The International Astronomical Union (IAU) is the major body recognized by astronomers and other scientists worldwide as the naming authority for astronomical bodies. In response to the need for unambiguous names for astronomical objects, it has created a number of systematic naming systems for bodies of various sorts.
A few star-naming companies sell the right to list stars in their private registries under whatever name the buyer so chooses. However, the IAU (and, therefore, most astronomers) do not recognize those names as "official" (although the companies themselves do). Some websites (especially those speaking for astronomers) say that the IAU is the only body allowed to officially name heavenly objects. The star-naming companies, naturally, disagree.
According to the IAU, apart from a limited number of bright stars with historic names, stars do not have proper names. Where historic names exist, these names are, with a few exceptions, taken from the Arabic language: this reflects the leading role of Arab culture in astronomy while Europe was experiencing the Middle Ages. See List of traditional star names for a list of some of these names.
There are no more than a few thousand stars that appear sufficiently bright in the Earth's sky to be visible to the naked eye, so this represents the limit of the possible number of stars available to be named by ancient cultures. This limit is approximate, as it varies by the acuity of any given observer's eyes, but ten thousand stars (the naked-eye stars to visual magnitude six) seems to be an upper bound to what is physiologically possible.
Estimates of the number of stars with recognised proper names range from 300 to 350 different stars. These tend to be the brightest stars, or stars that form part of constellation patterns with the brightest stars. The number of proper names for stars is greater than the number of stars with proper names, as many different cultures named stars independently. For example, the star known as Polaris has also at various times and places been known by the names Alruccabah, Angel Stern, Cynosura, the Lodestar, Mismar, Navigatoria, Phoenice, the Pole Star, the Star of Arcady, Tramontana and Yilduz.
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 such as combining constellation names with Greek letters. Multiple sky catalogues meant that some stars had more than one designation. For example, the star with the Arabic name of Rigil Kentaurus also has the Bayer designation of Alpha Centauri.
As the resolving power of telescopes increased, numerous objects that were thought to be a single object were found to be multiple star systems that were too closely spaced in the sky to be discriminated by the human eye. These and other confusions make it essential that great care is taken in using designations. For example, Rigil Kentaurus contains three stars in a triple star system, labelled Rigil Kentaurus A, B and C respectively.
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.
The star nearest to Earth, our Sun, 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 sometimes called by its Latin name, Sol.
Finally, there are a very few stars named after people.
In the 19th century, the exact nature of galaxies was not yet understood, and the early catalogs such as the Messier catalog simply grouped together open clusters, globular clusters, nebulas, and galaxies, 110 in total. The Andromeda Galaxy is Messier object 31, or M31; the Whirlpool Galaxy is M51. The New General Catalogue (NGC) (J.L.E. Dreyer 1888) was much larger and contained nearly 8,000 objects.
At least two more bodies were discovered later, and called planets:
All of these planets were given names from Greek or Roman myth, to match the ancient planet names. However, this was only after some controversy. For example, Sir William Herschel discovered Uranus in 1781, and originally called it Georgium Sidus (George's Star) in honour of King George III of the United Kingdom. French astronomers began calling it Herschel before German Johann Bode proposed the name Uranus, after the Greek and Roman god. The name "Uranus" did not come into common usage until around 1850.
Starting in 1801, asteroids were discovered between Mars and Jupiter. The first few (Ceres, Pallas, Juno, Vesta) were initially considered minor planets and joined the ranks of the planets. As more and more were discovered, they were soon stripped of their planetary status. On the other hand, Pluto was considered to be a planet at the time of its discovery in 1930, as it was found far beyond any then-known asteroid's greatest distance from the Sun.
Some sixty years after the discovery of Pluto, a large number of large trans-Neptunian objects began to be discovered. Under the criteria of classifying these Kuiper belt objects (KBOs), it became dubious whether Pluto would have been called a planet were it discovered in the 1990s. Its mass is now known to be much smaller than what was once thought and, with the discovery of Eris, it is simply the second largest known trans-Neptunian object. In 2006, Pluto was reclassified to a different class of astronomical bodies known as dwarf planets.
