Detail of Bayer's chart forOrion showing thebelt stars andOrion Nebula region, with both Greek and Latin letter labels visible
ABayer designation is astellar designation in which a specificstar is identified by aGreek orLatin letter followed by thegenitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1564 stars. The brighter stars were assigned their first systematic names by the GermanastronomerJohann Bayer in 1603, in his star atlasUranometria. Bayer catalogued only a few stars too far south to be seen from Germany, but later astronomers (includingNicolas-Louis de Lacaille andBenjamin Apthorp Gould) supplemented Bayer's catalog with entries for southern constellations.
Bayer assigned a lowercaseGreek letter (alpha (α), beta (β), gamma (γ), etc.) or a Latin letter (A, b, c, etc.) to each star he catalogued, combined with the Latin name of the star's parent constellation ingenitive (possessive) form. The constellation name is frequently abbreviated to a standard three-letter form.[1][a] For example,Aldebaran in the constellationTaurus (the Bull) is designatedα Tauri (abbreviatedα Tau, pronouncedAlpha Tauri), which means "Alpha of the Bull".[b][2]
Bayer used Greek letters for the brighter stars, but the Greek alphabet has only twenty-four letters, while a single constellation may contain fifty or more stars visible to the naked eye. When the Greek letters ran out, Bayer continued with Latin letters: uppercaseA, followed by lowercaseb throughz (omittingj andv, buto was included), for a total of another 24 letters.[3]
Bayer did not label "permanent" stars with uppercase letters (except forA, which he used instead ofa to avoid confusion withα). However, a number of stars in southern constellations have uppercase letter designations, likeB Centauri andG Scorpii. These letters were assigned by later astronomers, notably Lacaille in hisCoelum Australe Stelliferum and Gould in hisUranometria Argentina. Lacaille followed Bayer's use of Greek letters, but this was insufficient for many constellations. He used first the lowercase letters, starting witha, and if needed the uppercase letters, starting withA, thus deviating somewhat from Bayer's practice. Lacaille used the Latin alphabet three times over in the large constellationArgo Navis, once for each of the three areas that are now the constellations ofCarina,Puppis andVela. That was still insufficient for the number of stars, so he also used uppercase Latin letters such asN Velorum andQ Puppis. Lacaille assigned uppercase letters between R and Z in several constellations, but these have either been dropped to allow the assignment of those letters to variable stars or have actually turned out to be variable.[4]
In most constellations, Bayer assigned Greek and Latin letters to stars within a constellation in rough order ofapparent brightness, from brightest to dimmest. The order is not necessarily a precise labeling from brightest to dimmest: in Bayer's day stellar brightness could not be measured precisely. Instead, stars were traditionally assigned to one of six magnitude classes (the brightest to first magnitude, the dimmest to sixth), and Bayer typically ordered stars within a constellation by class: all the first-magnitude stars (in some order), followed by all the second-magnitude stars, and so on. Within each magnitude class, Bayer made no attempt to arrange stars by relative brightness.[5] As a result, the brightest star in each class did not always get listed first in Bayer's order—and the brightest star overall did not necessarily get the designation "Alpha". A good example is the constellationGemini, wherePollux is Beta Geminorum and the slightly dimmerCastor is Alpha Geminorum.
In addition, Bayer did not always follow the magnitude class rule; he sometimes assigned letters to stars according to their location within a constellation, or the order of their rising, or to historical or mythological details. Occasionally the order looks quite arbitrary.[3]
Of the 88 modern constellations, there are at least 30 in which Alpha is not the brightest star, and four of those lack a star labeled "Alpha" altogether. The constellations with no Alpha-designated star includeVela andPuppis—both formerly part ofArgo Navis, whose Greek-letter stars were split among three constellations.Canopus, the former α Argus, is now α Carinae in the modern constellationCarina.Norma's Alpha and Beta were reassigned toScorpius and re-designatedN andH Scorpii respectively, leaving Norma with no Alpha.Francis Baily died before designating an Alpha inLeo Minor, so it also has no Alpha. (The star46 Leonis Minoris would have been the obvious candidate.)
InOrion, Bayer first designatedBetelgeuse andRigel, the two 1st-magnitude stars (those of magnitude approximately 1.5 or less), as Alpha and Beta from north to south, with Betelgeuse (the shoulder) coming ahead of Rigel (the foot), even though the latter is usually the brighter. (Betelgeuse is a variable star and can at its maximum occasionally outshine Rigel.)[6] Bayer then repeated the procedure for the stars of the 2nd magnitude, labeling them fromgamma throughzeta in "top-down" (north-to-south) order. Letters as far as Latinp were used for stars of the sixth magnitude.
Although Bayer did not use uppercase Latin letters (exceptA) for "fixed stars", he did use them to label other items shown on his charts, such as neighboring constellations, "temporary stars", miscellaneous astronomical objects, or reference lines like the Tropic of Cancer.[7]: p. 131 InCygnus, for example, Bayer's fixed stars run throughg, and on this chart Bayer employsH throughP as miscellaneous labels, mostly for neighboring constellations. Bayer did not intend such labels as catalog designations, but some have survived to refer to astronomical objects:P Cygni for example is still used as a designation for Nova Cyg 1600. Tycho's Star (SN 1572), another "temporary star", appears as B Cassiopeiae. In charts for constellations that did not exhaust the Greek letters, Bayer sometimes used the leftover Greek letters for miscellaneous labels as well.[7]: p. 131
Bayer assigned two stars duplicate names by mistake:Xi Arietis (duplicated asPsi Ceti) andKappa Ceti (Kappa1 andKappa2) (duplicated asg Tauri). He corrected these in a later atlas, and the duplicate names were no longer used.[7]: p. 23
Other cases of multiple Bayer designations arose when stars named by Bayer in one constellation were transferred by later astronomers to a different constellation. Bayer's Gamma and Omicron Scorpii, for example, were later reassigned fromScorpius toLibra and given the new namesSigma andUpsilon Librae.[7]: p. 196 (To add to the confusion, the star now known asOmicron Scorpii was not named by Bayer but was assigned the designation o Scorpii (Latin lowercase 'o') by Lacaille—which later astronomers misinterpreted as omicron once Bayer's omicron had been reassigned to Libra.)[7]: p. 278
A few stars no longer lie (according to the modern constellation boundaries) within the constellation for which they are named. Theproper motion ofRho Aquilae, for example, carried it across the boundary intoDelphinus in 1992.[9][10]
A further complication is the use of numeric superscripts to distinguish neighboring stars that Bayer (or a later astronomer) labeled with a common letter. Usually these aredouble stars (mostly optical doubles rather than truebinary stars), but there are some exceptions such as the chain of starsπ1,π2,π3,π4,π5 andπ6Orionis. The most stars given the same Bayer designation but with an extra number attached to it isPsi Aurigae. (ψ1,ψ2,ψ3,ψ4,ψ5,ψ6,ψ7,ψ8,ψ9,ψ10, although according to the modern IAU constellation boundaries, ψ10 lies inLynx).
^Hirshfeld, A.; et al. (August 1992). "Book-Review - Sky Catalogue 2000.0 - V.1 - Stars to Magnitude 8.0 ED.2".Journal of the Royal Astronomical Society of Canada.86 (4): 221.Bibcode:1992JRASC..86..221L.
