Variable stars are stars that change brightness. They are very common in the Galaxy and, for that matter, in other galaxies too. Some behave in a completely predictable manner, while others are always liable to take us by surprise.

The Mira Star System
The Chandra x-ray image shows Mira (right) and its companion, VZ Ceti (left). To the right of the image is an artist’s conception of the Mira star system. NASA/CXC/SAO/M. Karovska et al./M. Weiss

The first star to be discovered to vary in a periodic manner was Omicron Ceti, better known as “Mira” (“the Wonderful”). In August 1596, the German astronomer David Fabricius recorded a 3rd-magnitude star in the constellation of Cetus the Whale, not far from the celestial equator. By October the same year, the star had disappeared. Bayer saw it again in 1603, when he was drawing up his star catalogue, and gave it the Greek letter Omicron, but shortly afterwards it vanished once more.

Not until some time later was it found that the star appears with fair regularity; it takes approximately 332 days to pass from maximum to maximum, and is visible to the naked eye for many weeks at a time. When at its best, Mira alters the whole aspect of the constellation Cetus.

Mira’s mean range in brightness is +3 to +10 magnitudes. At some maxima, the star has been known to exceed magnitude +2, though at others it barely rises above +4. Although the period of variation is well determined, the magnitude of a future maximum or minimum is not predictable.

Mira is the archetype for an important class of variable stars, with long periods and large-amplitude variations in brightness. All are pulsating red giants or supergiants, usually of spectral type M, S or C.

Mira Ceti lies at a distance of approximately 300 light years. In 1923, the American astronomer Robert Grant Aitken discovered a faint companion to Mira, now known as VZ Ceti. This star orbits Mira in 500 years and also shows variations in brightness, in the range of magnitude +9.5 to +12. The variations seem to be related to accretion of matter from Mira’s stellar wind, which makes VZ Ceti a symbiotic star.

Finder map – field width 90°, stars to magnitude +5.