SB (r) : This is the typical
``-shaped'' barred spiral. It
has a fairly large, elongated nucleus
with weak spiral dark lane in a strong, narrow bar along the major axis
of an elliptical ring marking the edge of the lens. Two main arms start
tangentially from the ring near the extremities of the bar (i.e. at
right angles to the bar); one or two fainter arms branch out from breaks
in the ring near its minor axis. In early stages the main arms tend to
form an outer, circular (R) structure, as in NGC 1433 (25); in
later types the breaks in the ring tend to produce slightly
hexagonal shapes. Examples are NGC 1433 (illustrated from S 74"), Sa
or Sab, NGC 3185, NGC 3351, Sab (26), NGC 2523, Sbc
13. NGC 4262 (P 200"): SB (r) 0
o. Small nucleus in short, stubby bar (stringer near
extremities) in bright diffuse lens. The nucleus appears large on this
image taken in yellow light. Compare with 9, 15 and 16.
14. NGC 1512 (S 30"): SB (r) 0
+. Small, elongated nucleus on narrow, bright bar
marking the major axis of elliptical ring; the ring is stronger near
the extremities of the major axis. Traces of spiral arms or arcs in
weak envelope outside ring. Compare with 8 and 25; NGC 1510
(E0) is at left near edge of field.
16. NGC 2859 (P 200"; inset P 48"): (R)
SB (r) 0 o. Fairly large nucleus on weak bar marking
major axis of little elongated lens, slightly stronger near rim where
bar appears brighter. Faint outer ring structure from which emerge two
very weak spiral arms or arcs. No spiral structure in lens. Compare
with 12, 14 and 15.
25. NGC 1433 (S 74"): (R') SB (r)
a. A small, elongated
nucleus whose major axis is tilted at 45° to a long, narrow bar
along which run filaments of dark matter. The bar marks the major axis
of an elliptical ring from which emerge two faint arms near the
extremities of the bar and two weaker branches forming an incomplete
outer ring (inset). Compare with 14, 26 and 29.
26. NGC 3351 = M95 (P 200"): (R') SB (r)
ab. The nucleus and bar are enhanced on this photograph
taken in yellow light; the elliptical ring appears circular or
slightly hexagonal by projection foreshortening; note the two main
arms emerging from ring near the extremities of the bar and weaker
branches. Compare with 25, 27 and 30.
27. NGC 2523 (P 200"): SB (r)
nucleus and bar are still strong, but the ring is narrower and knotty;
two main spiral arms, narrow and knotty (one with branching) emerge
from the ring near the extremities of the bar; additional, weaker arms
start near minor axis of elliptical ring (here appearing circular in
the projection). Note slight asymmetry. Compare with 26, 28 and 31.
28. NGC 3367 (P 200"): SB (r)
nucleus and bar are smaller and fainter; the elliptical ring is weaker
and begins to break up into knots; the arms are branching and
filamentary; note strong asymmetry. Compare with 24, 27 and 31.
SAB (r) : has a fairly small,
little elongated nucleus in a fairly broad and faint bar marking the
major axis of a little elongated ring from which several spiral arms
branch out. The main arms have a slight tendency to ``return'' inwards.
Examples are NGC 1832, NGC 7531, Sab, NGC 6744, Sbc
(illustrated from S 30"). Some, like NGC 6902, NGC 6935, Sa,
NGC 6937, have only very faint traces of a bar and
mark the transition towards SA (r).
15. NGC 3032 (P 200"): SAB (r) 0
o. Fairly large, elongated nucleus on short, bright
stubby bar, surrounded by dark ring or arcs in bright lens. The
envelope does not show up on this print. Compare with 3, 7 and 13.
SAB (rs) : this is the most
general mixed type involving all possible transitions between the main
typical patterns. An almost
infinite variety is possible here, but for classification purposes the
main characteristics of this hybrid type are: a small bright nucleus
in a broad, diffuse bar with some spiral structure in the lens. The
bar crosses a nearly circular or often hexagonal pseudo-ring formed by
the inner sections of the spiral arms. A very good example is NGC 4303 (M61), Sbc (illustrated from IR 20"); other
examples are NGC 3145,
NGC 6814. At a later stage this mixed structure is
well illustrated by
the central regions of NGC 5457 (M101) and NGC 6946, both Scd.
11. NGC 6873 (S 74"): SAB (rs) 0
o. Fairly small nucleus in elongated lens with dark
ring or crescent, surrounded by extensive, bright envelope. Compare
with 3, 7 and 9.
Examples of irregular later stages of
the spiral sequences are:
37. NGC 1613 (P 48"): I Bm.
structure includes the well resolved axial bar and asymmetrical bright
patch rich in supergiants and emission objects. Note characteristic
asymmetry. Compare with 36 and 38.
38. Dwarf I in Andromeda (2h31, + 40°19') (P 200"):
dIB (s) m. The characteristic structure of the
late-type SB (s) spirals is still recognizable, including axial
bar (foreshortened in the projection), with embryonic spiral arms
emerging at its extremities; one of the arms can be faintly traced to
the left and lower part of the field. Compare with 36 and 37.
Examples of peculiar lenticulars and
non-magellanic irregulars are:
56. NGC 1947 (S 74"): SA0p.
elongated, smooth nebulosity, rather similar to elliptical galaxy or
to spheroidal bulge of early spiral, cut by two slightly curved lanes
of dark matter on one side of nucleus. There is no trace of spindle or
outer spiral structure.
57. NGC 3077 (P 200"): I (0) or
SAB0p. Little elongated, smooth nebulosity marked by
irregular patches of dark matter; two of them emerging on both sides
near minor axis simulate embryonic spiral pattern as seen in nuclear
regions of SAB (s) 0/a. Has contrasting early spectral type and
advanced color index. Compare with 15.
58. NGC 4753 (W 100"): I (0) or
SB ? (s:)
0 +p. The inner isophotes (inset) are
elongated at right angles to the direction of the major axis of the
outer isophotes. The nuclear region is crossed by complex dark lanes
which appear to be on the near side below the small nucleus and on the
far side above it (see inset). This could be interpreted as the near
end-on (``along'') view of the bar and lens of the armless SB (s)
0/a object. Compare with 55 and 56.
59. NGC 4691 (P 200"): (R) SB (s:)
The weak outer whorls in the envelope (inset) show that this system is
seen nearly face-on; note the complex structure of the bar crossed and
surrounded by patches and curved lanes of dark matter. Compare with
52, 57, 58 and 60/61.
60/61. NGC 3034 = M82 (P 200"): I
(0) or SB
(s:) 0p. This is interpreted tentatively as the
``broad-side'', nearly edge-on view of the bar and lens of an armless
SB (s) 0/a object. Has contrasting early-type spectrum and
advanced color index. Compare with 53, 57 and 59. Note especially the
great similarity between 59 and 61 and the characteristic
``crossing-over'' of the main dark lane in the nucleus 1.
1 This interpretation of the non-magellanic irregulars is
consistent with their luminosities, colors and dimensions). It also
makes it possible to understand the rapid, solid-body rotation observed
along the major axis of M82 (N. U. Mayall, in ``Problems of Cosmic
Aerodynamics'', U.S. Central Air Documents Office, p. 181, 1951).