When satellites are first discovered, they are given provisional designations such as "S/2000 J 11" (the 11th new satellite of Jupiter discovered in 2000) or "S/2003 S 1" (the 1st new satellite of Saturn discovered in 2003). The initial "S/" stands for "satellite", and distinguishes from such prefixes as "D/", "C/", and "P/", used for comets. The designation "R/" is used for planetary rings. These designations are sometimes written like "S/2003 S1", dropping the second space. The letter following the category and year identifies the planet (Jupiter, Saturn, Uranus, Neptune, Pluto; although no occurrence of the other planets is expected, Mars and Mercury are disambiguated through the use of Hermes for the latter). When the object is found around a minor planet, the identifier used is the latter's number in parentheses. Thus, Dactyl, the moon of 243 Ida, was at first designated "S/1993 (243) 1". Once confirmed and named, it became (243) Ida I Dactyl.
Note: The assignation of "H" for Mercury is specified by the USGS Gazetteer of Planetary Nomenclature; since they usually follow IAU guidelines closely, this is very likely the IAU convention, but confirmation is needed.
After a few months or years, when a newly discovered satellite's existence has been confirmed and its orbit computed, a permanent name is chosen, which replaces the "S/" provisional designation. However, in the past, some satellites remained unnamed for surprisingly long periods after their discovery. See Naming of natural satellites for a history of how some of the major satellites got their current names.
The Roman numbering system arose with the very first discovery of natural satellites other than Earth's Moon: Galileo referred to the Galilean moons as I through IV (counting from Jupiter outward), in part to spite his rival Simon Marius, who had proposed the names now adopted. Similar numbering schemes naturally arose with the discovery of moons around Saturn and Mars. Although the numbers initially designated the moons in orbital sequence, new discoveries soon failed to conform with this scheme (e.g. "Jupiter V" is Amalthea, which orbits closer to Jupiter than does Io). The unstated convention then became, at the close of the 19th century, that the numbers more or less reflected the order of discovery, except for prior historical exceptions (see the Timeline of discovery of Solar System planets and their natural satellites).
In the early days, only a very limited number of features could be seen on other solar system bodies other than the Moon. Craters on the Moon could be observed with even some of the earliest telescopes, and 19th century telescopes could make out some features on Mars. Jupiter had its famous Great Red Spot, also visible though early telescopes.
In 1919 the IAU was formed, and it appointed a committee to regularize the chaotic lunar and Martian nomenclatures then current. Much of the work was done by Mary Adela Blagg, and the report Named Lunar Formations by Blagg and Muller (1935), was the first systematic listing of lunar nomenclature. Later, "The System of Lunar Craters, quadrants I, II, III, IV" was published, under the direction of Gerard P. Kuiper. These works were adopted by the IAU and became the recognized sources for lunar nomenclature.
The Martian nomenclature was clarified in 1958, when a committee of the IAU recommended for adoption the names of 128 albedo features (bright, dark, or colored) observed through ground-based telescopes (IAU, 1960). These names were based on a system of nomenclature developed in the late 19th century by the Italian astronomer Giovanni V. Schiaparelli (1879) and expanded in the early 20th century by Eugene M. Antoniadi (1929), a Greek-born astronomer working at Meudon, France.
However, the age of space probes brought high-resolution images of various solar system bodies, and it became necessary to propose naming standards for the features seen on them.
Minor planets are initially assigned provisional designations when observed, of the form "" (the first part is a year; the second part defines a sequential order of discovery within that year, see provisional designation for details). If enough sightings are obtained of the same minor planet to calculate an orbit, the object is assigned a sequential number - its 'designation' - and it can then be cited as, for instance, . After the designation is assigned, the discoverer is given an opportunity to propose a name, which, if it is accepted by the IAU, replaces the provisional designation. Thus for instance, was given the name Ixion and is now known as (28978) Ixion, which is often abridged to 28978 Ixion. The name becomes official after its publication in the Minor Planet Circular with a brief citation explaining its significance. This may be a few years after the initial sighting, or in the case of "lost" asteroids, it may take several decades before they are spotted again and finally assigned a designation. If a minor planet remains unnamed ten years after it has been given a designation, then the right to name it is given also to identifiers of the various apparitions of the object, to discoverers at apparitions other than the official one, to those whose observations contributed extensively to the orbit determination, or to representatives of the observatory at which the official discovery was made. The CSBN has the right to act on its own in naming a minor planet, which often happens when the number assigned to the body is an integral number of thousands.
