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There are two terms that we have to define before we can continue: name and designation. Throughout this Theme, and in other IAU publications, name refers to the alphabetical name used for a star in everyday speech (e.g. “Vega”), while the term designation is always alphanumerical and almost exclusively found in the official catalogues used by professional astronomers (e.g. “HD 172167”).
Development of Stellar Nomenclature
Tracking stars and their attributes, or cataloguing stars, has a long history. Since prehistory, cultures and civilizations worldwide have given their own unique names to the brightest and most prominent stars in the night sky.
The first star catalogues we are aware of were made in the ancient Greek and ancient Chinese cultures, almost at the same time, roughly 2000 years ago. In Greece, a catalogue was developed over centuries by scholars like Eudoxus (4th century BCE), Eratosthenes (3rd century BCE), Hipparchus (2nd century BCE) and others that were compiled by Ptolemy (2nd century CE), cf. Dekker (2013) and Hoffmann (2017). In China, three big schools developed star catalogues in the centuries BCE: the Gan De, Shi Shen, and (the earlier) Wu Xian. During the Han dynasty (between roughly 200 BCE and 200 CE), these three catalogues were combined (Sun & Kistemaker 1997); the origins are indicated in the earliest known map, the Dunhuang star chart (7th century CE). The list of 27 stars in India marking the positions of planets and moon and sun along the zodiac has been in use for the last 3000 years or so. The names are written down in almost all texts (e.g., Burgess, et al., 1935). While the Chinese star catalogues contained more stars (roughly 1400±100) but not all of them with coordinates, Ptolemy’s Almagest (2nd century CE) preserves a list of 1029 stars and other objects with complete ecliptic coordinates. Most of the stars in both catalogues do not have proper names. The Chinese system enumerates the stars within one constellation (producing stellar designators like Fang 1, Fang 2, Fang 3, and Fang 4 within the constellation Fang) while Ptolemy and his Greek predecessors describe stellar positions within the figure, e.g. “the star at the right shoulder of Orion”, where Orion is a constellation. Only a few star names are preserved unchanged since Greek antiquity: Sirius (found in Hesiod’s Works and Days from the 8th century BCE, but not in the Almagest), Antares, Arcturus, Canopus, Capella, Procyon, Regulus, Spica, Vindemiatrix and the two Aselli in Cancer. Most of the modern star names are a product of the processes of translation, misunderstandings, abbreviations and transformation from the original Greek texts to Arabic to mediaeval Latin and modern languages.
As precession (the wobble of the Earth’s axis) has been known since antiquity, Ptolemy gave the coordinates in the ecliptic system in order to ease the computation of their shift over the course of the next centuries. Thus, many generations of astronomers simply used the Almagest star catalogue almost unchanged: an appropriate longitude correction for their own epoch was often used. For many purposes, the accuracy of the coordinates in the Almagest transferred with the so-called “precession constant” from the Almagest was sufficient and for a millennium only few improvements were made, in particular by Islamic astronomers. For instance, Abd Al-Rahman al-Sufi (10th century) added some indigenous Arabic star names to the translated Greco-Roman ones and other scholars, like Al-Zarkali (11th century), improved the accuracy of instrumentation for navigation and other measurements. Still, it was only in the Early Modern Age when several scholars attempted to re-observe the whole catalogue, namely the Samarkand prince astronomer Ulugh Beg (1394-1449), the German astronomer Christoph Rothmann and the Danish nobleman Tycho Brahe, who both made their observations in the 1580s and 1590s. Tycho’s catalogue was first published in 1602, shortly after his death, by Johannes Kepler but Rothmann’s catalogue was not published until 1666. Arab astronomers in the previous centuries (Al-Sufi in 964 and Ulugh Beg in 1437) only partially revised Ptolemy’s data. Similar developments are observed in the Far East (cf. Débarbat et al., eds. 1988): the original Chinese star catalogue was occasionally transformed with precession and/or extended by astronomers in the Chinese empire and in the kingdoms of Korea and Japan.
Early Modern Expansions
In the era of European maritime expansion and the Golden Age of Netherlandish cartography when navigators brought extended lists of southern stars from their exploratory voyages, the German lawyer and amateur astronomer Johann Bayer created a new nomenclature of systematic short star names. In his star atlas Uranometria (1603), Bayer labelled the stars in each constellation with lowercase Greek letters, in the approximate order of their (apparent) brightness. The brightest star in a constellation was usually — but not always — labelled Alpha, the second brightest, labelled Beta, and so on. For example, the brightest star in Cygnus (the Swan) is Alpha Cygni (note the use of the genitive of the Latin constellation name), also named Deneb, and the brightest star in Leo (the Lion) is Alpha Leonis, also named Regulus.
