting the odd Eta Carinae, it is the brightest periodic variable in the sky that is not visible during part of its cycle. Mira was discovered (or at itself) have reliable observations stretching back well over a century.
Variable stars