Mira stars(Mira Ceti variables) Long-period pulsating variables, either red giants or red supergiants, that have periods ranging from about 80 to 1000 days and a range in brightness beginning at about 2.5 magnitudes and sometimes exceeding 10 magnitudes. They are numerous, Mira Ceti being the prototype. The shape of the light curve is not constant, the maximum brightness varying quite considerably between periods (see Mira). Because of the large amplitude they are easily recognizable, their high luminosity permitting detection at great distances.
Although their visual range is large, the range in bolometric and infrared magnitudes is very much less, many of them being infrared sources. In terms of their spectra 90% can be classified as Me stars, the rest being either carbon stars (Ce) or zirconium (Se) stars, i.e. there are bright emission lines present in the spectra in addition to molecular bands. The pulsations of these huge stars are not very stable. There is evidence of shock waves developing within the tenuous atmosphere and traveling outward, thus heating the gas and causing the production of emission lines. The expanding envelopes often contain condensed dust grains, which produce detectable infrared emission, and simple molecules: Mira stars often show maser emission from hydroxyl, water, and silicon monoxide molecules in the outer atmosphere. See also OH/IR star.