Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||00h 08m 23.2586s|
|Declination||+29° 05′ 25.555″|
|Apparent magnitude (V)||2.22||4.21|
|U-B color index||−0.46|
|B-V color index||−0.11|
|R-I color index||−0.10|
|Radial velocity (Rv)||−10.6 ± 0.3 km/s|
|RA (μα cos δ)||135.68 mas/yr|
|Dec. (μδ)||−162.95 mas/yr|
|Parallax (π)||33.60 ± 0.73 mas|
|Distance||97 ± 2 ly
(29.8 ± 0.6 pc)
|Absolute magnitude (MV)||−0.19 ± 0.30||2.00 ± 0.30|
|Mass||3.60 ± 0.20 M☉||1.78 ± 0.08 M☉|
|Radius||2.7 ± 0.4 R☉||1.65 ± 0.3 R☉|
|Surface gravity (log g)||4.15 ± 0.16||4.28 ± 0.20|
|Luminosity (bolometric)||240 ± 90 L☉||13 ± L☉|
|Metallicity||[M/H] = 0.2||[M/H] = 0.2|
|Rotational velocity (v sin i)||52km/s||110 ± 5km/s|
|Period (P)||96.7015 ± 0.0044 days|
|Semimajor axis (a)||24.0 ± 0.13 mas|
|Eccentricity (e)||0.535 ± 0.0046|
|Inclination (i)||105.6 ± 0.23°|
|Longitude of node (Ω)||284.4 ± 0.21°|
|Periastron epoch (T)||MJD 47374.563 ± 0.095|
|Argument of periastron (ω)
|257.4 ± 0.31°|
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.
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  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.
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 peculiarities, 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 mare". (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 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.
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.
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||00h 08m 16.626s|
|Declination||+29° 05′ 45.49″|
|Apparent magnitude (V)||10.8|
|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° |
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
- V* alf And — Spectroscopic binary, SIMBAD. August 12, 2008.
- 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.
- 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.
- 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.
- Value is for the center of mass of the system.
- Kochukhov, O.; et al. (2007). "Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae". Nature Physics 3 (8): 526. arXiv:0705.4469. Bibcode:2007NatPh...3..526K. doi:10.1038/nphys648.
- 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.
- Entry 00084+2905, discoverer code MKT 11, components Aa, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line September 3, 2008.
- Alpheratz (Alpha Andromedae), 2/14/2013
- Alpheratz, Kaler Stars  2/14/2013
- 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.
- Ludendorff, H. (1907). "Provisorische Bahnelemente des spektroskopischen Doppelsterns α Andromedae". Astronomische Nachrichten 176 (20): 327. Bibcode:1907AN....176..327L. doi:10.1002/asna.19071762007.
- Baker, R. H. (1910). "The orbit of α Andromedae". Publications of the Allegheny Observatory of the Western University of Pennsylvania 1: 17. Bibcode:1910PAllO...1...17B.
- 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.
- 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.
- Baxandall, F. E. (1914). "Stars, Spectra of, on the enhanced lines of Manganese in the spectrum of a Andromedae". Monthly Notices of the Royal Astronomical Society 74: 250. Bibcode:1914MNRAS..74..250B.
- 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.
- 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.
- Smith, K. C. (1996). "Chemically peculiar hot stars". Astrophysics and Space Science 237: 77. Bibcode:1996Ap&SS.237...77S. doi:10.1007/BF02424427.
- Michaud, G. (1970). "Diffusion Processes in Peculiar a Stars". The Astrophysical Journal 160: 641. Bibcode:1970ApJ...160..641M. doi:10.1086/150459.
- "alf And * (entry 019001)". General Catalogue of Variable Stars. Sternberg Astronomical Institute. Retrieved August 12, 2008.
- 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.
- Adelman, S. J.; et al. (2002). "The Variability of the Hgiiλ3984 Line of the Mercury‐Manganese Star α Andromedae". The Astrophysical Journal 575: 449. Bibcode:2002ApJ...575..449A. doi:10.1086/341140.
- Allen, R. A. (1899). Star-names and Their Meanings. G. E. Stechert. p. 35. LCCN 99004138.
- Goldstein, B. R. (1985). "Star Lists in Hebrew". Centaurus 28 (3): 185. Bibcode:1985Cent...28..185G. doi:10.1111/j.1600-0498.1985.tb00745.x.
- 陳久金 (2005). 中國星座神話. 台灣書房出版有限公司. p. 170. ISBN 978-986-7332-25-7. (Chinese)
- "香港太空館 - 研究資源 - 亮星中英對照表". Hong Kong Space Museum. Archived from the original on 25 October 2008. Retrieved November 26, 2008. (Chinese)
- Olcott, W. T. (1911). Star Lore of All Ages. G.P. Putnam's Sons. p. 26. LCCN 11031153.
- "Alpheratz". MSN Encarta. Archived from the original on 2009-10-31. Retrieved August 19, 2008.
- BD+28 4B — Star in double system, SIMBAD. Accessed on line August 12, 2008.
- Burnham, R. (1978). Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System 1. Courier Dover Publications. p. 111. ISBN 0-486-23567-X.
- 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. doi:10.1098/rstl.1782.0014. JSTOR 106455.