Alpha Andromedae (Alpha And, α And, α Andromedae), which has the traditional names Alpheratz (or Alpherat) and Sirrah (or Sirah), is the brightest star in the constellation of Andromeda. Located immediately northeast of the constellation of Pegasus, it is the northeastern star of the Great Square of Pegasus. As a connecting star to Pegasus, it is also known as δ Pegasi, though this name is no longer used (another such doubly-named connecting star is β Tauri). It is located 97 light-years from Earth. 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. It is the brightest mercury-manganese star known.
This star has for long been treated as being in Pegasus and simultaneously in Andromeda, and Johann Bayer catalogued it as both α Andromedae and δ Pegasi.
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. A preliminary orbit was published by Hans Ludendorff in 1907, and a more precise orbit was later published by Robert Horace Baker.
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. 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.
The two stars are now known to orbit each other with a period of 96.7 days. 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.
In 1906, Norman Lockyer and F. E. Baxandall reported that α Andromedae had a number of unusual lines in its spectrum. 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. In 1931, W. W. Morgan identified 12 additional stars with lines from manganese appearing in their spectra. Many of these stars were subsequently identified as part of the group of mercury-manganese stars, a class of chemically peculiar stars which have an excess of elements such as mercury, manganese, phosphorus, and gallium in their atmospheres., §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. Ryabchikova, Malanushenko, and Adelman have observed that the secondary has excess barium in its atmosphere and suggest that it be classified as a metallic-line star.
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., §4. This theory has successfully explained many observed chemical pecularities, including those of mercury-manganese stars., §4.
Variability of primary
α Andromedae has been reported to be slightly variable, but observations from 1990 to 1994 found its brightness to be constant to within less than 0.01 magnitude. 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. Subsequent Doppler imaging studies, published in 2007, showed that these clouds drift slowly over the star's surface.
Etymology and cultural significance
The names Alpheratz and Sirrah both derive from the Arabic name, سرة الفرس surrat al-faras "the navel of the horse". (سرة alone is surra.) The word horse reflects the star's historical placement in Pegasus. Another term for this star used by medieval astronomers writing in Arabic was راس المراة المسلسلة rās al-mar'a al-musalsala "the head of the woman in chains", the chained woman here being Andromeda. Other Arabic names include al-kaff al-khaḍīb and kaff al-naṣīr.
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.
In Chinese, 壁宿 (Bì Sù), meaning wall, refers to an asterism consisting of α Andromedae and γ Pegasi. Consequently, α Andromedae itself is known as 壁宿二 (Bì Sù èr, English: the second star of the wall.)
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.
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.
ADS 94 B Observation data
Epoch J2000.0 Equinox J2000.0 (ICRS)
Right ascension 00h 08m 16.626s
Declination +29° 05′ 45.49″
Apparent magnitude (V) 10.8
Spectral type G5
B−V color index 1.0
Proper motion (μ) RA: −3.9 mas/yr
Dec.: −24.0 mas/yr
Position (relative to A)
Epoch of observation 2000
Angular distance 89.3″ 
Position angle 284° 
H 5 32B, BD+28°4B, CCDM J00083+2905B, IDS 00032+2832 B, PPM 89439, WDS 00084+2905B.
The binary system described above has an optical visual companion, discovered by William Herschel on July 21, 1781. 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. Although by coincidence it appears near to the other two stars in the sky, it is not close to them in space.
Notes and references
1. ^ a b c d e f V* alf And -- Spectroscopic binary, database entry, SIMBAD. Accessed on line August 12, 2008.
2. ^ a b c d Astrometric data, mirrored by SIMBAD from the Hipparcos catalogue, pertains to the center of mass of the α Andromedae binary system. See §2.3.4, Volume 1, The Hipparcos and Tycho Catalogues, European Space Agency, 1997, and the entry in the Hipparcos catalogue (CDS ID I/239.)
3. ^ a b c d Entry, WDS identifier 00084+2905, Sixth Catalog of Orbits of Visual Binary Stars, William I. Hartkopf & Brian D. Mason, U.S. Naval Observatory. Accessed on line August 12, 2008.
