Astronomical Naming Conventions

In ancient times, only the Sun and Moon, a few 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. Astronomers need to be able to assign systematic designations to unambiguously identify all of these objects, and at the same time give names to the most interesting objects, and where relevant, features of those objects.

The International Astronomical Union (IAU) is the recognized authority in astronomy for assigning designations to celestial bodies such as stars, planets, and minor planets, including any surface features on them. In response to the need for unambiguous names for astronomical objects, it has created a number of systematic naming systems for objects of various sorts.

Stars

There are no more than a few thousand stars that appear sufficiently bright in Earth's sky to be visible to the naked eye. This represents the number of stars available to be named by ancient cultures. The upper boundary to what is physiologically possible to be seen with the unaided eye is an apparent magnitude of 6, or about ten thousand stars. With the advent of the increased light-gathering abilities of the telescope, many more stars became visible, far too many to all be given names. The earliest naming system which is still popular is the Bayer designation using the name of constellations to identify the stars within them.

The IAU is the only internationally recognized authority for assigning astronomical designations to celestial objects and surface features on them. The purpose of this is to ensure that names assigned are unambiguous. There have been many historical star catalogues, and new star catalogues are set up on a regular basis as new sky surveys are performed. All designations of objects in recent star catalogues start with an "initialism", which is kept globally unique by the IAU. Different star catalogues then have different naming conventions for what goes after the initialism, but modern catalogs tend to follow a set of generic rules for the data formats used.

The IAU does not recognize the commercial practice of selling fictitious star names by commercial star-naming companies. The IAU's website uses the word charlatanry in this context.

Proper names

There are about 300 to 350 stars with traditional or historical proper names. They tend to be the brightest stars in the sky and are often the most prominent ones of the constellation. Examples are Betelgeuse, Rigel and Vega. Most such names are derived from the Arabic language ''(see List of Arabic star names § History of Arabic star names)''.

Stars may have multiple proper names, as many different cultures named them independently. Polaris, for example, has also been known by the names ''Alruccabah'', ''Angel Stern, Cynosura'', the ''Lodestar'', ''Mismar'', ''Navigatoria'', ''Phoenice'', the ''Pole Star'', the ''Star of Arcady'', ''Tramontana'' and ''Yilduz'' at various times and places by different cultures in human history.

In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN (on 30 June and 20 July 2016) together with names of stars adopted by the IAU Executive Committee Working Group on Public Naming of Planets and Planetary Satellites during the 2015 NameExoWorlds campaign and recognized by the WGSN. Further batches of names were approved on 21 August 2016, 12 September 2016 and 5 October 2016. These were listed in a table included in the WGSN's second bulletin issued in October 2016. The next additions were done on 1 February, 30 June, 5 September and 19 November 2017, and on 6 June 2018. All are included on the current List of IAU-approved Star Names, last updated on 1 June 2018.

The star nearest to Earth is typically referred to simply as "the Sun" or its equivalent in the language being used (for instance, if two astronomers were speaking French, they would call it ''le Soleil''). However, it is usually called by its Latin name, Sol, in science fiction.

Named after people

There are about two dozen stars such as Barnard's Star and Kapteyn's Star that have historic names and which were named in honor after astronomers. As a result of the NameExoWorlds campaign in December 2015 the IAU approved the names Cervantes (honoring the writer Miguel de Cervantes) and Copernicus (honoring the astronomer Nicolaus Copernicus) for the stars Mu Arae and 55 Cancri A, respectively.NameExoWorlds The Approved Names
/ref> In July 2016, the IAU WGSN approved the name ''Cor Caroli'' (Latin for 'heart of Charles') for the star Alpha Canum Venaticorum, so named in honour of King Charles I of England by Sir Charles Scarborough, his physician. In 2019, IAU held the NameExoWorlds campaign. Spain named the star HD 149143 as Rosalíadecastro after the writer Rosalía de Castro.

Catalogues

With the advent of the increased light-gathering abilities of the telescope, many more stars became visible, far too many to all be given names. Instead, they have designations assigned to them by a variety of different star catalogues. Older catalogues either assigned an arbitrary number to each object, or used a simple systematic naming scheme based on the constellation the star lies in, like the older Ptolemy's Almagest in Greek from 150 and Al-Sufi's Book of Fixed Stars in Arabic from 964. The variety of sky catalogues now in use means that most bright stars currently have multiple designations.

In 1540 the Italian astronomer Piccolomini released the book ''De le Stelle Fisse'' (On the Fixed Stars) which include star maps of 47 constellations where he numbered the stars in magnitude order using latin letters.

