Alpha Andromedae

Alpha Andromedae (α Andromedae, abbreviated Alpha And or α And), officially named Alpheratz /ælˈfɪəræts/,[12][13] is 97 light-years from the Sun and is the brightest star in the constellation of Andromeda when Beta Andromedae undergoes its periodical dimming. Immediately northeast of the constellation of Pegasus, it is the upper left star of the Great Square of Pegasus.

"21 Andromedae" redirects here. For the galaxy, see Andromeda XXI.

α Andromedae
Location of α Andromedae (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Andromeda
Right ascension 00h 08m 23.25988s[1]
Declination +29° 05′ 25.5520″[1]
Apparent magnitude (V) 2.06 (2.22 + 4.21)[2]
Characteristics
U−B color index −0.46[3]
B−V color index −0.11[3]
R−I color index −0.10[3]
Primary
Spectral type B8IVpMnHg[4]
Secondary
Spectral type A3V[4]
Astrometry
Primary
Radial velocity (Rv)−10.6 ± 0.3[5] km/s
Proper motion (μ) RA: 135.68[6] mas/yr
Dec.: −162.95[6] mas/yr
Parallax (π)33.62 ± 0.35 mas[1]
Distance97 ± 1 ly
(29.7 ± 0.3 pc)
Absolute magnitude (MV)−0.19 ± 0.30[7]
Secondary
Absolute magnitude (MV)2.00 ± 0.30[7]
Orbit[2]
Period (P)96.7015 ± 0.0044 days
Semi-major axis (a)24.0 ± 0.13 mas
Eccentricity (e)0.535 ± 0.0046
Inclination (i)105.6 ± 0.23°
Longitude of the node (Ω)284.4 ± 0.21°
Periastron epoch (T)MJD 47374.563 ± 0.095
Argument of periastron (ω)
(secondary)		257.4 ± 0.31°
Details
Primary
Mass3.8 ± 0.2[8] M☉
Radius2.7 ± 0.4[8] R☉
Luminosity (bolometric)240[8] L☉
Surface gravity (log g)3.75[8] cgs
Temperature13,800[8] K
Metallicity[M/H] = 0.2
Rotation2.38195 d[9]
Rotational velocity (v sin i)52 km/s
Age60[8] Myr
Secondary
Mass1.85 ± 0.13[8] M☉
Radius1.65 ± 0.3[8] R☉
Luminosity (bolometric)13[8] L☉
Surface gravity (log g)4.0[8] cgs
Temperature8,500[8] K
Metallicity[M/H] = 0.2
Rotational velocity (v sin i)110 ± 5 km/s
Age70[8] Myr
Other designations
Alpheratz, Sirrah, Sirah, α And, Alpha Andromedae, Alpha And, δ Pegasi, δ Peg, Delta Pegasi, Delta Peg, 21 Andromedae, 21 And, H 5 32A, MKT 11, ADS 94 A, BD+28°4, CCDM J00083+2905A, FK5 1, GC 127, HD 358, HIP 677, HR 15, IDS 00032+2832 A, LTT 10039, NLTT 346, PPM 89441, SAO 73765, WDS 00084+2905A/Aa[6][10][11]
Database references
SIMBADdata

Although it appears to the naked eye as a single star, with overall apparent visual magnitude +2.06, it is actually a binary system composed of two stars in close orbit. The chemical composition of the brighter of the two stars is unusual as it is a mercury-manganese star whose atmosphere contains abnormally high levels of mercury, manganese, and other elements, including gallium and xenon.[14] It is the brightest mercury-manganese star known.[14]

Nomenclature
Alpha Andromedae is the brightest star in the constellation of Andromeda (right).

α Andromedae (Latinised to Alpha Andromedae) is the star's Bayer designation. Ptolemy considered the star (system) to be shared by Pegasus and Johann Bayer assigned it a designation in both constellations: Alpha Andromedae (α And) and Delta Pegasi (δ Peg). Since the IAU standardized constellation boundaries and widely published them two years after in 1930, the Pegasi alternate name has dropped from use, putting it slightly outside of that constellation.[15]

To most European centres of learning the star bore names Alpheratz (/ælˈfiːræts/[16]) or the cognate simplification Alpherat or the other part of the fabled description: Sirrah /ˈsɪrə/.

The origin of these three, the Arabic phrasal name, is سرة الفرس surrat al-faras "navel of the mare/horse", attracting a hard consonant not present above due to a following vowel. The horse corresponds equivalently to the winged horse of the Greeks, Pegasus. The star is in almost all depictions part of the main asterism of Pegasus and Andromeda.[17] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[18] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[19] confirmed Alpheratz as the name for the main star.