4. ^ a b c d Spectroscopic detection of the secondaries of the Hyades interferometric spectroscopic binary θ2 Tauri and of the interferometric spectroscopic binary α Andromedae, Jocelyn Tomkin, Xiaopei Pan, and James K. McCarthy, Astronomical Journal 109, #2 (February 1995), pp. 780–790, Bibcode: 1995AJ....109..780T, doi:10.1086/117321.
5. ^ a b c HR 15, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line August 12, 2008.
6. ^ Value is for the center of mass of the system.
7. ^ a b c d e f g h i j k l m n o p q r s t Orbital elements and abundance analyses of the double-lined spectroscopic binary alpha Andromedae, T. A. Ryabchikova, V. P. Malanushenko, and S. J. Adelman, Astronomy and Astrophysics 351 (November 1999), pp. 963–972, Bibcode: 1999A&A...351..963R. See §4 for component parameters and Table 3, §5 for elemental abundances.
8. ^ a b Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae, Oleg Kochukhov, Saul J. Adelman, Austin F. Gulliver, and Nikolai Piskunov, Nature Physics 3, #8 (2007), pp. 526–529, Bibcode: 2007NatPh...3..526K, doi:10.1038/nphys648.
9. ^ a b c d e Entry 00084+2905, discoverer code H 5 32, components Aa-B, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line August 12, 2008.
10. ^ Entry 00084+2905, discoverer code MKT 11, components Aa, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line September 3, 2008.
11. ^ a b Alpheratz (Alpha Andromedae), The Internet Encyclopedia of Science, David Darling. Accessed on line August 13, 2008.
12. ^ HR 15, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50.
13. ^ Alnath (Beta Tauri), The Internet Encyclopedia of Science, David Darling. Accessed on line August 13, 2008.
14. ^ a b Alpheratz, Stars, Jim Kaler. Accessed on line August 13, 2008.
15. ^ A list of five stars having variable radial velocities, V. M. Slipher, Astrophysical Journal 20 (September 1904), pp. 146–148, Bibcode: 1904ApJ....20..146S.
16. ^ (German) Provisorische Bahnelemente des spektroskopischen Doppelsterns α Andromedae, H. Ludendorff, Astronomische Nachrichten 176 (December 1907), p. 327, Bibcode: 1907AN....176..327L.
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19. ^ Some Stars with Peculiar Spectra, Norman Lockyer and F. E. Baxandall, Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 77, #520 (June 21, 1906), pp. 550–553, Bibcode: 1906RSPSA..77..550L.
20. ^ On the enhanced lines of Manganese in the spectrum of α Andromedae, F. E. Baxandall, Monthly Notices of the Royal Astronomical Society 74 (January 1914), pp. 250–256, Bibcode: 1914MNRAS..74..250B.
21. ^ Studies in Peculiar Stellar Spectra. I. The Manganese Lines in α Andromedae, W. W. Morgan, Astrophysical Journal 73 (March 1931), pp. 104–117, Bibcode: 1931ApJ....73..104M.
22. ^ Table II, A study of the λ3984 feature in the mercury-manganese stars, C. R. Cowley and G. C. L. Aikman, Publications of the Astronomical Society of the Pacific 87 (August 1975), pp. 513–521, Bibcode: 1975PASP...87..513C.
23. ^ a b c Chemically peculiar hot stars, Keith C. Smith, Astrophysics and Space Science 237, #1-2 (March 1996), pp. 77–105, Bibcode: 1996Ap&SS.237...77S, doi:10.1007/BF02424427.
24. ^ Diffusion Processes in Peculiar A Stars, Georges Michaud, Astrophysical Journal 160 (May 1970), pp. 641–658, Bibcode: 1970ApJ...160..641M.
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26. ^ uvby photometry of the chemically peculiar stars Alpha Andromedae, HD 184905, HR 8216, and HR 8434, S. J. Adelman et al., Astronomy and Astrophysics Supplement 106 (August 1994), pp. 333–337, Bibcode: 1994A&AS..106..333A.
27. ^ The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae, Saul J. Adelman, Austin F. Gulliver, Oleg P. Kochukov, and Tanya A. Ryabchikova, The Astrophysical Journal 575, #1 (August 10, 2002), pp. 449–460, Bibcode: 2002ApJ...575..449A, doi:10.1086/341140.
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