Bayer designation
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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. Instead, they're usually either 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.
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.