Other terms for this star used by some medieval astronomers writing were راس المراة المسلسلة rās al-mar'a al-musalsala (head of the woman in chains),[17] al-kaff al-khaḍīb and kaff al-naṣīr (palm of the faithful). The chained woman referenced Andromeda. [20]

In the Hindu lunar zodiac, this star, together with the other stars in the Great Square of Pegasus (α, β, and γ Pegasi), makes up the nakshatras of Pūrva Bhādrapadā and Uttara Bhādrapadā.[17]

In Chinese, 壁宿 (Bì Sù), meaning wall, refers to an asterism consisting of α Andromedae and γ Pegasi.[21] Consequently, the Chinese name for α Andromedae itself is 壁宿二 (Bì Sù èr, English: the second star of the wall.)[22]

It is also known as one of the "Three Guides" that mark the prime meridian of the heavens, the other two being Beta Cassiopeiae and Gamma Pegasi. It was believed to bless those born under its influence with honour and riches.[23]

System
α Andromedae

The radial velocity of a star away from or towards the observer can be determined by measuring the red shift or blue shift of its spectrum. The American astronomer Vesto Slipher made a series of such measurements from 1902 to 1904 and discovered that the radial velocity of α Andromedae varied periodically. He concluded that it was in orbit in a spectroscopic binary star system with a period of about 100 days.[24] A preliminary orbit was published by Hans Ludendorff in 1907,[25] and a more precise orbit was later published by Robert Horace Baker.[26]

The fainter star in the system was first resolved interferometrically by Xiaopei Pan and his coworkers during 1988 and 1989, using the Mark III Stellar Interferometer at the Mount Wilson Observatory, California, United States. This work was published in 1992.[27] Because of the difference in luminosity between the two stars, its spectral lines were not observed until the early 1990s, in observations made by Jocelyn Tomkin, Xiaopei Pan, and James K. McCarthy between 1991 and 1994 and published in 1995.[4]

The two stars are now known to orbit each other with a period of 96.7 days.[2] The larger, brighter star, called the primary, has a spectral type of B8IVpMnHg, a mass of approximately 3.6 solar masses, a surface temperature of about 13,800 K, and, measured over all wavelengths, a luminosity of about 200 times the Sun's. Its smaller, fainter companion, the secondary, has a mass of approximately 1.8 solar masses and a surface temperature of about 8,500 K, and, again measured over all wavelengths, a luminosity of about 10 times the Sun's. It is an early-type A star whose spectral type has been estimated as A3V.[4]

Chemical peculiarities

In 1906, Norman Lockyer and F. E. Baxandall reported that α Andromedae had a number of unusual lines in its spectrum.[28] In 1914, Baxandall pointed out that most of the unusual lines came from manganese, and that similar lines were present in the spectrum of μ Leporis.[29] In 1931, W. W. Morgan identified 12 additional stars with lines from manganese appearing in their spectra.[30] Many of these stars were subsequently identified as part of the group of mercury-manganese stars,[31] a class of chemically peculiar stars which have an excess of elements such as mercury, manganese, phosphorus, and gallium in their atmospheres.[32], §3.4. In the case of α Andromedae, the brighter primary star is a mercury-manganese star which, as well as the elements already mentioned, has excess xenon.

In 1970, Georges Michaud suggested that such chemically peculiar stars arose from radiative diffusion. According to this theory, in stars with unusually calm atmospheres, some elements sink under the force of gravity, while others are pushed to the surface by radiation pressure.[32], §4.[33] This theory has successfully explained many observed chemical peculiarities, including those of mercury-manganese stars.[32], §4.

Variability of primary

α Andromedae has been reported to be slightly variable,[34] but observations from 1990 to 1994 found its brightness to be constant to within less than 0.01 magnitude.[35] However, Adelman and his co-workers have discovered, in observations made between 1993 and 1999 and published in 2002, that the mercury line in its spectrum at 398.4 nm varies as the primary rotates. This is because the distribution of mercury in its atmosphere is not uniform. Applying Doppler imaging to the observations allowed Adelman et al. to find that it was concentrated in clouds near the equator.[36] Subsequent Doppler imaging studies, published in 2007, showed that these clouds drift slowly over the star's surface.[9]

Observation
Andromeda constellation. α Andromedae, labeled Sirrah, is at the lower right of the constellation, bordering Pegasus.

