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:

BrightnessSize
eFExcessively fainteSExcessively small
vFVery faintvSVery small
FFaintSSmall
cFConsiderably faintcSConsiderably small
pFPretty faintpSPretty small
pBPretty brightpLPretty large
cBConsiderably brightcLConsiderably large
BBrightLLarge
vBVery brightvLVery large
eBExtremelyeLExcessively large

Next normally comes a description of object’s general shape. This lies on a scale ranging from “round” to “extremely extended”, as follows:

CodeShape
RRound
vlEVery little extended
EElliptic or oval
cEConsiderably extended
pmEPretty much extended
mEMuch extended
vmEVery much extended
eEExtremely 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..............aboutn…...........north
alm.............almostneb............nebula
am..............amongnf.............north following
app.............appendednp.............north preceding
att.............attachednr.............near
b...............brighterN..............Nucleus, or to a
Nucleus
be..............betweenp..............pretty (before
F,B,L,S)
biN.............binuclearp..............preceding
bn..............brightest towardspg.............pretty gradually
the north sidepm.............pretty much
bs..............brightest towardsps.............pretty suddenly
the south sideP..............poor
bp..............brightest towardsquad...........quadrilateral
the preceding sidequar...........quartile
bf..............brightest towardsr..............resolvable
the following side(mottled,not resolved)
B...............brightrr.............partially relolved,
c...............considerablysome stars seen
ch..............chevelurerrr............well resolved, clearly
co..............coarse, coarselyconsisting of stars
com.............cometicR..............round
cont............in contactRR.............exactly round
C...............compressedRi.............rich
C.G.H...........Cape of Good Hopes..............suddenly
Cl..............clusters..............south
d...............diametersp.............south preceding
def.............definedsf.............south following
dif.............diffusedsc.............scattered
diffic..........difficultst.............stars
dist............distance orsev............several
distantsusp...........suspected
D...............doublesh.............shaped
e...............extremely,stell..........stellar
excessivelyS..............small
ee..............most extremelysm.............smaller
er..............easily resolvabletriN...........trinuclear
exc.............excentrictrap...........trapezium
E...............extendedv..............very
f...............followingvv.............very, very
F...............faintvar............variable
g...............gradually*..............a star: *10, a star of
gr..............group10th 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...............muchotherwise interesting
mm..............mixed magnitudesobject
mn..............milky nebulosityst 9...........stars from the 9th
M...............middle, or in themagnitude downwards
middlest 9 13........stars from the 9th
to 13th magnitude

See also NGC Abbreviations from the NGC/IC Project.


Bill Arnett; last updated: 2000 Jun 20