Star maps and catalogues

Already in the previous chapters, mention has been made of the names of some of the constellations and stars. Inspection of star maps shows that areas of the sky are divided into zones marked by constellations. Within a constellation it is usual to find an asterism (a pattern of stars) which is readily recognizable. Also some of the stars may carry a name, usually with an Arabic origin, a Greek letter or a number to provide individual identification. At first sight, the nomenclature may seem to be haphazard but this is simply the legacy of history. In the early days, the stars were listed according to their places within a constellation and designated by a letter or number. The constellation zones are quite arbitrary in relation to the stellar distribution but their designations persist for the ease of identification of the area of sky under observation. It is, however, important to have some appreciation of the background to the system and its current use.
To describe the philosophy behind the development in star catalogues we can do no better than to
quote the relevant section of Norton’s Star Atlas.

The origin of most of the constellation names is lost in antiquity. Coma Berenices was added to the old list (though not definitely fixed till the time of Tycho Brah´e), about 200 BC; but no further addition was made till the 17th century, when Bayer, Hevelius, and other astronomers, formed many constellations in the hitherto uncharted regions of the southern
heavens, and marked off portions of some of the large ill-defined ancient constellations into new constellations. Many of these latter, however, were never generally recognized, and are now either obsolete or have had their rather clumsy names abbreviated into more convenient forms. Since the middle of the 18th century when La Caille added thirteen names in the southern hemisphere, and sub-divided the unwieldy Argo into Carina, Malus (now Pyxis), Puppis, and Vela, no new constellations have been recognized. Originally, constellations had no boundaries, the position of the star in the ‘head’, ‘foot’, etc, of the figure answering the needs of the time; the first boundaries were drawn by Bode in 1801.
The star names have, for the most part, being handed down from classical or early medieval times, but only a few of them are now in use, a system devised by Bayer in 1603 having been found more convenient, viz., the designation of the bright stars of each constellation by the small letters of the Greek alphabet, α, β, γ, etc, the brightest star being usually made α, the second brightest β—though sometimes, as in Ursa Major, sequence, or position in the constellation figure, was preferred. When the Greek letters were exhausted, the small Roman letters, a, b, c, etc, were employed, and after these the capitals, A, B, etc—mostly in the Southern constellations. The capitals after Q were not required, so Argelander utilized R, S, T, etc, to denote variable stars in each constellation, a convenient index to their peculiarity.
The fainter stars are most conveniently designated by their numbers in some star catalogues. By universal consent, the numbers of Flamsteed’s British Catalogue (published 1725) are adopted for stars to which no Greek letter has been assigned, while for stars not appearing in that catalogue, the numbers of some other catalogue are utilized. The usual method of denoting any lettered or numbered star in a constellation is to give the letter, or Flamsteed number, followed by the genitive case of the Latin name of the constellation: thus α of Canes Venatici is described as α Canum Venaticorum.
Flamsteed catalogued his stars by constellations, numbering them in the order of their right ascension. Most modern catalogues are on this convenient basis (ignoring constellations), as the stars follow a regular sequence. But when right ascensions are nearly the same, especially if the declinations differ much, in time ‘precession’ may change the order: Flamsteed’s 20, 21, 22, 23 Herculis, numbered 200 years ago, now stand in the order 22, 20, 23, 21.
For convenience of reference, the more important star catalogues are designated by recognized contractions.
 
With the application of detectors on telescopes replacing the unaided eye, the resulting catalogues are forced to provide extensive listings of stars containing hundreds and thousands of entries. A famous catalogue based on photographic records of the sky made by Harvard University is known as the Henry Draper Catalogue. Each successive star is given the number according to increasing right ascension with the prefix HD. For example, HD 172167 is at once known by astronomers to denote the star numbered 172167 in that catalogue. This particular star is bright and well-known, being Vega or α Lyrae. It also appears in all other catalogues and may, for example, be known as 3 Lyrae (Flamsteed’s number), Groombridge 2616 and AGK2 + 38 1711 (from Zweiter Katalog der Astronomischen Gesellschaft fu¨r das A¨quinoktum 1950). Returning to the quotation from Norton’s Star Atlas:

Bode’s constellation boundaries were not treated as standard, and charts and catalogues issued before 1930 may differ as to which of two adjacent constellations a star belongs. Thus, Flamsteed numbered in Camelopardus several stars now allocated to Auriga, and by error he sometimes numbered the star in two constellations. Bayer, also, sometimes assigned to the same star a Greek letter in two constellations, ancient astronomers having stated that it belonged to both constellation figures: thus β Tauri = γ Auriga and α Andromedae = δ Pegasi. To remedy this inconvenience, in 1930 the International Astronomical Union standardized the boundaries along the Jan 1st, 1875, arcs of right ascension and declination, having regard, as far as possible, to the boundaries of the best star atlases. The work had already been done by Gould on that basis for most of the S. Hemisphere constellations. The IAU boundaries do not change in their positions among the stars and so objects can always be correctly located, though, owing to precession, the arcs of right ascension and declination of today no longer follow the boundaries, and are steadily departing from them. After some 12 900 years, however, these arcs will begin to return towards the boundaries, and 12 900 years after this, on completing the 25 800-year precessional period will approximate to them, but not exactly coincide.

Nowadays, as well as recording the stars’ positions for a particular epoch, a general catalogue will also list various observed parameters of each star. For example, the annual changes in right ascension and declination may be given. Other headings might include proper motion, annual parallax, radial velocity, apparent magnitude, colour index and spectral type. Special peculiarities may also be supplied—for example, if the star is a binary system. It may be noted, too, that according to

Figure 1. The hand as a means of estimating angles.

the IAU convention, the names of constellations are usually referred to by a standardized three-letter
abbreviation.