There are thousands of stars visible to the naked eye and millions visible in a telescope.
It is vital when communicating results to other astronomers
to give a umambiguous designation to a star. There are several systems for
giving a star a designation, and many times a star will have more than one
name by which it can be called. Astronomers catalog stars by their characteristics, so a catalog
designation by itself can tell something about the star.
Names: Some very bright or otherwise unusual stars have names: these come from Greek,
Arabic: e.g. Vega, Castor, Aldebaran
Bayer letters: The stars in a constelllation are ordered by brightness, then given a Greek or Roman
letter, + the genitive of the constellation name, e.g.
a Lyrae = Vega. These are naked eye
stars. The Roman letters are not usually used nowadays, except
for a few famous cases, e.g. P Cygni.
The highest Roman letter used was Q.
Flamsteed numbers: The stars in a constellation are ordered by increasing right ascension
(= the east-west celestial coordinate) and given a number + the genitive of the
constellation name, e.g. 17 Tauri = a star in the Pleiades. Some of these stars are
1-2 magnitudes fainter than naked eye visibity since Flamsteed used a small telescope.
In any constellation the Flamsteed numbers go up only to about 200.
Variable stars: Variable stars are given a single or double Roman letter + the genitive
of the constellation name starting with R: R, S...Z; RR, RS...RZ; SS, ST...SZ; ...ZZ;
AA, AB ...AZ; BB, BC ...BZ; ...QZ; e.g. RR Lyrae. When these 334 letter combinations are
used up the next is designated with a V plus a running number; e.g. V335 Aquilae.
BD numbers: These come from a catalog made in the middle of the 19th century at Bonn
(Bonner Durchmusterung). It includes several hundred thousand stars brighter
than 10th mag. The catalog gives the position of the star, which can also then by found on
an accompanying chart. The catalog number was assigned by counting stars in declination
(the north-south celestial coordinate) zones. Thus a BD number will consist of a
declination plus a running number, e.g. BD+31o216 =
the 216th star in the declination band between
+31o and +32o dec,
starting at 0 hr right ascension. The BD extends from +90o to -22o dec;
the CD (Cordoba Durchmusterung) and CPD (Cape Photographic Durchmusterung) [done at
Cordoba, Argentina, and Cape Observatory, South Africa, later in the 19th century]
similarly for more southerly zones.
The Bright Star Catalog: Stars brighter than 6.5 mag are given a running number by increasing RA, with HR or BS before it; e.g. HR1099.
The Henry Draper Catalog: Gives an objective prism spectral type for stars brighter than
about 8.5 mag, and some fainter; e.g. HD183143, a highly reddened B star in Cygnus.
Double stars are given a number and the name of the discoverer, or are referred to by their
number in the Aitken Double Star catalog (ADS) or the Burnham Double Star catalog (BDS) or the
Washington Double Star catalog; individual components of each system are
given capital letters: A, B, etc.
The Guide Star Catalog contains stars with well measured positions;
it was made to be used by the guidance sensors on the
Hubble Space Telescope. It goes as faint as about 13th magnitude
(but is incomplete for stars that the faint), and
does not include the brightest stars (which were overexposed on the
photographs which were measured), and stars which overlapped on the
photographs. The sky is
divided into zones, and the stars are numbered in each zone, for
example GSC 4068/1167, also called HD25443 or BD+61 669
(and several other designations), a star observed by WUPPE
Catalogs of stars have been made giving new data such as proper motion or
position or spectral type.
These stars are designated by some code number and a word or letter to identify the catalog,
frequently the author; e.g. Ross 614, Luyten 726-8, 2 nearby stars which were first found by
looking for high proper motion stars. Very unusual stars are informally named after their
discoverer; e.g. Barnard's Star.
Bright nonstellar objects (which look fuzzy in the telescope) [= nebulae and star clusters
in the Milky Way, and other galaxies] are given their number in the Messier (M), New General
Catalog (NGC) or Index Catalog (IC); e.g. M31, the Great Galaxy in Andromeda.
Faint galaxies and quasars are found in various catalogs, and some faint companions of
the Milky Way are designated by the constellation they are found in;
e.g. Markarian 421 (MKN 421) = B2 1101+38.4 = several other similar identifiers using its coordinates
(it is at 11 hr 01 min, +38 deg); Sextans A.