Last updated: July-7-99

Spiral Arm Types

Smooth-armed spirals as a class were first introduced by [177]; see also [178] and [207] for a preliminary list. Later, [35, 38] the systematic variations in the orderliness of spiral arms were categorized (see the table below); broadly considered, Arm Classes 1 through 4 were termed flocculent, while Arm Classes 5 through 12 were said to be grand design. Discussions of Arm Classes and their correlation with other parameters can be found in [33, 34, 36, 37, 47, 124, 147, 152, 175]. Arm widths are discussed by [15].

**Arm Classification for Spiral Galaxies** [35, 38]
Arm Class	Description	Prototype
1	Chaotic appearance; no symmetry; fragmented arms with differing pitch angles.	NGC 2500
2	Short, fragmented spiral arm pieces defined mostly by HII regions; no regular pattern; loose, open arms.	NGC 7793
3	Fragmented arms, uniformly distributed around the galaxy center.	NGC 2841 NGC 5055
4	Only one prominent arm; otherwise fragmented spiral arms with no regular pattern in the rest of the galaxy.	NGC 2403
5	Two symmetric short arms in the inner regions; irregular arms in the outer regions.	NGC 1313 NGC 0598
6	Two symmetric arms; feathery appearance in the outer arms, which tend to wrap tightly and form a ring-like structure.	NGC 2935 NGC 1637
7	Two symmetric, long arms in the outer regions, and irregular or feather arms in the inner regions.	NGC 2903
8	Tightly wrapped arms forming a ring-like structure with over-all symmetry.	NGC 3992
9	Multiple, long and continuous arms in the outer parts, two symmetric and continuous arms in the inner parts.	NGC 5457 NGC 1232
10	Two long arms extending from either end of a prominent bar (abandoned in later applications [38]).	NGC 1300 NGC 5383
11	Two long symmetric arms in the inner regions with a long drawn-out arm or arms in the outer regions; near-by galaxy apparently interacting (abandoned in later applications [38]).	NGC 7753 NGC 5194
12	Two long, sharply defined symmetric arms dominating the total appearance.	NGC 4321