Bayer designation

Scheme

Bayer assigned a lowercase Greek letter (alpha (α), beta (β), gamma (γ), etc.) or a Latin letter (A, b, c, etc.) to each star he catalogued, combined with the Latin name of the star's parent constellation in genitive (possessive) form. The constellation name is frequently abbreviated to a standard three-letter form. For example, Aldebaran in the constellation Taurus (the Bull) is designated α Tauri (abbreviated α Tau, pronounced Alpha Tauri), which means "Alpha of the Bull".

Bayer used Greek letters for the brighter stars, but the Greek alphabet has only twenty-four letters, while a single constellation may contain fifty or more stars visible to the naked eye. When the Greek letters ran out, Bayer continued with Latin letters: uppercase A, followed by lowercase b through z (omitting j and v, but o was included), for a total of another 24 letters.

Bayer did not label "permanent" stars with uppercase letters (except for A, which he used instead of a to avoid confusion with α). However, a number of stars in southern constellations have uppercase letter designations, like B Centauri and G Scorpii. These letters were assigned by later astronomers, notably Lacaille in his Coelum Australe Stelliferum and Gould in his Uranometria Argentina. Lacaille followed Bayer's use of Greek letters, but this was insufficient for many constellations. He used first the lowercase letters, starting with a, and if needed the uppercase letters, starting with A, thus deviating somewhat from Bayer's practice. Lacaille used the Latin alphabet three times over in the large constellation Argo Navis, once for each of the three areas that are now the constellations of Carina, Puppis and Vela. That was still insufficient for the number of stars, so he also used uppercase Latin letters such as N Velorum and Q Puppis. Lacaille assigned uppercase letters between R and Z in several constellations, but these have either been dropped to allow the assignment of those letters to variable stars or have actually turned out to be variable.

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. The order is not necessarily a precise labeling from brightest to dimmest: in Bayer's day stellar brightness could not be measured precisely. Instead, 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 (in some order), followed by all the second-magnitude stars, and so on. Within each magnitude class, Bayer made no attempt to arrange stars by relative brightness. As a result, the brightest star in each class did not always get listed first in Bayer's order—and the brightest star overall did not necessarily get the designation "Alpha". A good example is the constellation Gemini, where Pollux is Beta Geminorum and the slightly dimmer Castor is Alpha Geminorum.

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.

Of the 88 modern constellations, there are at least 30 in which Alpha is not the brightest star, and four of those lack a star labeled "Alpha" altogether. The constellations with no Alpha-designated star include Vela and Puppis—both formerly part of Argo Navis, whose Greek-letter stars were split among three constellations. Canopus, the former α Argus, is now α Carinae in the modern constellation Carina. Norma's Alpha and Beta were reassigned to Scorpius and re-designated N and H Scorpii respectively, leaving Norma with no Alpha. Francis Baily died before designating an Alpha in Leo Minor, so it also has no Alpha. (The star 46 Leonis Minoris would have been the obvious candidate.)

Orion as an example

In Orion, 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.) Bayer then repeated the procedure for the stars of the 2nd magnitude, labeling them from gamma through zeta in "top-down" (north-to-south) order. Letters as far as Latin p were used for stars of the sixth magnitude.

Various 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 uppercase Latin letters (except A) for "fixed stars", he did use them to label other items shown on his charts, such as neighboring constellations, "temporary stars", miscellaneous astronomical objects, or reference lines like the Tropic of Cancer.{{cite book

Revised 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 (Nu1 and Nu2)(in Boötes and Hercules) and Fomalhaut (in Piscis Austrinus and Aquarius). Bayer assigned the first three of these stars a Greek letter from both constellations: , , and . (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 (Kappa1 and Kappa2) (duplicated as g Tauri). He corrected these in a later atlas, and the duplicate names were no longer used.

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. (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 lowercase 'o') by Lacaille—which later astronomers misinterpreted as omicron once Bayer's omicron had been reassigned to Libra.)

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, carried it across the boundary into Delphinus in 1992.

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. The most stars given the same Bayer designation but with an extra number attached to it is Psi Aurigae. (ψ1, ψ2, ψ3, ψ4, ψ5, ψ6, ψ7, ψ8, ψ9, ψ10, although according to the modern IAU constellation boundaries, ψ10 lies in Lynx).

References

Notes

References

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2. Bayer, Johann. (1987). "Uranometria". Archival Facsimiles.
3. Ridpath, Ian. (1989). "Star Tales". Lutterworth Press.
4. (1763). "Coelum australe stelliferum ...". H.L. Guerin & L.F. Delatour.
5. Swerdlow, N. M.. (August 1986). "A Star Catalogue Used by Johannes Bayer". Journal for the History of Astronomy.
6. Patrick Moore. (1996). "Brilliant Stars". Cassell.
7. Moore, Patrick. (2005). "The Observer's Year: 366 Nights in the Universe".
8. (1930). "Délimitation scientifique des constellations". At the University press.
9. Patrick Moore. (29 June 2013). "The Observer's Year: 366 Nights of the Universe". Springer Science & Business Media.
10. (August 1992). "Book-Review - Sky Catalogue 2000.0 - V.1 - Stars to Magnitude 8.0 ED.2". Journal of the Royal Astronomical Society of Canada.