In astronomy, a variable-star designation is a unique identifier given to variable stars. It uses a variation on the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive 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. The identifying label can be one or two Latin letters or a V plus a number (e.g. V399). Examples are R Coronae Borealis, YZ Ceti, V603 Aquilae.
The current naming system is:
Stars with existing Greek letter Bayer designations are not given new designations.
Otherwise, start with the letter R and go through Z.
Continue with RR...RZ, then use SS...SZ, TT...TZ and so on until ZZ.
Use AA...AZ, BB...BZ, CC...CZ and so on until reaching QZ, omitting J in both the first and second positions.
Abandon the Latin script after 334 combinations of letters and start naming stars with V335, V336, and so on.
The second letter is never nearer the beginning of the alphabet than the first, e.g., no star can be BA, CA, CB, DA and so on.
In the early 19th century few variable stars were known, so it seemed reasonable to use the letters of the Latin script. Because very few constellations contained stars with uppercase Latin-letter Bayer designation greater than Q, the letter R was chosen as a starting point so as to avoid confusion with letter spectral types or the (now rarely used) Latin-letter Bayer designations. Although Lacaille had used uppercase R to Z letters in a few cases, for example X Puppis (HR 2548), these designations were either dropped or accepted as variable star designations. The star T Puppis was accepted by Argelander as a variable star and is included in the General Catalogue of Variable Stars with that designation but is now classed as non-variable.
This variable star naming convention was developed by Friedrich W. Argelander. There is a widespread belief according to which Argelander chose the letter R for German rot or French rouge, both meaning "red", because many variable stars known at that time appear red.
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A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either: Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.
In astronomy, stars have a variety of different stellar designations and names, including catalogue designations, current and historical proper names, and foreign language names. Only a tiny minority of known stars have proper names; all others have only designations from various catalogues or lists, or no identifier at all. Hipparchus in the 2nd century BC enumerated about 850 naked-eye stars. Johann Bayer in 1603 listed about twice this number. Only in the 19th century did star catalogues list the naked-eye stars exhaustively.
A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. The brighter stars were assigned their first systematic names by the German astronomer Johann Bayer in 1603, in his star atlas Uranometria. Bayer catalogued only a few stars too far south to be seen from Germany, but later astronomers (including Nicolas-Louis de Lacaille and Benjamin Apthorp Gould) supplemented Bayer's catalog with entries for southern constellations.
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