Mira

Mira
Mira 1997.jpg
Mira as seen by Hubble. NASA image.
Mira the star.jpg
Mira as seen from the Earth.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation 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 8,400–9,360[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,[7] while Hipparcos data suggests a distance of 418 light-years, albeit with a margin of error of ~14%.

Contents

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.

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 is currently an Asymptotic Giant Branch (AGB) star, in the thermally pulsing AGB phase.[10][11] Each pulse lasts a decade or more, and an amount of time 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.[12]

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.[11]

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 ca 6,000 to 7,000 known stars of this class[13]) are all red giants whose surfaces oscillate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1,000 days.

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 1,700 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.[14]

Mass loss

Ultraviolet mosaic of Mira's bow shock and tail

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.[15][16] 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.[17][18] The tail consists of material stripped from the head of the bow-wave, which is also visible in ultra-violet observations. Mira's lost material will eventually be illuminated in a planetary nebula, the shaping of which will be considerably affected by the motion through the interstellar medium (ISM).[19]

Component 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 could eventually 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.[20]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 "V* omi Cet -- Variable Star of Mira Cet type". SIMBAD. Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-id?protocol=html&Ident=V*+omi+Cet. Retrieved 2006-08-10. 
  2. ↑ Castelaz, Michael W.; Luttermoser, Donald G. (1997). "Spectroscopy of Mira Variables at Different Phases.". The Astronomical Journal 114: 1584–1591. doi:10.1086/118589. http://adsabs.harvard.edu/abs/1997AJ....114.1584C. Retrieved 2007-12-10. 
  3. ↑ 3.0 3.1 Celis S., L. (1982). "Red variable stars. I — UBVRI photometry and photometric properties". Astronomical Journal 87: 1791–1802. doi:10.1086/113268. http://adsabs.harvard.edu/abs/1982AJ.....87.1791C. Retrieved 2007-12-10. 
  4. ↑ 4.0 4.1 Wyatt, S. P.; Cahn, J. H. (1983). "Kinematics and ages of Mira variables in the greater solar neighborhood". Astrophysical Journal, Part 1 275: 225–239. doi:10.1086/161527. http://adsabs.harvard.edu/abs/1983ApJ...275..225W. Retrieved 2007-12-17. 
  5. ↑ 5.0 5.1 5.2 Woodruff, H. C.; Eberhardt, M.; Driebe, T.; Hofmann, K.-H.; Ohnaka, K.; Richichi, A.; Schert, D.; Schöller, M.; Scholz, M.; Weigelt, G.; Wittkowski, M.; Wood, P. R. (2004). "Interferometric observations of the Mira star o Ceti with the VLTI/VINCI instrument in the near-infrared" (PDF). Astronomy & Astrophysics 421: 703–714. doi:10.1051/0004-6361:20035826. http://www.eso.org/~mwittkow/publications/conferences/SPIECWo5491199.pdf. Retrieved 2007-12-07. 
  6. ↑ Allen, Richard H. (1963). Star Names: Their Lore and Meaning. New York: Dover Publications. ISBN 0486210790. 
  7. ↑ Robert Burnham, Jr.,Burnham's Celestial Handbook, Vol. 1 (New York: Dover Publications Inc., 1980), 634.)
  8. ↑ Wilk, Stephen R (1996). "Mythological Evidence for Ancient Observations of Variable Stars". The Journal of the American Association of Variable Star Observers 24 (2): 129–133. http://adsabs.harvard.edu/abs/1996JAVSO..24..129W. Retrieved 2007-12-07. 
  9. ↑ Hoffleit, Dorrit. "History of Mira's Discovery". http://www.aavso.org/vstar/vsots/mirahistory.shtml. Retrieved 2007-08-16. 
  10. ↑ Pogge, Richard (January 21, 2006). "Lecture 16: The Evolution of Low-Mass Stars". Ohio State University. http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit2/lowmass.html. Retrieved 2007-12-11. 
  11. ↑ 11.0 11.1 Lopez, B. (1999). "AGB and post-AGB stars at high angular resolution". Proceedings IAU Symposium #191: Asymptotic Giant Branch Stars. pp. 409. http://adsabs.harvard.edu/abs/1999IAUS..191..409L. Retrieved 2007-12-11. 
  12. ↑ De Loore, C. W. H.; Doom, C (1992). Structure and Evolution of Single and Binary Stars. Springer. ISBN 0792317688. 
  13. ↑ GCVS: vartype.txt from the GCVS catalogue (statistics at the end of the file indicate 6,006 mirae and 1,237 probable mirae)
  14. ↑ Braune, Werner. "Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne". http://www.bav-astro.de/index_e.html. Retrieved 2007-08-16. 
  15. ↑ Martin, Christopher; Seibert, M; Neill, JD; Schiminovich, D; Forster, K; Rich, RM; Welsh, BY; Madore, BF et al. (August 17, 2007). "A turbulent wake as a tracer of 30,000 years of Mira's mass loss history". Nature 448 (7155): 780–783. doi:10.1038/nature06003. PMID 17700694. 
  16. ↑ Minkel, JR. "Shooting Bullet Star Leaves Vast Ultraviolet Wake", "The Scientific American", August 15, 2007 Accessed August 21, 2007.
  17. ↑ Wareing, Christopher; Zijlstra, A. A.; O'Brien, T. J.; Seibert, M. (November 6, 2007). "It's a wonderful tail: the mass-loss history of Mira". Astrophysical Journal Letters 670 (2): L125–L129. doi:10.1086/524407. http://www.iop.org/EJ/article/1538-4357/670/2/L125/22252.html. 
  18. ↑ Clavin, W. (August 15, 2007). "GALEX finds link between big and small stellar blasts". California Institute of Technology. http://www.galex.caltech.edu/MEDIA/2007-04/images.html. Retrieved 2007-08-16. 
  19. ↑ Wareing, Christopher (December 13, 2008). "Wonderful Mira". Philosophical Transactions of the Royal Society A 366 (1884): 4429–4440. doi:10.1098/rsta.2008.0167. PMID 18812301. 
  20. ↑ Than, Ker. "Dying star's dust helping to build new planets". http://www.msnbc.msn.com/id/16564325/. Retrieved 2007-08-16. 

External links

Stars of Cetus
1 • 2 • 3 • 6 • 7 • ι • 9 • 10 • 11 • 12 • 13 • 14 • 15 • β (Deneb Kaitos/Diphda) • φ1 • 18 • φ2 • 20 • 21 • φ3 • φ4 • 25 • 26 • 27 • 28 • 29 • 30 • η • 32 • 33 • 34 • 35 • 36 • 37 • 38 • 39 • 40 • 41 • 42 • 43 • 44 • θ • 46 • 47 • 48 • 49 • 50 • τ • χ • ζ (Baten Kaitos) • υ1 • 57 • 58 • υ2 • 61 • 62 • 63 • 64 • ξ1 • 66 • 67 • ο A (Mira) • ο B (VZ Ceti) • 69 • 70 • 71 • ρ • ξ2 • 75 • σ • 77 • ν • 79 • 80 • 81 • δ • ε • 84 • γ (Kaffaljidhma) • μ • π • λ • α (Menkar) • 93 • 94 • 95 • Kappa¹ Ceti • κ2 • 25 Ari
List