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Mira, pronounced /ˈmaɪrə/, also known as Omicron Ceti (ο Ceti, ο Cet), is a red giant star estimated 200-400 light years away in the constellation Cetus. Mira is a binary star, consisting of the red giant Mira A along with Mira B. Mira A is also an oscillating variable star and was the first non-supernova variable star discovered, with the possible exception of Algol. Apart from the unusual Eta Carinae, Mira is the brightest periodic variable in the sky that is not visible to the naked eye for part of its cycle. Its distance is uncertain; pre-Hipparcos estimates centered around 220 light-years,[7] while Hipparcos data suggests a distance of 418 light-years, albeit with a margin of error of ~14%.


Observation history

Evidence that the variability of Mira was known in ancient China, Babylon or Greece is at best only circumstantial.[8] What is certain is that the variability of Mira was recorded by the astronomer David Fabricius beginning on August 3, 1596. Observing the planet Mercury, he needed a reference star for comparing positions and picked a previously unremarked third-magnitude star nearby. By August 21, however, it had increased in brightness by one magnitude, then by October had faded from view. Fabricius assumed it was a nova, but then saw it again on February 16, 1609.[9]

In 1638 Johannes Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful" or "astonishing," in Latin) in 1662's Historiola Mirae Stellae, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days, and perfectly forgivable, as Mira is known to vary slightly in period, and may even be slowly changing over time. The star is estimated to be a 6-billion-year-old red giant.

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