Unfortunately, this visual sorting scheme ran into some difficulties. Misestimates and other irregularities meant it wasn’t always accurate. For example, the brightest star in Gemini (the Twins) is Beta Geminorum (Pollux), not Alpha Geminorum (Castor; actually the second brightest star in the constellation), as we might expect. Additionally, the Greek alphabet only has 24 letters, and many constellations contain far more stars, even if the naming system is restricted to those visible to the naked eye. Bayer attempted to fix this problem by introducing letters from the modern Latin alphabet, starting with an uppercase A followed by lowercase letters (b, c, d, etc. to z). Uppercase letters (B to Z) were also used for denoting non-stellar objects such as position circles, bright stars of neighbouring constellations visible on the same plate and in a few isolated cases for temporary or variable stars (B for SN1572 in Cassiopeia, P for the new star of 1600 in Cygnus and H for a star in Gemini).
The Golden Age of Celestial Atlases
Nearly 200 years after Bayer’s Greek letter system was introduced, another popular scheme arose, known as the Flamsteed numbers and named after the first English Astronomer Royal, John Flamsteed. While observing at Greenwich, Flamsteed made the first significant star catalogue with a telescope, which was published posthumously in 1725 (Stellarum Inerrantium Catalogus Britannicus, British Catalogue of Fixed Stars) and again in 1729 as Atlas Coelestis, containing 2936 entries in the final version. Flamsteed’s 1729 catalogue was the foundation for many great and famous celestial atlases of the 18th century, namely by the French “engineer of the king” Jean Fortin (editor of the Atlas Céleste; 1776) and the German astronomer Johann E. Bode (Vorstellung der Gestirne; 1782), cf. Latußeck and Hoffmann (2017). What we now know as Flamsteed numbers were not allocated by Flamsteed himself, but by a French astronomer, Jérôme Lalande, in a French edition of Flamsteed’s catalogue published in 1783 (Éphémérides des Mouvements Célestes pour le Méridien de Paris). In this scheme, stars are numbered based on their position on the celestial sphere. More specifically, they are numbered in order of increasing right ascension within each constellation; for example, the westernmost star in Cygnus is labelled 1 Cygni (this is incredibly similar to the traditional Chinese nomenclature). Today, the Flamsteed numbers don’t always produce the right ordering because the precession of the Earth changes stellar coordinates in Flamsteed’s equatorial coordinate system (this is why Ptolemy had used the ecliptic system, as he argued in the Almagest, book seven).
It makes sense to designate stars with regard to their constellations because the shape of the constellations is rather stable over the course of the few centuries of written cultures. On geological and astronomical timescales, the individual motions of stars will progressively change their positions on the celestial sphere in various different directions. However, the timescale necessary to recognize this deformation is one or many precession cycles. For fast, nearby stars, this is on the order of 20,000 years, whereas the history of star catalogues is only a tenth of this age.
Other designation schemes for bright stars have been introduced but have not received the same level of popularity. One such scheme, built on Flamsteed numbering, was introduced by the American astronomer Benjamin Gould in 1879 (Uranometria Argentina). However, only a handful of stars are occasionally referenced with the Gould scheme today: for example, 38G Puppis.
As global scientific astronomy advanced, astronomers not only observed the locally visible stars but conducted systematic surveys from observatories built all over the world. For example, astronomers from China, Europe, and the United States built observatories in the southern hemisphere. Additionally, from the 19th century onwards, anthropological, ethnographic, and cultural astronomy studies of many of the world's Indigenous cultures have brought Traditional Knowledge of the night sky to the forefront. With globalization came the need for a universal cataloguing system in which the brightest stars are known by the same labels, regardless of country or culture.
With the advent of new observing technologies, stars discovered in the modern era will inevitably be much fainter than those catalogued under the Bayer or Flamsteed schemes. Bode’s second atlas, the Uranographia (1801) contained already 17240 objects and Gould’s Uranometria Argentina (1879) had 32448 objects. As astronomers discover these new stars and nebulae, it is standard practice to identify them using an alphanumeric designation. These designations are practical as modern star catalogues typically contain thousands, millions, or even billions of objects, such as those released from the European Space Agency’s satellites HIPPARCOS and Gaia.
