MIRA

'Mira', pronounced , also known as 'Omicron Ceti' (or ο Ceti / ο Cet), is a red giant star approximately 418 light-years away in the constellation Cetus. Mira is a binary star, Mira A being the giant, 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 odd Eta Carinae, Mira is the brightest periodic variable in the sky that is not visible during part of its cycle. Its distance is uncertain; pre-Hipparcos estimates centered around 220 light-years,(1) while Hipparcos data suggests a distance of 417 light-years, albeit with a margin of error of ~14%.
The companion star was resolved by the Hubble Space Telescope in 1995, when it was 70 astronomical units from the primary; results were announced in 1997. The HST ultraviolet images and later X-ray images by the Chandra space telescope show a spiral of gas rising off Mira in the direction of Mira B. The companion's orbital period around Mira is approximately 400 years.
Mira became the "type" star of the long-period "Mira variables". It — and the other 6000 or so known stars of this class — are all red giants whose surfaces oscillate in such a way as to increase and decrease in brightness over periods ranging from about 80 days to more than 1000. In the particular case of Mira, its increases in brightness take it up to about magnitude 3.5 on average, which is a fairly noticeable star. Individual cycles vary too; well-attested maxima go as high as magnitude 2.0 in brightness and as low as 4.9, a range almost 15 times in brightness, and there are historical suggestions that the real spread may be three times this or more. Minima range much less, and have historically been between 8.6 and 10.1, a factor of four times in luminosity. The total swing in brightness from absolute maximum to absolute minimum (two events which did not occur on the same cycle) is 1700 times. Interestingly, since Mira emits the vast majority of its radiation in the infrared, its variability in that band is only about two magnitudes.(2) The shape of its light curve is of an increase over about 100 days, and a return twice as long. A recent lightcurve is available from the BAV ^[1].
In 2007, observations showed a protoplanetary disc around the companion, Mira B. This disc is being accreted from material in the solar wind from Mira and may eventually go on to form new planets. These observations also revealed that the companion is most likely a main sequence star of around 0.7 solar masses and spectral type K, instead of a white dwarf as previously believed ^[1].
Ultra-violet studies of Mira by NASA's Galaxy Evolution Explorer (Galex) space telescope have revealed that it sheds a trail of material from the outer envelope, creating a tail 13 light-years in length, formed over the last 30,000 years ^[3][4]. It is thought that the hot bow-wave of compressed plasma/gas (resulting from Mira's extremely high speed of 130km/sec) is the cause of the shed material ^[5]. This bow wave is also visible in ultra-violet observations.

Contents

Discovery

References

See also

External links

Discovery

Mira was discovered (or at least noted as special for the first time) after a series of observations 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 ^[1].
Eventually, Johann Holwarda determined a period of the star's reappearances, eleven months; Johannes Hevelius was observing it at the same time and named it "Mira" (meaning "wonderful, astonishing") 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).
There is considerable speculation as to whether Mira had been observed prior to Fabricius. Certainly Algol's history (known for certain as a variable only in 1667, but with legends and such dating back to antiquity showing that it had been observed with suspicion for millennia) suggests that Mira might have been known too. Karl Manitius, a translator of Hipparchus' ''Commentary on Aratus'', has suggested that certain lines from that second century BC text may be about Mira. The other pre-telescopic Western catalogs of Ptolemy, al-Sufi, Ulugh Beg, and Tycho Brahe turn up no mentions, even as a regular star. There are three observations from Chinese and Korean archives, in 1596, 1070, and the same year when Hipparchus would have made his observation (134 BC) that are suggestive, but the Chinese practice of pinning down observations no more precisely than within a given Chinese constellation makes it difficult to be sure.

References

1.

2.

3.
A turbulent wake as a tracer of 30,000 years of Mira's mass loss history, , Christopher, Martin, Nature, 2007

4.
Minkel, JR.
"Shooting Bullet Star Leaves Vast Ultraviolet Wake", "The Scientific American", August 15 2007. Accessed August 21 2007.

5.
GALEX finds link between big and small stellar blasts
6.



★

★

★

★ Robert Burnham Jr.,''Burnham's Celestial Handbook'', Vol. 1, (New York: Dover Publications, Inc., 1978), 634.

★ James Kaler, ''The Hundred Greatest Stars'', (New York: Copernicus Books, 2002), 121.

See also

★ Mira in fiction

External links

★ Mira – Bizarre Star at NASA

★ History of Mira's Discovery at AAVSO

★ Mira has tail nearly 13 light years in length (BBC)

★ Speeding Bullet Star Leaves Enormous Streak Across Sky at Caltech

★ Astronomy Picture of the Day:
1998-10-11, 2001-01-21, 2006-07-22, 2007-02-21, 2007-08-17