Astronomical indexing

Astronomical Object Indexing is a way of controlling the literature by collecting together celestial objects with designations which at least correspond to recognised lists and in acceptable formats.

History of indexing

The naming and numbering of astronomical objects have a long and colourful history, older and richer than the science of astronomy itself. The very first star catalogues used the constellations and to this day the brightest star in the night sky, Sirius, remains known as alpha Canis Majoris or α CMa. When the Greek and Roman letters ran out, numbering systems were used but remained constellation-based. As time went by, fainter stars, as well as “nebulous” objects in and beyond our own Galaxy, were found.

Catalogues

The larger catalogues presented problems for the enumeration of millions of new objects. The solution was to use an abbreviation (such as “NGC” for the “New General Catalogue”) together with a sequential number. Later with radio and X-ray detections, a positional “format” became popular. Today, with the launch of ever-increasingly sophisticated space-borne instrumentation and deep surveys with large ground-based telescopes, huge catalogues are often only available in machine-readable form and only the most unusual objects within each collection receive sufficient study to result in publication.

To make matters worse, an object in one catalogue, say an infrared source, may later be found to be identical to a previously catalogued X-ray source. So an object can have several designations. The literature is now reasonably constrained and authors are encouraged to avoid duplication and have their suggested catalogue abbreviation checked by the IAU (the International Astronomical Union – astronomy’s governing body) before publication. Nevertheless, many objects have received a variety of aliases or positional truncations in the past and the literature is awash with ambiguous designations to trap the unwary astronomer.