Several catalogues of faint stars have been around for many years, such as the Bonner Durchmusterung (BD; Argelander 1859-1862), the Henry Draper Catalogue (HD; 1918-1924) and the Boss General Catalogue (GC; 1910). The Cordoba Durchmusterung (CD; Thome 1892–1932) and the Cape Durchmusterung (Kapteyn 1895–1900) supplement the BD for stars in the southern hemisphere. Other catalogues commonly used are the Bright Star Catalogue (Harvard Revised Photometry, HR; originally Yale University 1908, first published by Schlesinger 1930, more recently Hoffleit 1991), the Smithsonian Astrophysical Observatory Catalogue (SAO; 1966), or the Positions and Proper Motions Catalogue (PPM; Roeser S. and Bastian U., ARI Heidelberg, 1991). The same star can appear in several catalogues, each with a different designation. For example, Betelgeuse (the red giant in the constellation Orion) is known as Alpha Orionis (Bayer), 58 Orionis (Flamsteed), BD +7 1055, HD 39801, HR 2061, SAO 113271, and PPM 149643.
As the number of star surveys grew, there arose a need to provide guidelines for creating new designations consisting of acronyms (“identifiers”) and either indices or stellar positions. The IAU Working Group on Designations & Nomenclature provides guidelines on creating new alphanumeric designations for astronomical objects outside the solar system.
Binary and multiple systems
Stars in binary or multiple systems are labelled in various ways: by capital letters from the Latin alphabet if the star has a common colloquial name; by Bayer name; by Flamsteed designation; or by a catalogue number. For example, the brightest star in the sky, Sirius (Canis Major), has a white dwarf companion that is catalogued as each of the following: Sirius B, Alpha Canis Majoris B, and HD 48915 B.
In 1862, a cataloguing scheme for variable stars — whose brightness appear to fluctuate over time — was proposed by German astronomer Friedrich Wilhelm Argelander (of Bonner Durchmusterung fame). Building on Bayer’s scheme, Argelander suggested using the letters R through Z for the variable stars within each constellation (and occasionally Q).
Initially, the nine available letters seemed more than sufficient to label the small number of variable stars in each constellation. However, as the number of variable stars grew, Argelander’s scheme was extended to two-letter names and then to include numbers.
Today, variable stars are catalogued in a few slightly different ways according to their order of discovery. The first variable to be discovered in each constellation was assigned the letter R and the Latin genitive of the constellation name, such as R Andromedae. The second discovered variable is dubbed S, continuing in this fashion up to Z, beyond which two-letter names are introduced, such as RR Lyrae. The RR designation is followed by RS up to RZ, and then SS to SZ, and so forth up to ZZ. As more variable stars are discovered, the scheme returns to AA to AZ, BB to BZ, and up to QQ to QZ. Interestingly, the letter J is omitted in this scheme to avoid confusion with the letter I.
This system provides 334 possible unique designations for variable stars in a constellation. If even more are discovered, the catalogue turns to designations whereby the constellation name is preceded by the letter V and a number (e.g., V1500 Cygni), which can continue indefinitely. The exceptions are those variable stars already assigned a Bayer name, which are not given a new name according to this scheme (for example, Delta Cephei, Beta Lyrae, Beta Persei, or Omicron Ceti). Additionally, the type of variable star is classified based on a well-known prototype. Such examples include Mira, RR Lyrae, or Delta Cephei stars (also known as Cepheids).
Novae and supernovae
Stellar explosions called novae and supernovae are transient brightening events that may appear regularly or irregularly, or appear for a short time and disappear. These stellar explosions employ a slightly different alphanumeric system than variable stars. Novae are assigned designations according to their constellation, together with the year their superlumination event occurred (e.g. Nova Cygni 1975), and are later given variable star designations. Indeed, Nova Cygni 1975 refers to the same object as V1500 Cygni, mentioned above, but V1500 Cyg designates the variable stellar system, a cataclysmic binary, and Nova Cygni 1975 refers to its 1975 eruption. Supernovae are also named for their year of occurrence, along with the letters SN and an additional uppercase letter, as in the supernova SN 1987A. A double-lowercase designation is used (e.g., SN 1997bs) if a single year is particularly flush with supernova events. When double-lowercase designations are exhausted, triple-lowercase (e.g. SN 2022hrs) and even quadruple-lowercase designations (e.g. SN 2021afsj) are used. Since 2016 Jan. 1, the Transient Name Server (TNS, https://www.wis-tns.org/) gives an AT (Astronomical Transient) designation (like as AT 2016A) when a suspected supernova is reported, and if it is confirmed as a supernova spectroscopically, it is renamed to the SN designation (like as SN 2016A).
