Variable star designation
Encyclopedia
Variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

s are named using a variation on the Bayer designation
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...

 format of an identifying label (as described below) combined with the Latin genitive
Genitive case
In grammar, genitive is the grammatical case that marks a noun as modifying another noun...

 of the name of the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 in which the star lies. See List of constellations for a list of constellations and the genitive forms of their names.

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.
  • Finally give up on the Roman alphabet after 334 combinations of letters and start naming stars with V335, V336, and so on.


Sample designations are R Coronae Borealis
R Coronae Borealis
R Coronae Borealis is a yellow supergiant star, and is the prototype of the RCB class of variable stars, which fade by several magnitudes at irregular intervals...

, YZ Ceti
YZ Ceti
YZ Ceti is a red dwarf star in the constellation Cetus. Although it is relatively close to the Sun at just 12 light years, this star cannot be seen with the naked eye. It is classified as a flare star that undergoes intermittent fluctuations in luminosity...

, and V603 Aquilae
V603 Aquilae
V603 Aquilae was a bright nova occurring in Aquila in 1918.It reached a magnitude of −1.4, and was the brightest nova of modern times.-References:* -External links:* * *...

.

Note that the first letter is never further up the alphabet
Alphabet
An alphabet is a standard set of letters—basic written symbols or graphemes—each of which represents a phoneme in a spoken language, either as it exists now or as it was in the past. There are other systems, such as logographies, in which each character represents a word, morpheme, or semantic...

 than the second, that is to say no star can be BA, CA, CB, DA or so on.

History

In the early 19th century few variable stars were known, so it seemed reasonable to use the letters of the Roman alphabet, starting from the letter R so as to avoid confusion with letter spectral types or the (now rarely used) Latin-letter Bayer designations. This system of astronomical naming convention
Astronomical naming conventions
In ancient times, only the Sun and Moon, a few hundred stars and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year...

 was developed by Friedrich W. Argelander
Friedrich Wilhelm Argelander
Friedrich Wilhelm August Argelander was a German astronomer. He is known for his determinations of stellar brightnesses, positions, and distances.- Life and work :...

. There is a widespread belief according to which Argelander chose the letter R for German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

 rot or French
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 rouge, both meaning "red", because many variable stars known at that time appear red. However, Argelander's own statement disproves this.

By 1836, even the letter S had only been used in one constellation, Serpens
Serpens
Serpens is a constellation of the northern hemisphere. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations defined by the International Astronomical Union....

. With the advent of photography the number of variables piled up quickly, and variable star names soon fell into the Bayer trap of reaching the end of the alphabet while still having stars to name. After two subsequent supplementary double-lettering systems hit similar limits, numbers were finally introduced.
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