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Mira, Omicron Ceti

Omicron Ceti, Mira Variable in Cetus

[Mira amateur image]
Right Ascension 02 : 19 : 20.7 (h:m:s)
Declination -02 : 58 : 39 (deg:m:s)
Distance 400 (ly)
Visual brightness 2.0 .. 10.1 (mag)
Period 331.96 (days)
Spectra type M7IIIe

Mira was the second variable and the first pulsating one to be discovered, on August 13, 1596 by David Fabricius (1564-1617), a disciple of Tycho Brahe and amateur astronomer, and then regarded as a nova because it appeared and faded from view later. Before its discovery, prediscovery sightings have been recorded, first by Hipparchus (134 B.C.), then in 1070 by Chinese observers, and possibly in 1592 and/or 1594 by Korean observers. In 1603, it was seen, measured and cataloged by Johann Bayer in his Uranometria as star of 4th magnitude, and labelled Omicron Ceti by him. R.H. Allen (1899) reports that its discoverer, Fabricius, observed it again on February 15, 1609. Its periodic variability was only discovered by Jan Fokkens (Johann Phocylides) Holwarda (1618-1651) of Friesland/Holland, who discovered it again in 1638 and observed it subsequently in the years following; he derived a period of about 11 months (first derived 1639). Johann Hevelius observed it from 1659 to 1682, inserted it in his monumental work, Prodomus Astronomiae, and christened it "Mira", "the Wonderful", in his Historiola Mirae Stellae of 1662. The period of Mira was first determined more acurately by Ismail Bouillaud of Paris to be 333 days, only about 1 day deviating from the modern value, no wonder because this period is subject to small irregular variations.

William Herschel found its brightness "almost equal to Aldebaran" (0.85 mag) on November 6, 1779, and its color being a deep garnet like Mu Cephei in 1783. According to Allen, S.C. Chandler gave its maxima in the 1890s as varying between 1.7 and 5.0 mag (1897: 3.0 mag), and the minima between 8,0 and 9.5.

Mira is the brightest and most famous long-period pulsating variable in the sky, and gave the name to this whole class of stars. It changes its brightness normally between maxima of about 3rd magnitude and minima of about mag 10, but occasionally brighter maxima up to mag 2.0 are observed (e.g. by William Herschel), or fainter when Mira stays at about magnitude 5. At a distance of about 400 light years, this corresponds to absolute magnitudes of about -2.5 near the maxima and +4.7 near its minima, so giant cool Mira is only about as, or even less luminous than our sun near its minima, but brightens up to about 700 and occasionally even over 1500 solar luminosities near the maximum of its cycle.

Mira is also the dominant component of a double star, which is separated by only 0.6 arc seconds. As the companion orbits Mira in about 400 years, it has now just once orbited the star since Fabricius discovered its variability. The linear distance was given as about 70 Astronomical Units, i.e. 70 times the distance between Earth and Sun. The companion is probably a white dwarf in interaction with Mira, which is surrounded by an accretion disc of material which it has captured away from the red giant Mira, and which may well be brighter than the companion star itself. This companion has a brightness which also varies, between 9.5 and 12 visual magnitudes (its variable star designation is VZ Ceti). Its variation is rather complicated: A slow variation of about 13 years period is superimposeds by rapid fluctuations over minutes, and occasionally a rare flare of some minutes duration. CZ is currently coming even closer to Mira, to about 0.1 arc seconds at its periastron in 2001; their separation has been about 1.7 arc seconds around 1800. Would the companion be closer, this system would be classified as a symbiotic star (like R Aquarii).

The membership in a binary makes Mira interesting because it allows for a variety of physical investigations, such as mass determination.

Astronomers using the Hubble Space Telescope have determined Mira's angular diameter as about 60 milli-arc seconds, corresponding to 700 times the linear diameter of our sun.

In 2007, Mira was photographed by the Galaxy Evolution Explorer (GALEX) spaceborne observatory, in the ultraviolet light. These photos revealed, as a big surprise, that Mira possesses a comet-like tail of about 13 light-years length. This tail is probably composed of material that was once ejected by Mira, over the past 30,000 years. While it is not immediately certain, the formation of a tail may have to do with Mira's comparatively high velocity with respect to the surrounding stars and part of the Milky Way galaxy: Mira moves at about 130 km/s.

Our Mira image was obtained by amateur Jack Schmidling.

Because of the form of its light curve, Mira is roughly visible to the naked eye (i.e., brighter than about 6th mag) for 1.5 months before and 2.5 to 3 months after its maximum. Recent and upcoming Mira maxima: References:


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Hartmut Frommert
Christine Kronberg
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