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.

Most of the brighter stars were assigned their first scientific names by the German astronomer Johann Bayer during the early 17th Century, in 1603, in his star atlas Uranometria (named after Urania, the Greek Muse of Astronomy). Johann Bayer traveled by sailing ship to various parts of the world, including the southern hemisphere, to conduct his astronomical observations and apply his data. Bayer assigned a lower-case Greek letter, such as alpha (α), beta (β), gamma (γ), etc, to each star he cataloged. Bayer then attached to each star’s Greek letter the Latin name of the star’s parent constellation in genitive or possessive form to indicate that the star belonged exclusively to that constellation. (See List of constellations for a list of constellations and the genitive forms of their names.) For example, Bayer assigned the Greek letter alpha (α) to a specific star in the constellation Taurus) the Bull, and added to the star’s Greek letter (α) the name Tauri, which is the genitive form of the Latin name Taurus, to indicate that the star belongs exclusively to Taurus the Bull. As a result, the star’s scientific name turned out to be α Tauri (that is to say Alpha Tauri), which means "Alpha of Taurus" or "Alpha of the Bull". A single constellation may contain fifty or more stars, but there are only twenty-four letters of the Greek alphabet, so, when he ran out of Greek letters to use for identifying the stars of a specific constellation, Bayer began using lower-case Latin letters. For example, Bayer assigned three stars in the constellation Carina as s Carinae, and another star in Centaurus as d Centauri, to indicate "s of the Keel" and "d of the Centaur", respectively. Within constellations having an extremely large number of stars, Bayer eventually advanced to upper-case Latin letters, ending with the upper-case letter "Q". For example, Bayer assigned a star in Scorpius the Scorpion the scientific name G Scorpii, which means "G of Scorpius" or "G of the Scorpion". Another example would be a star in Vela the Sails, called N Velorum, which means "N of Vela" or "N of the Sails".

Is Alpha Always the Brightest Star?

For the most part, Bayer assigned Greek and Latin letters to stars in order of apparent brightness, from brightest to dimmest, within a particular constellation. The brightest star in a particular constellation was assigned alpha (α), the second brightest beta (β), the third brightest gamma (γ), and so on. Since the brightest star in many constellations is designated as 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. 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).

Bayer Designations in Orion

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. (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.

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.

Various Bayer Designation Arrangements

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.

Revised Bayer Designations

When the International Astronomical Union (IAU) outlined the official 88 constellations with definite boundaries in 1930, some stars became borderlined between constellations. The International Astronomical Union (IAU) declared that stars and other celestial objects can only be assigned one, and only one, constellation. Consequently, these borderlined stars that lie very close to constellation boundaries were reassigned to one new official constellation and were given new Bayer designations in the process. For example, β Tau, formerly known as (γ Aur), and α And, formerly known as (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.

Bayer Designation Styles

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 CMaj) are rarely used today.

Other Bayer Designations

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 rarely 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.