Mira variable

Mira variable. Credit: Sky & Telescope

A Mira variable is a cool giant star of spectral type Ke, Me, Se, or Ce (with molecular bands) that pulsates with a period of 80 to 1,000 days and varies in brightness by 2.5 to 11 magnitudes. Mira stars, also known as long-period variables, are named for their prototype, Mira (Omicron Ceti). Another well known example is R Leonis.


The brightness, large amplitude, and distinctive properties of Mira stars make them so easy to find that more of them are known than any other type of variable star. They occupy the high luminosity portion of the asymptotic giant branch in the Hertzsprung-Russell diagram, along with semi-regular variables.


Mira stars have masses similar to that of the Sun but, owing to their much greater size, have a feeble gravitational hold on the material in their outer layers which, as a consequence, escapes into space in the form of a strong stellar wind at a rate of about 10-7 to 10-6 solar mass per year. Material thus shed accumulates around the star as an extensive circumstellar shell. The rate of mass loss is such that the Mira stage can only last about a million years before the aging star evolves to become a white dwarf surrounded by a planetary nebula. The Sun itself is destined to become a Mira star within the next few billion years.