Alpheratz is a prominent star system in the constellation ofAndromeda. Pronounced/ælˈfɪəræts/,[13][14] it has theBayer designationAlpha Andromedae,Latinised fromα Andromedae, and abbreviatedAlpha And orα And, respectively. Alpheratz is thebrightest star in the constellation whenMirach (βAndromedae) undergoes its periodical dimming. Immediately northeast of the constellation ofPegasus, it is the upper left star of the Great Square of Pegasus. It is located at a distance of 97light-years from Earth.
Alpha Andromedae is the brightest star in the constellation of Andromeda (right).
α Andromedae (Latinised toAlpha Andromedae) is the star'sBayer designation.Ptolemy considered the star (system) to be shared byPegasus andJohann Bayer assigned it a designation in both constellations: Alpha Andromedae (α And) andDelta 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.[16]
To most European centres of learning the star bore namesAlpheratz (/ælˈfiːræts/[17]) or the cognate simplificationAlpherat or the other part of the fabled description:Sirrah/ˈsɪrə/. The name has no relation with the word "alpha".
The origin of these three, theArabic phrasal name, isسرة الفرسsurrat 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 Pegasusand Andromeda.[18] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[20] confirmedAlpheratz as the name for the main star.
Other terms for this star used by some medieval astronomers writing were راس المراة المسلسلةrās al-mar'a al-musalsala (head of the woman in chains),[18]al-kaff al-khaḍīb andkaff al-naṣīr (palm of the faithful). The chained woman referencedAndromeda.[21]
InChinese,壁宿 (Bì Sù), meaningwall, refers to an asterism consisting of α Andromedae andγ Pegasi.[22] Consequently, theChinese name for α Andromedae itself is壁宿二 (Bì Sù èr, English:the second star of the wall.)[23]
It is also known as one of the "Three Guides" that mark the prime meridian of the heavens, the other two beingBeta Cassiopeiae andGamma Pegasi. It was believed to bless those born under its influence with honour and riches.[24]
Theradial velocity of a star away from or towards the observer can be determined by measuring thered shift or blue shift of itsspectrum. The American astronomerVesto 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 inorbit in aspectroscopic binarystar system with a period of about 100 days.[25] A preliminary orbit was published byHans Ludendorff in 1907,[26] and a more precise orbit was later published byRobert Horace Baker.[27]
The fainter star in the system was first resolvedinterferometrically by Xiaopei Pan and his coworkers during 1988 and 1989, using theMark III Stellar Interferometer at theMount Wilson Observatory,California, United States. This work was published in 1992.[28] Because of the difference in luminosity between the two stars, itsspectral 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.[29]
The two stars are now known toorbit each other with a period of 96.9 days.[7] The larger, brighter star, called theprimary, has aspectral type of B8IV-VHgMn,[30] amass of approximately 3.6solar masses,[7] asurface temperature of about 11,900K (or 13,850 K[10]), and, measured over allwavelengths, aluminosity of about 160 times theSun's.[5] Its smaller, fainter companion, thesecondary, has a mass of approximately 1.9 solar masses[7] and a surface temperature of about 7,900 K, and, again measured over all wavelengths, a luminosity of about 15 times theSun's. It is a late-type A star whose spectral type is estimated as A7V.[5]
In 1906, Norman Lockyer and F. E. Baxandall reported that α Andromedae had a number of unusual lines in itsspectrum.[31] In 1914, Baxandall pointed out that most of the unusual lines came frommanganese, and that similar lines were present in the spectrum ofμ Leporis.[32] In 1931,W. W. Morgan identified 12 additional stars with lines from manganese appearing in their spectra.[33] Many of these stars were subsequently identified as part of the group ofmercury-manganese stars,[34] a class ofchemically peculiar stars which have an excess ofelements such asmercury,manganese,phosphorus, andgallium in their atmospheres.[35], §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 excessxenon.
In 1970,Georges Michaud suggested that suchchemically peculiar stars arose from radiative diffusion. According to this theory, in stars with unusually calmatmospheres, someelements sink under the force ofgravity, while others are pushed to the surface byradiation pressure.[35], §4.[36] This theory has successfully explained many observed chemical peculiarities, including those of mercury-manganese stars.[35], §4.
α Andromedae has been reported to be slightlyvariable,[37] but observations from 1990 to 1994 found its brightness to be constant to within less than 0.01 magnitude.[38] However, Adelman and his co-workers have discovered, in observations made between 1993 and 1999 and published in 2002, that themercury line in its spectrum at 398.4nm varies as the primary rotates. This is because the distribution of mercury in its atmosphere is not uniform. ApplyingDoppler imaging to the observations allowed Adelman et al. to find that it was concentrated in clouds near the equator.[39] Subsequent Doppler imaging studies, published in 2007, showed that these clouds drift slowly over the star's surface.[9]
Andromeda constellation. α Andromedae, labeledSirrah, 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.[40]
^Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.".The Astronomical Journal.126 (4): 2048.arXiv:astro-ph/0308182.Bibcode:2003AJ....126.2048G.doi:10.1086/378365.
^abcdRyabchikova, 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.
^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.ISBN978-1-931559-44-7.
^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.
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