Specifications concerning designations
for astronomical radiation sources outside the solar system

How to refer to a source or designate a new one from the Working Group Designations of IAU Commission B2


Short title: IAU Recommendations for Nomenclature
Key words: Designations IAU
(Version August 2018)

A look at the current literature reveals that unclear, ambiguous or confusing designations of astronomical sources of radiation are too often encountered. Therefore, all contributors to databases, and authors of papers, catalogs and surveys, are urged to adhere to the following set of specifications (developed and endorsed by the International Astronomical Union); otherwise, significant data may be irretrievably lost.

1. General recommendations

All source listings should always contain positional information and/or a second designation next to a principal designation in order to avoid ambiguities that can arise with a single designation.

2. Case of existing designations

3. Creation of new designations

The designation of an astronomical source should consist of the following parts :

Acronym ^ Sequence ^ (Specifier)

Note that the ^ is used here to denote a blank. It is used for emphasis in showing where spaces occur in a designation. (Users are expected to use an actual blank and not this character.) Parentheses are required if a specifier is included. Acronym and sequence are essential, specifier is optional; the number of blanks may be larger in machine-readable files to right justify numerical or tabular data.

The following examples illustrate the recommended form of astronomical designations :


3.1 Acronym

The acronym (earlier called origin) is a code (i.e., alphanumerical string of characters) that specifies the catalog or collection of sources. It may be constructed from catalog names (e.g., NGC, BD), the names of authors (RCW), instruments or observatories used for large surveys (VLA, IRAS, 3C, 51W), etc – see helpful hints on creating acronyms.

The following rules apply to the construction of new acronyms:

3.2 Sequence

The sequence (or numbering) is an alphanumeric string of characters, normally only numerical, that uniquely determines the source within a catalog or collection. It may be a sequence number within a catalog (e. g., HD^224801), a combination of fields, or it may be based on coordinates. The way the sequence is constructed is called the format of the sequence; the symbols used are summarized in the Inventory of the Formats, a document which also includes examples of use and misuse.

3.2.1 Use of coordinates

Coordinate-based designations are just ``names'' and should have enough significant figures to unambiguously identify the sources. It is expected that precise coordinates will be provided in the paper (e.g., in a table), accompanied by any needed explanations and other relevant information. When sexagesimal coordinates are used, the right ascension portion must have one significant digit more than the declination portion. For decimal degree coordinates, RA and DEC should contain the same number of significant figures.

The number of significant digits in the sequence must not exceed the accuracy of the positions, e.g., subarcsecond positions should not be used in the sequence when the positional uncertainty is no better than an arcminute.

If coordinates in any form are used to encode a source of radiation, a set of rules applies, which we will illustrate with a source, namely the QSO with coordinates:

(J2000.0)     00h51m09.38s –42° 26' 33.8'' = 012.78908 –42.44272
(B1950.0)     00h48m48.97s –42° 42' 52.1'' = 012.20404 –42.7144

3.3 Specifier

Although obsolete, the specifier was optional and allowed one to indicate other source parameters. However, they were not required syntax and were enclosed in parentheses.

3.4 Punctuation and special characters

If the designation requires the use of punctuation or special characters, the recommendations are the following:

If, at some stage, subcomponents or multiplicity of sources is recognized, the current practice is to name the subcomponents with letters or numerals such as W 51 A. Alternatively the subcomponent receives a standard designation which may be added to the sequence of the parent source with a colon; e.g., ABELL^1644:[D80]^053    where D80 refers to Dressler's catalog of morphological types in 55 rich clusters of galaxies (1980ApJS...42..565D). For further elaboration on designating subcomponents and also on finding the reference to a subcomponent refer to current practices regarding subcomponents.

3.5 Examples

3.5.1 Examples of complete designations

Designation Position
Acronym^Sequence^(Specifier) RA(J2000.0) Dec(J2000.0)
  h m s ° ' ''
RX^J1426.8+6950 14 26 49.3 +69 50 21
HESS^J1302–638 13 02 47.72 63 50 08.5
PN^G001.2–00.3 17 49 36.9 28 03 59
TYC^1234–545–1 03 32 53.6417 +15 32 59.314
1E^2127+119 21 29 58.312 +12 10 02.67

The examples in the table above are from pre-existing designations. A look at the Dictionary of Nomenclature reveals that unique 2-letter combinations for acronyms are nearly exhausted. That is the reason for the change in the rule for new acronyms where at least three characters are now required.

3.5.2 Examples of improper designations

BD^4°14 use of ``deg symbol'', declination sign missing
N221 no space, unclear source : NGC or N in LMC ?
GRO^J317-85 leading zero in right ascension missing
P^43578 one letter acronym is ambiguous
RC^0401+0456 missing flag letter J for Julian 2000 equatorial coordinates.

3.6 Helpful Hints

There are further documents which provide help on:

4. Advice on designations

Advice on specific problems may be obtained from the following IAU Working Group Designations representatives:

Marion Schmitz - Chair
IPAC - Caltech
MS 100-22
Phone: +1 626 395 1873   ⋅  Fax: .

Heinz J. Andernach
Depto. de Astronomia, Univ. Guanajuato
Apdo. Postal 144
Guanajuato, C.P. 36000, Mexico
Phone: +52 473 732 9548
or 473 732 9607   ⋅  Fax: +52 473 732 0253

Kirk Borne
George Mason University
Computational and Data Sciences Department, MS 6A2
Fairfax, VA 22030, USA
Phone: +1 703.993.8402   ⋅  Fax: +1 703.993.9300

Oleg Malkov
Centre for Astronomical Data
Institute of Astronomy of the Russian Academy of Sciences
48 Pyatnitskaya St.
109017 Moscow, Russia
Phone: -—   ⋅  Fax: -—

Marianne Brouty
Observatoire astronomique de Strasbourg
11, rue de l'Université
F-67000 Strasbourg, France
Phone: +33 368 852 2403   ⋅  Fax: -—

Working Group members: Please send any updated email addresses or changes of institutes to the IAU Secretariat at:

5. Further information

For general information, in particular about existing designations, consult the following references :