Look through any issue of Sky & Telescope magazine and you'll find a wide variety of seemingly incomprehensible names for celestial objects: Arp 220, MWC 560, QSO 0957+561, 1E 1740.7–2942, 3C 273, PSR 1257+12. Some apply only to galaxies, others to nebulae and star clusters, and still others to pulsars, quasars, radio-wave emitters, X-ray sources — everything in the astrophysical zoo. Luckily, though, backyard astronomers with ordinary telescopes needn't memorize the names of hundreds of authors, acronyms for space-borne experiments, or codes for sources at invisible wavelengths.

M is for Messier

Deep-sky objects such as star clusters and nebulae began to draw attention as soon as telescopes were pointed to the night sky. But it wasn't until the late 18th century that the French astronomer Charles Messier began tallying them in substantial numbers, publishing several lists that were to become the now-familiar catalog bearing his name.

Messier (pronounced mess-YAY") was a comet hunter, and the main purpose of his list was to provide himself and others with a roster of diffuse cometlike objects to ignore. Although Messier discovered a large portion of those in his list, many had already been found by others (sometimes unbeknown to him), especially by his colleague Pierre Méchain.

Although the Messier catalog is 200 years old it is still the most commonly used list, simply because it contains most of the bright diffuse objects in the northern two-thirds of the sky. Practically every amateur who becomes interested in viewing clusters and galaxies gets started by working from the Messier list. All 103 of the M objects (or 107, 109, or 110, depending on which later additions you accept) can be seen in a 6-inch telescope even under suburban skies. Some observers have bagged all the Messier objects with a 2.4-inch refractor, and from a very dark site they're all visible in 8 x 50 binoculars.

The NGC and Beyond

The next big catalog to appear that remains in wide use today was the NGC . The New General Catalogue of Nebulae and Clusters of Stars by John L. E. Dreyer (pronounced "dryer") appeared in 1888. It is a compendium of all the lists of nonstellar objects compiled by the many 19th-century observers who had been ransacking the sky. Chief among these lists was John Herschel's monumental General Catalogue of Nebulae, published in 1864. The NGC contains 7,840 objects of many types, numbered in order of equinox-1860 right ascension.

As discoveries kept pouring in, Dreyer published two supplements to the NGC in 1895 and 1908 titled the Index Catalogues, abbreviated IC. They brought the total to 13,226. These three lists should really be considered a single work. They include nearly every extended (non-pointlike) telescopic object beyond the solar system that is visible with, say, an 8- to 12-inch telescope from a backyard observing site with slight to moderate light pollution. Almost any NGC object can be detected with a sharp 12-inch telescope working from a first-class observing site. The IC is a different story, however. About half the IC objects were discovered visually, but much of the second IC catalog (objects numbered IC 1530 and up) consists of photographic discoveries. Many of these are difficult or impossible to see visually.

Huge numbers of objects, galaxies in particular, continued to be recorded and cataloged in works such as the southern photographic surveys by Harvard Observatory. For amateurs who push beyond the limits of the NGC and IC, the next designation usually encountered is UGC — for the Uppsala General Catalogue of Galaxies by Peter Nilson (Uppsala Astronomical Observatory, 1973). It includes the 12,940 brightest galaxies north of declination –2½° (1950.0). Delving still deeper, one soon meets the Master List of Nonstellar Optical Astronomical Objects (MOL) by Robert S. Dixon and George Sonneborn (Ohio State University Press, 1980). This huge, diverse compendium packs 185,000 brief listings from 270 catalogs into a single volume.