Dreyer's NGC "Shorthand"

Adapted from Bill Arnett's NGC Abbreviations web page.

The majority of NGC and IC objects in the SAC deep sky database include the visual descriptions used by Johann Dreyer in his New General Catalogue, published in 1888. These descriptions are remarkable for the way they pack a lot information into a few letters, but they can be somewhat daunting for the beginning amateur. For example, the Dreyer description of the open cluster M103 in Cassiopeia is:

Cl,pL,B, R,Ri,*10..11

This translates as "[open] Cluster, pretty large, bright, round, rich, with stars of 10th to 11th magnitude." - a pretty good description packed into only 21 characters!

Here is how the great galaxy in Andromeda (M31/NGC224) is described:

!!!eeB, eL, vmE

This means "magnificent object, most extremely bright, excessively large, very much extended".

The descriptions are fairly consistent in layout. First, a few standout objects (like NGC 224 above) are prefixed with one to three exclamation points.

The object's brightness and size is normally next. Dreyer used the scale specified by Sir John Herschel, and this scale may be confusing to modern observers; for example, "considerably bright" is fainter than "bright" on this scale. Another possible source of confusion is the fact that that 19th-century astronomers often called a faint star "small" and a bright star "large", so one has to use the context while reading part of a description that says "pretty small" to figure out if JD was referring to size or brightness.

Below are the scales used for brightness and size:

Next normally comes a description of object's general shape. This runs from "round" to "extremely extended":

The most cryptic part of the description can be the group of letters specifying Herschel's "degree and rate of condensation." A simple example is gbM, meaning "gradually brighter towards the middle". Easy, right? How about the description for NGC 4725, a galaxy in Coma Berenices, which is vsvmbMeBN? Even this jumble can be fairly easily translated as "very suddenly very much brighter in the middle, with an extremely bright nucleus".

When the descriptions give directions on the sky, "preceding" means "west" and "following" means "east." These can be remembered by picturing the way an object drifts across the field of view of a telescope with the drive turned off. Use of these terms is much more natural at the telescope eyepiece than the very confusing West and East, given the way that optical systems invert and/or reflect the field of view.

Quite often the notes refer to groups. The "1st of 4" is the first member of a group of four nebulae to drift across the field of view, i.e., the most western one, preceding all the others. All members of a group will have very nearly the same declination.

The Codes and Descriptions

The complete list of abbreviations used in the Dreyer description of an object appears below:

() - Items questioned by Dreyer are enclosed in parentheses.

" - arc seconds (two "not-equals" in published catalog)
' - arc minutes (one "not-equals" in published catalog)