Bayer designation

Many of the brighter stars are given names which are known as Bayer designations. These designations, which were introduced by Johann Bayer in his star atlas Uranometria (named after Urania) in 1603, consist of a Greek letter followed by the genitive (in Latin) of the name of the constellation in which the star lies. See List of constellations for a list of constellations and the genitive forms of their names.

Because the brightest star in many constellations is designated as Alpha, many people wrongly assume that Bayer meant to put the stars 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. However within each magnitude class, there was no attempt to arrange stars by relative brightness. Bayer did not always assign Greek and Roman letters to stars in this manner. Bayer 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 which a constellation’s stars rise in the east, according to the historical or mythological information on specific stars within a constellation, or according to his own random choosing. Usually they were roughly ordered from the head to the feet (or tail) of the figure (like the stars in the Big Dipper).

Orion provides a good example of Bayer's method. (Remember that the lower the magnitude, the brighter the star. Additionally a "2nd-magnitude" star has a more precise magnitude 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 knee), even though the latter is the brighter.

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

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" route.

The belt of Orion is composed of three bright stars Delta Orionis, Epsilon Orionis and Zeta Orionis, however, brightest star in the belt is not Delta but Epsilon. Instead, Bayer named the stars of Orion's belt in the order in which they rise in the east, first delta, then the middle star epsilon, then zeta. This same method is done 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" to Castor and "beta" to Pollux.

Although the brightest star in Draco is Eltanin (gamma draconis), Thuban was assigned "alpha" by Bayer because Thuban, in history, was once the North Star.

Sometimes, indeed, there's no apparent order. (The letters of the Greek alphabet were used in antiquity to represent the successive integers; so Bayer's scheme might be regarded as equivalent to a numbering system.)

So Alpha isn't always the brightest star in a constellation. And there are even cases where a star has a designation for a constellation in which it does not lie (according to the modern constellation boundaries). Nonetheless, these designations have proved useful and are widely used today.

Two stars lie very close to constellation boundaries and have double designations: β Tau (γ Aur) and α And (δ Peg). Another star, σ Lib, was formerly known as γ Sco; however it is not on the boundary of Libra and Scorpius but well inside Libra.

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. Earlier 4-letter abbreviations (such as α UMaj) are rarely used today.

Although most common Bayer letters are Greek, it should also be mentioned that the system was extended, first by using lowercase Latin letters, and then by using uppercase Latin letters. Most of these are little used, but there are some exceptions such as h Persei (which is actually a star cluster) and P Cygni. Note that uppercase Latin Bayer designations never went beyond Q, and 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.

See also