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

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