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A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars.
Most of the brighter stars were assigned their first systematic names by the German astronomer Johann Bayer in 1603, in his star atlas Uranometria. Bayer assigned a lower-case Greek letter, such as alpha (α), beta (β), gamma (γ), etc., to each star he catalogued, combined with the Latin name of the star’s parent constellation in genitive (possessive) form. (See 88 modern constellations for the genitive forms.) For example, Aldebaran is designated α Tauri (pronounced Alpha Tauri), which means "Alpha of the constellation Taurus".
A single constellation may contain fifty or more stars, but the Greek alphabet has only twenty-four letters. When these ran out, Bayer began using Latin letters: upper case A, followed by lower case b through z (omitting j and v), for a total of another 24 letters. Bayer never went beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include s Carinae (s of the constellation Carina), d Centauri (d of the constellation Centaurus), G Scorpii (G of the constellation Scorpius), and N Velorum (N of the constellation Vela). The last upper-case letter used in this way was Q.
Is Alpha always the brightest star?
For the most part, Bayer assigned Greek and Latin letters to stars in rough order of apparent brightness, from brightest to dimmest, within a particular constellation. Since in a majority of constellations the brightest star is designated Alpha (α), many people wrongly assume that Bayer meant to put the stars exclusively in order of their brightness, but in his day there was no way to measure stellar brightness precisely. Traditionally, the stars were assigned to one of six magnitude classes, and Bayer's catalog lists all the first-magnitude stars, followed by all the second-magnitude stars, and so on. Within each magnitude class, Bayer made no attempt to arrange stars by relative brightness.
Bayer did not always follow this rule; he sometimes assigned letters to stars according to their location within a constellation (for example: the northern, southern, eastern, or western part of a constellation), according to either the order in which they rise in the east, to historical or mythological information on specific stars within a constellation, or to his own arbitrary choosing.
Of the 88 modern constellations, there are at least 30 in which "Alpha" is not the brightest star, and four of those lack an alpha star altogether. (Constellations with no alpha include Vela and Puppis, both formerly part of Argo Navis whose alpha is Canopus in Carina.)
Bayer designations in Orion
Orion provides a good example of Bayer's method. (The lower the magnitude, the brighter the star; additionally, there is a precise definition: a "2nd-magnitude" star ranks between 1.51 and 2.50, inclusive.) Bayer first designated the two 1st-magnitude stars, Betelgeuse and Rigel, as Alpha and Beta, with Betelgeuse (the shoulder) coming ahead of Rigel (the foot), even though the latter is usually the brighter. (Betelgeuse, a variable star, can at its maximum occasionally be brighter than Rigel.) He then repeated the procedure for the stars of the 2nd magnitude. As is evident from the map and chart, he again followed a "top-down" ("north-south") route.
Various Bayer designation arrangements
This "First to Rise in the East" method is done in a number of other instances, even for Castor and Pollux of Gemini. Although Pollux is brighter than Castor, the latter was assigned alpha because it rises in the east ahead of the former. Bayer may also have assigned the stars Castor and Pollux in terms of historical or mythological knowledge. Both historically and mythologically, Castor's name is almost always mentioned first (Castor and Pollux) whenever the twins are mentioned, and that may have compelled him to assign alpha (α) to Castor and beta (β) to Pollux.
Although the brightest star in Draco is Eltanin (Gamma Draconis), Thuban was assigned alpha (α) by Bayer because Thuban, in history, was once the north pole star, 4,000 years ago. Almost every star with a history of being the North Star, including Vega, Alderamin, and Polaris, was designated as the alpha (α) of its parent constellation by Bayer.
