Articles and References

The following is an explanation of J.L.E. Dreyer's abbreviations and symbols used in the Summary Description column of the New General Catalog of Nebulae and Star Clusters (NGC), as taken from the original version published in 1888. These shorthand cryptic descriptions are unparalleled in their information content, but can initially be somewhat difficult to decipher. For example, the NGC lists Dreyer's summary description for the magnificent globular cluster in the constellation Hercules (NGC6205, M13) as !!glob.Cl,eB,vRi,vgeCM,st11..., which translates to very remarkable globular cluster, extremely bright, very rich, very gradually extremely compressed towards the middle, consisting of stars of 11th magnitude and lower. As this example illustrates, a remarkable amount of descriptive information can be compacted into just a few characters.

Dreyer's summary descriptions in the NGC are adaptations of the original observer's descriptions in their own words or as nearly as possible. Dreyer also incorporated the progressive scale of brightness, size and form adopted by Sir John Herschel and calls attention to the scale in his introduction to the NGC catalog.

Dreyer's description codes can be generally broken into six sections: rating of the object, type of object, brightness, size, form and condensation. Based on the original observer's description and its conversion to the Dreyer code, not all sections may appear in the summary description. Dreyer used short codes to describe each of these sections and to include as much of the original observers description as possible.

For those objects deemed by the observer as remarkable, Dreyer began their summary description with a series of exclamation marks (!) to denote their special nature. This rating system identifies objects in the catalog that are showpieces in the sky. Dreyer used from one to three exclamation marks to separate these objects into levels with three exclamation marks denoting the most outstanding objects in the catalog.

All objects in the NGC catalog fall into one of four categories; nebulae, planetary nebulae, star clusters, and globular star clusters. Since the number of objects that did not fall into the nebulae category was small, Dreyer choose to identify only those objects that were not classified as nebulae. Dreyer inserted codes at the beginning of the description to identify these special cases. For example; glob.cl. is inserted at the beginning of a summary description for a globular cluster.

Today, we know most of the nebulae listed in the NGC catalog are not nebulae but galaxies far outside our own Milky Way galaxy. This knowledge was not available to Dreyer or the Herschel's when they compiled the NGC catalog and therefore no galaxy category exists in Dreyer's summary description. Some contemporary compilations of the NGC catalog expand the object categories to include galaxies.

Next, the description code normally contains a description of the object's brightness and size. Dreyer adopted the scale used by Sir John Herschel, the semantics of which may be confusing to some observers. Some observers intermix the descriptions of brightness and size often calling a faint star small and a bright star large. It is not always apparent as to whether a description such as pretty large is referring to the size of the object or its brightness.

For objects with where the observer listed a distinctive or notable shape, Dreyer included a section describing the form of the object. The form scale begins with round objects and proceeds through several steps to extremely extended (oval) objects.

Perhaps the most difficult section to decipher is Dreyer's abbreviation of what Sir John Herschel referred to as "the degree and rate of condensation". This description generally describes how the brightness changes throughout the object's surface. A simple example is gbM which translates to gradually brighter towards the middle. Dreyer writes the description of NGC 4725, a galaxy in Coma Berenices as vB,vL,E,vg,vsvmbMeBN, which looks somewhat overwhelming at first glance. Even this long cryptic description is easily translated as very bright, very large, extended very gradually, very suddenly very much brighter towards the middle with an extremely bright nucleus.

Some descriptions give directions in the sky using the terms preceding and following to describe the object. If you imagine the view through a telescope with no clock drive, objects would drift from east to west across the field of view as the Earth rotated on its axis. If two objects appear in the field of view simultaneously, the object that appeared first would be preceding (or to the west of) the second object. Conversely, the object that appears second would be following (or to the east of) the first object. Because not all optical systems in telescopes present the sky the same way, these terms are less confusing than the terms east and west.

When describing many objects close together, the description may refer to a group of objects and their position within the group. For example, a description may say 1st of 3 which means this is the first object (or the most western) to drift into the field of view and precedes all other objects in the group. Because all objects drift through the same field of view, all members of a particular group will have nearly the same declination.