The NGC on the Net

Here's where to find the NGC data: NGC 2000.0 is also available on paper or diskettes from Sky Publishing.

NGC description codes

The codes used to describe the objects in the NGC catalog are extremely compact and extremely helpful. But they are a bit cryptic :-)

Discussion

The helpfile for Chris Marriott's SkyMap v2.2 (an excellent observer-oriented program capable of printing great charts) contains the following information on the Dreyer Object Descriptions (which I have edited slightly to resolve upper/lower case ambiguities):

The majority of objects in the deep sky database include the visual descriptions used by Johann Dreyer in his "New General Catalogue", published in 1888. These descriptions are remarkable for their information content, but can be somewhat daunting at first. For example, the Dreyer description of the globular cluster M3 in Canes Venatici is as follows:

GCL,eB,vL,vsmbM,*11
This can be translated as "Globular cluster, extremely bright, very large, very suddenly much brighter towards the middle, composed of 11th magnitude stars" - a pretty good description in only 19 characters!

Similarly the galaxy NGC 2863 in Hydra is described as:

cF, S, E, bet 2 st 12, 16
which means "considerably faint, small, elongated, between two stars of magnitude 12 and 16".

The description normally starts with a description of the object's brightness and size. Dreyer adopted the scale used for this from Sir John Herschel, and the order used may be confusing to modern observers; for example, is "considerably faint" brighter or fainter than merely "faint"? The other possible source of confusion is that 19th century astronomers often called a faint star "small" and a bright star "large", so one always has to be careful to judge whether a description such as "pretty small" refers to size or brightness!

The scale used is as follows:


Brightness                              Size

eF      Excessively faint               eS      Excessively small
vF      Very faint                      vS      Very small
F       Faint                           S       Small
cF      Considerably faint              cS      Considerably small
pF      Pretty faint                    pS      Pretty small
pB      Pretty bright                   pL      Pretty large
cB      Considerably bright             cL      Considerably large
B       Bright                          L       Large
vB      Very bright                     vL      Very large
eB      Extremely                       eL      Excessively large
Next normally comes a description of object's general shape. This lies on a scale ranging from "round" to "extremely extended", as follows:
Code            Shape

R               Round
vlE             Very little extended
E               Elliptic or oval
cE              Considerably extended
pmE             Pretty much extended
mE              Much extended
vmE             Very much extended
eE              Extremely extended
By far the most cryptic part of the description, at first glance, is the group of letters giving what Sir John Herschel described as "the degree and rate of condensation". A simple example is "gbM", meaning "gradually brighter towards the middle". Looking, though, at NGC 4725, a galaxy in Coma Berenices, we find the dreadful looking "vsvmbMeBN"! Even this mouthful, though, is 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, the terms "preceding" and "following" are used for west and easy respectively. To see what is meant by this, picture the way an object drifts across the field of view of a telescope if the drive is switched 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 speak of groups. The "1st of 4" is the first member of a group of four nebulae to drift across the field of view ie, the most western one, preceding all the others. All members of a group will have very nearly the same declination.

The codes

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

                Ab..............about                 n..............north 
                alm.............almost                neb............nebula 
                am..............among                 nf.............north following 
                app.............appended              np.............north preceding 
                att.............attached              nr.............near 
                b...............brighter              N..............Nucleus, or to a
                                                                     Nucleus 
                be..............between               p..............pretty (before
                                                                     F,B,L,S) 
                biN.............binuclear             p..............preceding 
                bn..............brightest towards     pg.............pretty gradually 
                                the north side        pm.............pretty much 
                bs..............brightest towards     ps.............pretty suddenly 
                                the south side        P..............poor 
                bp..............brightest towards     quad...........quadrilateral 
                                the preceding side    quar...........quartile 
                bf..............brightest towards     r..............resolvable 
                                the following side                   (mottled,not resolved) 
                B...............bright                rr.............partially relolved,
                c...............considerably                         some stars seen 
                ch..............chevelure             rrr............well resolved, clearly
                co..............coarse, coarsely                     consisting of stars 
                com.............cometic               R..............round 
                cont............in contact            RR.............exactly round 
                C...............compressed            Ri.............rich 
                C.G.H...........Cape of Good Hope     s..............suddenly 
                Cl..............cluster               s..............south 
                d...............diameter              sp.............south preceding 
                def.............defined               sf.............south following 
                dif.............diffused              sc.............scattered 
                diffic..........difficult             st.............stars 
                dist............distance or           sev............several 
                                distant               susp...........suspected 
                D...............double                sh.............shaped 
                e...............extremely,            stell..........stellar 
                                excessively           S..............small 
                ee..............most extremely        sm.............smaller 
                er..............easily resolvable     triN...........trinuclear 
                exc.............excentric             trap...........trapezium 
                E...............extended              v..............very 
                f...............following             vv.............very, very 
                F...............faint                 var............variable 
                g...............gradually             *..............a star: *10, a star of
                gr..............group                                10th magnitude 
                i...............irregular             **.............double star 
                inv.............involved,involving    ***............triple star 
                iF..............irregular figure      !..............remarkable 
                l...............little,long           !!.............very remarkable 
                L...............large                 !!!............a magnificent or 
                m...............much                                 otherwise interesting 
                mm..............mixed magnitudes                     object 
                mn..............milky nebulosity      st 9...........stars from the 9th 
                M...............middle, or in the                    magnitude downwards 
                                middle                st 9 13........stars from the 9th
                                                                     to 13th magnitude

()              items questioned by Dreyer enclosed in parentheses

"               arc seconds (two "not-equals" in published catalog)
'               arc minutes (one "not-equals" in published catalog)
See also NGC Abbreviations from the NGC/IC Project.
Bill Arnett; last updated: 2000 Jun 20