Sometimes, indeed, there's no apparent order, as exemplified by the stars in Sagittarius, where Bayer's designations appear almost random to the modern eye. Alpha and Beta Sagittarii are perhaps the most anomalously designated stars in the sky; they are more than two magnitudes fainter than the brightest star (designated Epsilon), lie several degrees south of the main pattern (the "teapot" asterism), are more than 20 degrees off the ecliptic in a Zodiacal constellation, and do not even rise from Bayer's native Germany while Epsilon and several other brighter stars do. The order of the letters assigned in Sagittarius does correspond to the magnitudes as illustrated on Bayer's chart; but the latter don't agree with modern determinations of the magnitudes.
Bayer designations added by later astronomers generally were ordered by magnitude, but care was usually taken to avoid conflict with designations already assigned. In Libra, for example, the new designations sigma, tau, and upsilon were chosen to avoid conflict with Bayer's earlier designations, even though several stars with earlier letters are not as bright.
Bayer's miscellaneous labels
Although Bayer did not use upper-case Latin letters (except A) for "fixed stars", he did use them to label other items shown on his charts, such as neighboring constellations, miscellaneous astronomical objects, or reference lines like the Tropic of Cancer.:p. 131 In Cygnus, for example, Bayer's fixed stars run through g, and on this chart Bayer employs H through P 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. In charts for constellations that did not exhaust the Greek letters, Bayer sometimes used the left-over Greek letters for miscellaneous labels as well.:p. 131
Revised Bayer designations
Ptolemy designated three stars as "border stars", each shared by two constellations; and Bayer assigned these a Greek letter from both constellations: β Tauri = γ Aurigae, α Andromedae = δ Pegasi, and ν Boötis = ψ Herculis. When the International Astronomical Union (IAU) outlined the official 88 constellations with definite boundaries in 1930, it declared that stars and other celestial objects can be assigned to only one constellation. Consequently, the redundant Bayer designations for those three stars were scrapped in favor of Beta Tauri, Alpha Andromedae, and Nu Boötis.
Other cases of multiple Bayer designations arose when stars named by Bayer in one constellation were transferred to a different constellation. Bayer's Gamma and Omicron Scorpii, for example, were later reassigned from Scorpius to Libra and given the new names Sigma and Upsilon Librae. (To add to the confusion, the star now known as Omicron Scorpii was not named by Bayer but was assigned the designation o Scorpii (Latin lower case 'o') by Lacaille – which later astronomers misinterpreted as omicron once Bayer's omicron had been reassigned to Libra.)
A few stars no longer lie (according to the modern constellation boundaries) within the constellation for which they are named. The proper motion of Rho Aquilae, for example, recently carried it across the boundary into Delphinus. Nonetheless, these designations have proved useful and are widely used today.
Bayer designation styles
Greek letters can be used together with the standard 3-letter abbreviation of the constellation, as in α CMa or β Per. Or the two can be combined (α Canis Majoris). Earlier 4-letter abbreviations (such as α CMaj) are rarely used today.
Other Bayer designations
The Latin-letter extended designations are not as commonly used as the Greek-letter ones, but there are some exceptions such as h Persei (which is actually a star cluster) and P Cygni. Uppercase Latin Bayer designations in modern use do not go beyond Q; names such as R Leporis and W Ursae Majoris are variable star designations, not Bayer designations.
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 are double stars (mostly optical doubles rather than true binary stars), but there are some exceptions such as the chain of stars π1, π2, π3, π4, π5 and π6 Orionis.
- Bayer objects
- Flamsteed designation
- Gould designation
- List of constellations
- Star catalogue
- Star designation
- Table of stars with Bayer designations
- Variable star designation
- The letters of the Greek alphabet were used in antiquity as numerals, however in a different way.
- Ridpath, Ian (1989). "Bayer's Uranometria and Bayer letters". Star Tales. Lutterworth Press. ISBN 0718826957.
- Swerdlow, N. M. (August 1986). "A Star Catalogue Used by Johannes Bayer". Journal of the History of Astronomy 17 (50): 189–197. Bibcode:1986JHA....17..189S. See p. 192.
- Patrick Moore, Brilliant Stars, 1996.
- Wagmon, Morton (2003). Lost Stars. McDonald & Woodward. ISBN 0939923785.