IAU Working Group on Star Names (WGSN)
In 2016, the IAU mobilised the Working Group on Star Names (WGSN) under its Division C (Education, Outreach, and Heritage), whose purpose was to formally catalogue the names of stars, beginning with the brightest and best-known. The WGSN is an international coalition of astronomers and cultural astronomers, each bringing different perspectives and experiences. Further details on the group’s establishment can be found in this press release.
Alphanumeric designations are useful for astronomers to officially identify the stars they study. Nonetheless, in many instances, such as for bright stars and stars of historical, cultural, or astrophysical interest, it can be more convenient to refer to them by a memorable name. Many names are already in common parlance and have been for a long time, but until the WGSN was established, there was no official, IAU catalogue of names for the brightest stars in our sky.
The WGSN aims to solve the problems that have arisen over the centuries for global academic astronomy due to different cultures — as well as different astronomers — giving their own names to stars. Until recently, some of the most famous stars in the sky — including Sirius, Rigel and Betelgeuse — had no official spelling. In some instances, stars had multiple names, and sometimes different stars had identical names. For example, a cursory perusal through historical and cultural astronomy literature finds over 40 names for the star commonly known as Fomalhaut (e.g. cf. Allen, 1899; Kunitzsch, 1959, pp. 164–165; Johnson, Mahelona & Ruggles, in press). While this particular spelling has seen the most use over the centuries, references to the same star have shown up in the literature as Fom-al hut al-jenubi, Fomahandt, Fomahant, Fomal'gaut, Fomal'khaut, Fomalhani, Fomalhut, Formalhaut, Fumahant, Fumahaut, and Fumalhaut (cf. Allen 1899, pp. 345-346). By creating an IAU-endorsed stellar name catalogue, the Working Group greatly reduced this confusion. Similarly, the unique IAU star names will not be available for future use as potential names for asteroids, planetary satellites, or exoplanets in order to further reduce confusion.
The WGSN is delving into worldwide history and culture to produce a list of officially recognized star names that best represent and respect the global diversity of astronomical knowledge among human cultures past and present. Such an exercise will continue to be the Group’s main aim for the next few years. Once the names of the bright stars in the sky have been officially catalogued, the WGSN will focus on establishing the rules and process by which proposals for stellar names will be accepted by professional astronomers and the general public, alike. One such IAU project, NameExoWorlds, included the public in the process of naming stars. Its first call was in 2015, followed by a second call in 2019. The former was a worldwide open contest to name a series of well-known planets and their host stars. However, the rule, in the latter case, was that the stars concerned were not visible to the naked eye and that all countries on Earth were equally represented (that is, one name per country).
Further, Indigenous star names from all over the world are now being collected and assembled in a database that was launched in 2022. The goal of this work is to ensure the IAU's catalogue gives a fair representation across the sky with regard to human cultures, according to criteria that are in the process of being defined by the Working Group in collaboration with representatives of the world’s Indigenous communities.
Although there is no rigid format that star names must follow, since they have their roots in many varied cultures and languages, the WGSN has established some initial, basic guidelines, which build on the lessons from other IAU Working Groups. The guidelines outline a preference for shorter, one-word names that are not too similar to existing names for stars, planets or moons, as well as names that best represent the diverse living and ancestral cultural heritage from around the world. In order to choose among many suggestions from different origins (e.g., human cultures past and present, neologisms), further guidelines are currently being developed to ensure fair distribution among cultures with regard to such factors as brightness and location of stars in the sky.
Before the WGSN was established, the IAU had only officially approved the names of 14 stars, in connection with efforts to catalogue the names of newly discovered exoplanets. The WGSN has significantly increased this number. To date, the IAU WGSN has officially approved the names of roughly 449 stars, with more underway.
Current List of IAU Star Names (click on headers to sort)
To view this table as a GoogleSheets document, click here. To download this table as a comma-separated values (CSV) file, click here.