Mira stars

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.

References in periodicals archive ?
For example, Mira stars only change slowly in brightness, perhaps only one magnitude in a couple of weeks at most.
Named in honor of Omicron ([omicron]) Ceti, Mira stars are cool, highly evolved red giants that have nearly depleted their hydrogen fuel.
However, Mira stars spin slowly and still retain their oblong appearance.
concludes that the average mass of the Mira stars and their semiregular relatives is about 15 solar masses."
Its long observational history has proved invaluable in understanding the internal mechanisms of Mira stars, because at just about the time its variations were detected, it was entering a dramatic stage of its life.
Mira stars are old and highly evolved, and their supplies of nuclear fuel are close to exhaustion.
R Hydrae may now be recovering from its shell flash, but we expect to see similar behavior in other Mira stars - ultimately, perhaps, in all of them.
Mira stars are red giants of roughly the Sun's mass that are in their unstable old age, soon to shed their outer layers entirely to expose the white dwarf growing in their cores.
Harding succeeded in finding the third asteroid, Juno, in 1804, but his subsequent searches turned up only new long-period variable stars, also known today as Mira stars. His discoveries included R Virginis in 1809, R Serpentis in 1826, and S Serpentis in 1828.
On the later occasion the accretion disk was faint, so the Mira star could still be seen shining dimly through the dark cloud.
Another theory, advanced by Joanna Mikolajewska and Scott Kenyon in 1992, associates the 44-year interval not with an orbital period but with helium-shell flashes occurring deep inside the Mira star above its degenerate core.
The total mass of R Lep cannot be measured directly, but most Mira stars are thought to have only a few times the mass of the Sun.