The location of α Andromedae in the sky is shown on the left. It can be seen by the naked eye and is theoretically visible at all latitudes north of 60° S. During evening from August to October, it will be high in the sky as seen from the northern midlatitudes.[37]

Optical companion
ADS 94 B
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Andromeda
Right ascension 00h 08m 16.626s[38]
Declination +29° 05′ 45.49″[38]
Apparent magnitude (V) 10.8[38]
Characteristics
Spectral type G5[38]
B−V color index 1.0[38]
Astrometry
Proper motion (μ) RA: −3.9[38] mas/yr
Dec.: −24.0[38] mas/yr
Parallax (π)2.3990 ± 0.0369 mas[39]
Distance1,360 ± 20 ly
(417 ± 6 pc)
Position (relative to A)
Epoch of observation2000
Angular distance89.3″ [10]
Position angle284° [10]
Other designations
H 5 32B, BD+28°4B, CCDM J00083+2905B, IDS 00032+2832 B, PPM 89439, WDS 00084+2905B.[10][38]
Database references
SIMBADdata

The binary system described above has an optical visual companion, discovered by William Herschel on July 21, 1781.[10][40][41] Designated as ADS 94 B in the Aitken Double Star Catalogue, it is a G-type star with an apparent visual magnitude of approximately 10.8.[38] Although by coincidence it appears near to the other two stars in the sky, it's much more distant from Earth; the parallax observed by Gaia place this star more than 1,300 light years away.[40]

