The Mira variable stars are defined as M-type stars with periods of 80 - 1000 days and amplitudes greater than 2.5 magnitudes. The spectra of these giant pulsating stars show dramatic changes during the pulsation cycle, and even show spectral differences from cycle to cycle. Shock waves in the pulsating atmospheres of these stars produce emission lines in their spectra. The hydrogen lines are usually in emission, with the emission strength increasing toward maximum light, although the Balmer decrement is quite often unusual. Note that in S Leo, H and H are quite strong, but H is not visible. In R Leo, the hydrogen lines are not strongly in emission, but emission lines of Fe I (4202, 4308) and Mg I 4571 are visible. In many stars the hydrogen-line emission strength is in antiphase with the emission lines of Fe I and Mg I.
Mira variable stars can also show a number of other interesting phenomena. Many Mira stars show a "washed-out" appearance to their spectra at the faintest phases in which the TiO bands lose contrast, and the atomic lines appear broader and shallower. This phenomenon has been referred to as "veiling", and may be due to the formation of high-level atmospheric clouds. In addition, Miras can show spectral bands due to AlO at certain phases.
This figure uses 3.6 Å resolution flux spectra.