Deep Sky Object Designations

Jack Kramer

A detailed star atlas contains an abundance of galaxies, nebulae and star clusters, each object designated with a series of letters and numbers. Ever wonder what these designations mean and where they came from?

In a period of a little over one hundred years, most of the deep sky objects that we observe were listed in a catalog with the letters M, NGC, or IC plus a number. Every amateur astronomer is familiar with Charles Messier's list of deep sky objects, in which he made his final (103rd) entry in 1781. (Numbers 104 through 110 were added after his death.) Messier wasn't interested in star clusters and nebulae, but rather in comets.

A similar twist involves William Herschel, who achieved notoriety for his discovery of Uranus; he wasn't really interested in planets, but in clusters and nebulae. Herschel recorded his observations of about 2500 objects, but it was his son, John, who in 1864 published a catalog of deep sky objects, known as the "General Catalog". That catalog doubled the number of objects recorded by the elder Herschel. In 1888, John Dreyer augmented John Herschel's catalog, publishing the "New General Catalog" which included 7,840 objects. Then in 1895 and 1908, Dreyer published supplements to the NGC, which became known as the "Index Catalog". By this time there were 13,226 nebulae and star clusters recorded by Messier, Herschel and other observers, and included as NGC or IC objects. Galaxies were referred-to as "nebulae", since they were not yet known to be island universes beyond the Milky Way.

With the advent of new methods and larger instruments in the 20th Century, additional deep sky objects were observed and cataloged with designations that generally included the initials of the discoverer. Thus we have things such as the Collinder (Cr), Trumpler (Tr) and Melotte (Mel) star clusters, the Perek-Kohoutek (PK) planetary nebulae, Barnard (B) dark nebulae, and Uppsala General Catalog (UGC) galaxies. Most of these are fainter than the M, NGC, and IC objects. However, a few are not so faint, especially open star clusters that were more recently identified as true physical groupings (although these "newer" clusters generally appear pretty sparse through a telescope).

More recent catalogs include objects found with modern instruments. The Zwicky and Abell catalogs list galaxy clusters, and the Principal Galaxy Catalog (PGC) lists over 100,000 galaxies!

Published in the November 2002 issue of the NightTimes