Deep Sky Objects

Deep Sky Objects With the invention of the telescope, astronomers could now see objects much smaller and fainter than anything previously visible to the human eye. When turned toward the sky, the telescope revealed hundreds of faint, fuzzy new objects, not quite starlike, that had never been observed before. These objects are collectively referred to by amateur and professional astronomers alike as “deep sky objects”. Deep sky objects include star clusters, bright and dark nebulae, galaxies, and clusters of galaxies. Many of these objects have nicknames derived from the constellation where they are located, or from their appearance in a small telescope. Examples include the Great Andromeda Galaxy and the Lagoon Nebuls. A few deep sky objects have proper names handed down from historical times, like the Pleiades star cluster. As with stars, astronomers have developed classification systems and catalogs for deep sky objects. This section lists some of the most important and well-known.

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The Messier (M) Catalog

Charles Messier (1730-1817) was a French astronomer whose particular interest was comet hunting. In 1758, Messier was one of the first to see the predicted return of HaIley’s comet. In all, Messier discovered 15 new comets and claimed another six that were actually first seen by others. In 1758, Messier was deceived by the comet-like appearance of the Crab Nebula in his telescope, and made a special note to avoid it. From that point onward, he began cataloging fuzzy-looking objects in the sky that might be mistaken for comets. Messier produced his first list, containing 45 objects, in 1771; a second list in 1780 added another 23 objects; and his final list of 1781 (published in 1784) had 103 objects. Not all were Messier’s own discoveries; several had been seen first by other observers, notably the Swiss comet-hunter Jean Philippe Loys de Cheseaux (1718-1751). Another six objects were added to Messier’s list by the French astronomer Pierre Francois Andre Mechain (1744-1804). Charles Messier’s famous list of 110 deep sky objects includes some of the brightest star clusters, nebulae, and galaxies in the sky. Astronomers still refer to objects by their Messier, or M, numbers; for example, M 1 (the Crab Nebula) and M 31 (the Andromeda Galaxy). The English astronomer Sir William Herschel (1738-1822) was a contemporary of Charles Messier. He and his son, John Herschel (1792-1871) were also great discoverers of deep sky objects. The Herschels had better instruments, however, and were able to resolve many star clusters that appeared merely nebulous to Messier, and discovered many more hundreds of objects themselves. John Herschel’s so-called “General Catalogue” of nebulae and star clusters was published in 1846. As an admirer of Messier’s list, Herschel carefully avoided giving his own numbers to any object catalogued by the Frenchman. As a result, Herschel’s catalog of deep sky objects is rarely used today.

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The New General Catalog (NGC) and Index Catalog (IC)

The NGG and its supplements, the IC I and II, were originally compiled by J. L. E. Dreyer in the 1880s. Dreyer was born in Denmark, but emigrated to Ireland in 1874 to work at Lord Rosse’s great observatory in Parsonstown. Lord Rosse had built successively larger telescopes through the late 1830’s and early 1840’s. Rosse, his son, and his observers - such as Dreyer - spent years discovering and examining the known nebulae in the northern sky with the famous “Leviathan of Parsonstown”. During these observations, it became clear to Dreyer that it was time to update Herschel’s so-called “General Catalogue” of nebulae and star clusters. There were simply too many new nebulae being discovered, and too many different lists to consult. Preparing observing lists or simply finding if a nebula had been previously discovered had become a time-consuming chore. The “New General Catalogue of Nebulae and Star Clusters” appeared as Volume 49, Number 1 of the Memoirs of the Royal Astronomical Society in 1888, and contained more than 8,000 objects. Dreyer subsequently expanded the NGC with two Index Catalogues (IC I in 1895, and IC II in 1908), adding nearly 5,000 objects. The NGC and IC remain today one of the best-known catalogs of deep sky objects in astronomy. They are one of the most comprehensive general deep sky catalogs, as they include objects of all kinds - open and globular star clusters, diffuse and planetary nebulae, supernova remnants, and galaxies of all types. They also contains some erroneous entries corresponding to single or double stars, or to no object at all. The NGC and IC include both the northern and southern celestial hemispheres, and objects in them are numbered in order of right ascension. Nearly all 110 Messier objects, and almost every deep sky objects that can be seen in backyard telescopes, has an NGC or IC number. The Andromeda galaxy (M 31) is NGC 224; the Orion Nebula (M 42) is NGC 1976.

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Specialized Catalogs

Many other lists and catalogs of deep sky objects exist, but these are mostly specialized to a particular kind of object (e.g., galaxies). Some of the most commonly-used specialty catalogs are described below. Open Clusters are often referred to by their numbers on the lists of Trumpler (Tr), 1930; Melotte (Mel), 1915, and Collinder (Cr), 1931. Examples include Tr 24 in Scorpius, the Coma Star Cluster (Mel 111), and the “Coathanger” cluster in Vulpecula (Cr 399). Bright nebulae are frequently referred to by their numbers in the Sharpless catalog of HII regions (Sh 2), 1959; the Lynds Catalog of Brignt Nebulae (LBN), 1965; and the Cederblad Catalog of Bright Diffuse Galactic Nebulae (Ced), 1946. Examples include the Cave Nebula (Sh 2-155) and Cederblad 214 in Cepheus. Dark nebulae are most famously cataloged in E. E. Barnard’s 1927 “Catalogue of 349 Dark Objects in the Sky”. The Horsehead Nebula, for instance, is Barnard 33. Numbers from the The Lynds Catalog of Dark Nebulae (LDN), 1962, are also often used for dark nebulae. The Horsehead Nebula, B 33, is LDN 1630. For planetary nebulae, the Perek-Kohoutek Catalog of Galactic Planetary Nebulae (PK) is a compilation of all planetary nebulae known in the Milky Way Galaxy in 1964. There is an online version of the second edition (2000). The Ring Nebula, M 57, is also known as PK 63+13.1 in the Perek-Kohoutek catalog. The 1992 Strasbourg-ESO Catalog of galactic Planetary Nebulae (PN G) is another well known catalog of these objects. Many galaxies not listed in the NGC or IC are referred to by their number in the Uppsala General Catalogue of Galaxies (UGC), published by P. N. Nilson in 1973. Nilson also published the “Catalogue of Selected Non-UGC Galaxies” (UGCA) as an extension of the UGC in 1974. One of the largest and most comprehensive databases of galactic data is the Principal Galaxy Catalog (PGC), published by G. Paturel in 2003. It contains data for nearly 1 million galaxies, and is still being updated.