In recent years automated search efforts such as LINEAR or LONEOS have discovered so many thousands of new asteroids that the Center for Small Body Nomenclature has officially limited naming to a maximum of two names per discoverer every two months. Thus, the overwhelming majority of asteroids discovered from now on will never receive a name.
Under IAU rules, names must be pronounceable, preferably one word (such as Annefrank (5535 Annefrank)), although exceptions are possible (such as James Bond (9007 James Bond)), and since 1982 limited to a maximum length of sixteen characters, including spaces and hyphens. Letters with diacritics are accepted, although the diacritical marks are usually omitted in everyday usage. 4090 Říšehvězd is an asteroid with the most diacritics (four). Military and political leaders are unsuitable until they are dead for 100 years. Nowadays, names of pet animals are discouraged, but there are some from the past. Names after people, companies or products known only for success in business are not accepted, as well as citations that resemble advertising.
Whimsical names can be used for relatively ordinary asteroids (such as 26858 Misterrogers), but those belonging to certain dynamical groups are expected to follow more strictly defined naming schemes.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains today. A comet is named after up to its first three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument (for example, Comet IRAS-Araki-Alcock (C/1983 H1) was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock). Comet 105P/Singer Brewster, discovered by Stephen Singer-Brewster, should by rights have been named "105P/Singer-Brewster", but this would have led most readers to believe it had been a joint discovery by two astronomers named Singer and Brewster, respectively, so the hyphen was replaced by a space. Other comets with spaces in their names, however, reflect their discoverers' name spellings (e.g. 32P/Comas Solá).
Until 1994, the systematic naming of comets (the "Old Style") involved first giving them a provisional designation of the year of their discovery followed by a lower case letter indicating its order of discovery in that year (e.g. the first Comet Bennett is 1969i, the 9th comet discovered in 1969). In 1987, more than 26 comets were discovered, so the alphabet was used again with a "1" subscript, very much like what is still done with asteroids (an example is Comet Skorichenko-George, 1989e1). The record year was 1989, which went as high as 1989h1. Once an orbit had been established, the comet was given a permanent designation in order of time of perihelion passage, consisting of the year followed by a Roman numeral. For example, Comet Bennett (1969i) became 1970 II.
Increasing numbers of comet discoveries made this procedure difficult to operate, and in 1994 the International Astronomical Union approved a new naming system (the "New Style"). Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery, so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated (typically comets described in historical chronicles), D/ indicating a comet which has broken up or been lost, and A/ indicating an object at first thought to be a comet but later reclassified as an asteroid. Periodic comets also have a number indicating the order of their discovery. Thus Bennett's comet has the systematic designation C/1969 Y1. Halley's Comet, the first comet to be identified as periodic, has the systematic name 1P/1682 Q1. Comet Hale-Bopp's systematic name is C/1995 O1. The famous Comet Shoemaker-Levy 9 was the ninth periodic comet jointly discovered by Carolyn Shoemaker, Eugene Shoemaker, and David Levy (the Shoemaker-Levy team has also discovered four non-periodic comets interspersed with the periodic ones), but its systematic name is D/1993 F2 (it was discovered in 1993 and the prefix "D/" is applied, since it was observed to crash into Jupiter).
Some comets were first spotted as minor planets, and received a temporary designation accordingly before cometary activity was later discovered. This is the reason for such comets as (Catalina 2) or (Spacewatch-LINEAR). The MPECs and html version of IAUCs, because of their telegraphic style, "flatten out" the subscripts, but PDF version of IAUCs and some other sources such as the Yamamoto Circulars and the Kometnyj Tsirkular use them.