Mira

Mira as seen by Hubble. NASA image.
Observation data Epoch J2000.0
Constellation (pronunciation)	Cetus
Right ascension	02h 19m 20.7927s[1]
Declination	-02° 58′ 39.513″[1]
Apparent magnitude (V)	2.0 to 10.1
Characteristics
Spectral type	M7 IIIe[2]
U-B color index	+0.08[3]
B-V color index	+1.53[3]
Variable type	Mira variable
Astrometry
Radial velocity (Rv)	+63.8[1] km/s
Proper motion (μ)	RA: 10.33[1] mas/yr Dec.: -239.48[1] mas/yr
Parallax (π)	7.79 ± 1.07[1] mas
Distance	approx. 420 ly (approx. 130 pc)
Absolute magnitude (MV)	0.93
Details
Mass	1.18[4] M☉
Radius	332–402[5] R☉
Luminosity	8400–9360[5] L☉
Temperature	2918–3192[5] K
Age	6×109[4] years
Other designations
Stella Mira, Collum Ceti, Wonderful Star,[6] Omicron Ceti, 68 Ceti, HR 681, BD −03°353, HD 14386, LTT 1179, SAO 129825, HIP 10826.[1]

Mira, pronounced /ˈmaɪrə/, also known as Omicron Ceti (or 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,(1) 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.^[7] 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 ^[8].

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 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.

System

Component A

Mira A generated energy by nuclear fusion of hydrogen to form helium at its core. Once the supply of hydrogen at the core was exhausted, fusion of hydrogen continued along a shell surrounding the inert helium core. This shell generated more energy from fusion than did the core, so the luminosity of Mira A increases. However, at the same time, the outer atmosphere of Mira A expanded to many times its original size, producing a red giant.^[9]

As additional helium was generated by the hydrogen-burning shell, the dormant helium core of Mira A steadily increased in mass. Once the core reached a temperature and pressure sufficient to begin burning helium, Mira A underwent a runaway process called the helium flash. This initiates the fusion of helium in the core, producing an ash of carbon and oxygen. Gradually the core expanded and cooled, and decreased energy was generated from hydrogen-burning. This caused the luminosity of the star to decrease, while the outer atmosphere shrank and increased in temperature. This stage of the star's evolution is called the horizontal branch.^[9]

When the supply of helium at the core became exhausted, a new helium-burning shell forms around the inactive core of carbon and oxygen. Once more Mira A began to expand, increasing in luminosity while the surface temperature decreased. This stage is known as the Asymptotic Giant Branch (AGB),^[9] and Mira A is currently at this phase of its evolution.^[10]

Mira as seen from the Earth

Once the temperature above Mira's helium burning shell rose to about 10⁷K, hydrogen became ignited along a shell at that radius. However, because energy is now less readily transported through the hydrogen burning region, pressure builds up between the layers. This causes the hydrogen burning layer to rise until the fusion is shut off by lower temperatures. Mira then contracts and the hydrogen layer re-ignites. The result is an instability in Mira known as the thermally pulsing AGB phase. Each pulse lasts a decade or more, and on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods.^[11]

The overall shape of Mira A has been observed to change, exhibiting pronounced departures from symmetry. These appear to be caused by bright spots on the surface that evolve their shape on time scales of 3–14 months. Observations of Mira A in the ultraviolet band by the Hubble Space Telescope have shown a plume-like feature pointing toward the companion star.^[10]

Variability

Mira A is a well-known example of a category of variable stars known as Mira variables, which are named after this star. It—and the other 6000^{[citation needed]} 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, placing it among the brighter stars in the Cetus constellation. 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..^[12]

Mass loss

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 tens of thousands of years ^[13][14]. It is thought that a hot bow-wave of compressed plasma/gas is the cause of the tail; the bow-wave is a result of the interaction of the stellar wind from Mira A with gas in the interstellar space, through which Mira is moving at an extremely high speed of 130 kilometres/second ^[15][16]. The tail consists of material stripped from the head of the bow-wave, which is also visible in ultra-violet observations.

Component B

Main article: Mira B

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.

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 ^[17].

References

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
• Mira. Alcyone Ephemeris. Retrieved on 2006-08-25.
• Interferometric observations of the Mira star o Ceti with the VLTI/VINCI instrument in the near-infrared.
• 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.
• SEDS article
• A recent lightcurve of Mira from the BAV.

Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Mira". Image Credit.

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