Notes and references
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. Entry, WDS identifier 00084+2905, Sixth Catalog of Orbits of Visual Binary Stars Archived 2017-11-12 at the Wayback Machine, William I. Hartkopf & Brian D. Mason, U.S. Naval Observatory. Accessed on line August 12, 2008.
  3. Hoffleit, D.; Warren, W. H. Jr. "HR 15". The Bright Star Catalogue (5th Revised ed.). VizieR. and Hoffleit, D.; Warren, W. H. Jr. "Detailed Description of V/50". The Bright Star Catalogue (5th Revised ed.). Centre de Données astronomiques de Strasbourg.
  4. Tomkin, J.; Pan, X.; McCarthy, J. K. (1995). "Spectroscopic detection of the secondaries of the Hyades interferometric spectroscopic binary theta2 Tauri and of the interferometric spectroscopic binary alpha Andromedae". Astronomical Journal. 109: 780. Bibcode:1995AJ....109..780T. doi:10.1086/117321.
  5. Value is for the center of mass of the system.
  6. "* alf And". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved August 12, 2008.
  7. Ryabchikova, T. A.; Malanushenko, V. P.; Adelman, S. J. (1999). "Orbital elements and abundance analyses of the double-lined spectroscopic binary alpha Andromedae". Astronomy and Astrophysics. 351: 963. Bibcode:1999A&A...351..963R.
  8. Ryabchikova, T.; Malanushenko, V.; Adelman, S. J. (1998). "The double-lined spectroscopic binary alpha Andromedae: Orbital elements and elemental abundances". Contributions of the Astronomical Observatory Skalnate Pleso. 27 (3): 356. arXiv:astro-ph/9805205. Bibcode:1998CoSka..27..356R.
  9. Kochukhov, O.; et al. (2007). "Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae". Nature Physics. 3 (8): 526–529. arXiv:0705.4469. Bibcode:2007NatPh...3..526K. doi:10.1038/nphys648. S2CID 117951514.
  10. Entry 00084+2905, discoverer code H 5  32, components Aa-B, The Washington Double Star Catalog Archived 2011-08-16 at the Wayback Machine, United States Naval Observatory. Accessed on line August 15, 2017.
  11. Entry 00084+2905, discoverer code MKT  11, components Aa, The Washington Double Star Catalog Archived 2011-08-16 at the Wayback Machine, United States Naval Observatory. Accessed on line August 15, 2017.
  12. Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  13. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  14. Alpheratz, Kaler Stars 2/14/2013
  15. Bayer's Uranometria and Bayer letters
  16. Merriam-Webster Dictionary 2017 – Alpheratz
  17. Allen, R. A. (1899). Star-names and Their Meanings. G. E. Stechert. p. 35. LCCN 99004138.
  18. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  19. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  20. Goldstein, B. R. (1985). "Star Lists in Hebrew". Centaurus. 28 (3): 185–208. Bibcode:1985Cent...28..185G. doi:10.1111/j.1600-0498.1985.tb00745.x.
  21. 陳久金 (2005). 中國星座神話 (in Chinese). 台灣書房出版有限公司. p. 170. ISBN 978-986-7332-25-7.
  22. 香港太空館 - 研究資源 - 亮星中英對照表 (in Chinese). Hong Kong Space Museum. Archived from the original on 25 October 2008. Retrieved November 26, 2008.
  23. Olcott, W. T. (1911). Star Lore of All Ages. G.P. Putnam's Sons. p. 26. LCCN 11031153.
  24. Slipher, V. M. (1904). "A list of five stars having variable radial velocities". The Astrophysical Journal. 20: 146. Bibcode:1904ApJ....20..146S. doi:10.1086/141148.
  25. Ludendorff, H. (1907). "Provisorische Bahnelemente des spektroskopischen Doppelsterns α Andromedae". Astronomische Nachrichten. 176 (20): 327–328. Bibcode:1907AN....176..327L. doi:10.1002/asna.19071762007.
  26. Baker, R. H. (1910). "The orbit of α Andromedae". Publications of the Allegheny Observatory of the Western University of Pennsylvania. 1 (3): 17. Bibcode:1910PAllO...1...17B.
  27. Pan, X.; et al. (1992). "Determination of the visual orbit of the spectroscopic binary Alpha Andromedae with submilliarcsecond precision". The Astrophysical Journal. 384: 624. Bibcode:1992ApJ...384..624P. doi:10.1086/170904.
  28. Lockyer, N.; Baxandall, F. E. (1906). "Some Stars with Peculiar Spectra". Proceedings of the Royal Society of London. Series A. 77 (520): 550. Bibcode:1906RSPSA..77..550L. doi:10.1098/rspa.1906.0049. JSTOR 92668.
  29. Baxandall, F. E. (1914). "Stars, Spectra of, on the enhanced lines of Manganese in the spectrum of α Andromedae". Monthly Notices of the Royal Astronomical Society. 74 (3): 250. Bibcode:1914MNRAS..74..250B. doi:10.1093/mnras/74.3.250.
  30. Morgan, W. W. (1931). "Studies in Peculiar Stellar Spectra. I. The Manganese Lines in α Andromedae". The Astrophysical Journal. 73: 104. Bibcode:1931ApJ....73..104M. doi:10.1086/143299.
  31. Cowley, C. R.; Aikman, G. C. L. (1975). "A study of the lambda 3984 feature in the mercury-manganese stars". Publications of the Astronomical Society of the Pacific. 87: 513. Bibcode:1975PASP...87..513C. doi:10.1086/129801.
  32. Smith, K. C. (1996). "Chemically peculiar hot stars". Astrophysics and Space Science. 237 (1–2): 77–105. Bibcode:1996Ap&SS.237...77S. doi:10.1007/BF02424427. S2CID 189836091.
  33. Michaud, G. (1970). "Diffusion Processes in Peculiar a Stars". The Astrophysical Journal. 160: 641. Bibcode:1970ApJ...160..641M. doi:10.1086/150459.
  34. "alf And * (entry 019001)". General Catalogue of Variable Stars. Sternberg Astronomical Institute. Archived from the original on June 20, 2017. Retrieved August 12, 2008.
  35. Adelman, S. J.; et al. (1994). "uvby photometry of the chemically peculiar stars Alpha Andromedae, HD 184905, HR 8216, and HR 8434". Astronomy and Astrophysics Supplement. 106: 333. Bibcode:1994A&AS..106..333A.
  36. Adelman, S. J.; et al. (2002). "The Variability of the Hgiiλ3984 Line of the Mercury-Manganese Star α Andromedae". The Astrophysical Journal. 575 (1): 449–460. Bibcode:2002ApJ...575..449A. doi:10.1086/341140.
  37. "Alpheratz". MSN Encarta. Archived from the original on 2009-11-03. Retrieved August 19, 2008.
  38. "TYC 1735-3181-1". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved August 12, 2008.
  39. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051.
  40. Burnham, R. (1978). Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System. Vol. 1. Courier Dover Publications. p. 111. ISBN 0-486-23567-X.
  41. See p.140, entry 32 in Herschel, M.; Watson, D. (1782). "Catalogue of Double Stars. By Mr. Herschel, F. R. S. Communicated by Dr. Watson, Jun". Philosophical Transactions of the Royal Society of London. 72: 112–162. Bibcode:1782RSPT...72..112H. doi:10.1098/rstl.1782.0014. JSTOR 106455. S2CID 186209247.
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