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Mira variables are a class of pulsating variable stars characterized by very red colors, pulsation periods longer than 100 days, and light amplitudes greater than one magnitude. They are red giant stars in the very late stages of stellar evolution (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 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 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.
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