Bayer designation

The Bayer designations of about 1,500 brightest stars were first published in 1603. In this list, a star is identified by a lower-case letter of the Greek alphabet, followed by the Latin name of its parent constellation. The Bayer designation uses the possessive form of a constellation's name, which in almost every case ends in ''is'', ''i'' or ''ae''; ''um'' if the constellation's name is plural ''(see genitive case for constellations)''. In addition, a three-letter abbreviation is often used . Examples include Alpha Andromedae (''α And'') in the constellation of Andromeda, Alpha Centauri (''α Cen''), in the constellation Centaurus, Alpha Crucis (''α Cru'') and Beta Crucis (''β Cru''), the two brightest stars in the constellation Crux, the Southern Cross, Epsilon Carinae (''ε Car'') in Carina, Lambda Scorpii (''λ Sco'') in Scorpius and Sigma Sagittarii (''σ Sgr'') in Sagittarius. After all twenty-four Greek letters have been assigned, upper and lower case Latin letters are used, such as for A Centauri (''A Cen''), D Centauri (''D Cen''), G Scorpii (''G Sco''), P Cygni (''P Cyg''), b Sagittarii (''b Sgr''), d Centauri (''d Cen'') and s Carinae (''s Car'').

As the resolving power of telescopes increased, numerous objects that were thought to be a single object were found to be optical star systems that were too closely spaced in the sky to be discriminated by the human eye. This led to a third iteration, where numeric superscripts were added to distinguish those previously unresolved stars. Examples include Theta Sagittarii (''θ Sgr'') later distinguished as Theta¹ Sagittarii (''θ¹ Sgr'') and Theta² Sagittarii (''θ² Sgr''), each being their own (physical) star system with two and three stars, respectively.

Flamsteed designation

Flamsteed designations consist of a number and the Latin genitive of the constellation the star lies in. Examples include 51 Pegasi and 61 Cygni. About 2,500 stars are catalogued. They are commonly used when no Bayer designation exists, or when the Bayer designation uses numeric superscripts such as in Rho¹ Cancri. In this case, the simpler Flamsteed designation, 55 Cancri, is often preferred.

Modern catalogues

Most modern catalogues are generated by computers, using high-resolution, high-sensitivity telescopes, and as a result describe very large numbers of objects. For example, the Guide Star Catalog II has entries on over 998 million distinct astronomical objects. Objects in these catalogs are typically located with very high resolution, and assign designations to these objects based on their position in the sky. An example of such a designation is ''SDSSp J153259.96−003944.1'', where the initialism ''SDSSp'' indicates that the designation is from the " Sloan Digital Sky Survey preliminary objects", and the other characters indicate celestial coordinates (epoch 'J', right ascension 153259.96, declination −00°39′44.1″).

Variable stars

Variable stars are assigned designations in a variable star scheme that is based on a variation of the Bayer designation format, with an identifying label preceding the Latin genitive of the name of the constellation in which the star lies. Such designations mark them as variable stars. Examples include R Cygni, RR Lyrae, and V1331 Cygni. The International Astronomical Union delegates the task to the Sternberg Astronomical Institute in Moscow, Russia.

Compact stars

Pulsars

Pulsars such as PSR J0737-3039, are designated with a "PSR" prefix, that stands for ''Pulsating Source of Radio''. The prefix is followed by the pulsar's right ascension and degrees of declination. The right ascension is also prefixed with a "J" (Julian epoch) or a "B" ( Besselian Epochs) used prior to 1993, as in PSR B1257+12.

Black holes

Black holes have no consistent naming conventions. Supermassive black holes receive the designation of the galaxy whose core they reside in. Examples are NGC 4261, NGC 4151 and M31, which derive their designation from the '' New General Catalogue'' and the list of ''Messier objects''.

Other black holes, such as Cygnus X-1 – a highly likely stellar black hole, are cataloged by their constellation and the order in which they were discovered. A large number of black holes are designated by their position in the sky and prefixed with the instrument or survey that discovered them. Examples are (where SDSS stands for Sloan Digital Sky Survey), and RX J1131−1231, observed by the Chandra X-ray Observatory.

Supernovae

Supernova discoveries are reported to the IAU's ''Central Bureau for Astronomical Telegrams'' and are automatically given a provisional designation based on the co-ordinates of the discovery. Historically, when supernovae are identified as belonging to a "type", CBAT has also published circulars with assigned year–letter designations, and discovery details. A supernova's permanent designation is formed by the standard prefix "SN", the year of discovery, and a suffix composed of one or two letters of the Latin alphabet. The first 26 supernovae of the year receive a capital letter from ''A'' to ''Z''. Subsequent supernovae of that year are designated with pairs of lower-case letters from "aa" to "az", and then continuing with "ba" until "zz". For example, the prominent SN 1987A, was the first one to be observed in 1987. Several thousand supernovae have been reported since 1885. In recent years, several supernova discovery projects have retained their more distant supernova discoveries for in-house follow-up, and not reported them to CBAT. Starting in 2015, CBAT has scaled back its efforts to publish assigned designations of typed supernovae: By September 2014, CBAT had published names and details of 100 supernovae discovered in that year. By September 2015, CBAT had only published names of 20 supernovae discovered in that year. The Astronomer's Telegram provides some surrogate services independent from CBAT.

Four historical supernovae are known simply by the year they occurred: SN 1006 (the brightest stellar event ever recorded), SN 1054 (of which the remnant is the Crab Nebula and the Crab Pulsar

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