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 (named after Urania, the Greek Muse of Astronomy, along with Uranus, the Greek god of the sky and heavens). 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 List of constellations for the genitive forms.) For example, Aldebaran is designated α Tauri (pronounced Alpha Tauri), which means "Alpha of the Bull". (The letters of the Greek alphabet were used in antiquity as numerals, so Bayer's scheme might be regarded as a numbering system.)
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 lower-case Latin letters: hence s Carinae (s of the Keel) and d Centauri (d of the Centaur). Within constellations having an extremely large number of stars, Bayer eventually advanced to upper-case Latin letters, as in G Scorpii (G of the Scorpion) and N Velorum (N of the Sails). The last letter used by Bayer was Q.
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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.[1]
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 the order in they rise in the east, according to historical or mythological information on specific stars within a constellation, or according to his own random choosing. Usually the stars were roughly ordered from the head to the feet (or tail) of the figure (as in the Big Dipper).
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 Designation |
Apparent Magnitude |
Proper Name |
|---|---|---|
| α Ori | 0.45 | Betelgeuse |
| β Ori | 0.18 | Rigel |
| γ Ori | 1.64 | Bellatrix |
| δ Ori | 2.23 | Mintaka |
| ε Ori | 1.69 | Alnilam |
| ζ Ori | 1.70 | Alnitak |
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 very occasionally be brighter than Rigel.[2]) 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.
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, were designated as the alpha (α) of their parent constellations by Bayer.
Sometimes, indeed, there's no apparent order, as exemplified by the stars in Libra and Sagittarius, where Bayer assigned designations to stars at random.
Some stars were "border stars", falling on the boundaries between two constellations, and Bayer would assign them a Greek letter from each constellation. The two most prominent examples are β Tau/γ Aur, and α And/δ Peg. 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 two stars were scrapped, and now only Beta Tauri and Alpha Andromedae are used. Another star, σ Lib, was formerly known as γ Sco, though it is well inside Libra. A few stars still do not lie (according to the modern constellation boundaries) within the constellation for which they are named. Nonetheless, these designations have proved useful and are widely used today.
There are two common ways in which Bayer designations can be written. The designation can be written out in full, as in Alpha Canis Majoris or Beta Persei, or a lowercase Greek letter 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.
The Latin-letter extended designations are rarely used, but there are some exceptions such as h Persei (which is actually a star cluster) and P Cygni. Uppercase Latin Bayer designations never went 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 between stars with the same Bayer 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.