Bayer designation

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".^[1][2]

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.^[3] Bayer never went beyond z,^[3] 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.

Bayer catalogued only a few stars too far south to be seen from Germany, but later astronomers (notably Lacaille and Gould) supplemented Bayer's catalog with entries for southern constellations.

Order by magnitude class

In most constellations, Bayer assigned Greek and Latin letters to stars within a constellation in rough order of apparent brightness, from brightest to dimmest. Since the brightest star in a majority of constellations is designated Alpha (α), many people wrongly assume that Bayer meant to order the stars exclusively by brightness. In Bayer's day, however, stellar brightness could not be measured precisely. 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, followed by all the second-magnitude stars, and so on. Within each magnitude class, Bayer made no attempt to arrange stars by relative brightness.^[4] As a result, the brightest star in each class did not always get listed first in Bayer's order.

But 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 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 constellation map

Orion provides a good example of Bayer's method. Bayer first designated Betelgeuse and Rigel, the two 1st-magnitude stars (those of magnitude 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.)^[5] Bayer then repeated the procedure for the stars of the 2nd magnitude (those between magnitudes 1.51 and 2.5), labeling them from gamma through zeta in "top-down" (north-to-south) order.

Various Bayer designation arrangements

The "First to Rise in the East" order is used in a number of instances. Castor and Pollux of Gemini may be an example of this: Pollux is brighter than Castor, but the latter rises earlier and was assigned alpha. In this case, Bayer may also have been influenced by the traditional order of the mythological names "Castor and Pollux": Castor is generally named first whenever the twins are mentioned.

Although the brightest star in Draco is Eltanin (Gamma Draconis), Thuban was assigned alpha (α) by Bayer because, due to precession, Thuban was the north pole star 4,000 years ago.

Sometimes there is 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), they lie several degrees south of the main pattern (the "teapot" asterism), they are more than 20 degrees off the ecliptic in a Zodiacal constellation, and they 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 do not 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.^{[6]: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.^{[6]:p. 131}

Revised Bayer designations

Ptolemy designated four stars as "border stars", each shared by two constellations: Alpheratz (in Andromeda and Pegasus), Elnath (in Taurus and Auriga), Nu Boötis (in Boötes and Hercules), and Fomalhaut (in Piscis Austrinus and Aquarius).^{[6]:p. 23} Bayer assigned the first three of these stars a Greek letter from both constellations: Alpha Andromedae = Delta Pegasi, Beta Tauri = Gamma Aurigae, and Nu Boötis = Psi Herculis. (He catalogued Fomalhaut only once, as Alpha Piscis Austrini.) When the International Astronomical Union (IAU) assigned definite boundaries to the constellations in 1930, it declared that stars and other celestial objects can belong to only one constellation. Consequently, the redundant second designation in each pair above has dropped out of use.

Bayer assigned two stars duplicate names by mistake: Xi Arietis (duplicated as Psi Ceti) and Kappa Ceti (duplicated as g Tauri). He corrected these in a later atlas, and the duplicate names were no longer used.^{[6]: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 from Scorpius to Libra and given the new names Sigma and Upsilon Librae.^{[6]:p. 196} (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.)^{[6]:p. 278}

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.

Bayer designation styles

Bayer designations are most often written as the Greek or Latin letter followed by the standard 3-character constellation abbreviation: α CMa, β Per; or occasionally with the constellation genitive in full: α Canis Majoris, β Persei. Earlier 4-letter abbreviations (α CMaj, β Pers) are rarely used today. The Greek letter names are sometimes written out as well: Alpha Canis Majoris, Beta Persei.

Other Bayer designations

The Latin-letter extended designations are not as commonly used as the Greek-letter ones (especially in constellations with Flamsteed designations), 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.

See also

• Bayer objects
• Flamsteed designation
• Gould designation
• Lists of constellations
• Star catalogue
• Star designation
• Table of stars with Bayer designations
• Variable star designation

References