1. Standard classification. This
is the Mt. Wilson classification
as used by Hubble between 1925 and 1935; it has been so often
D] that only
a short summary will suffice here. It is
illustrated by Hubble's well known ``tuning-fork'' diagram
a) Description of types.
nebulae (E), range from circular or globular
objects, such as NGC 3379, to elongated, lenticular objects, such as
NGC 3115. As a rule they show no structural
details, besides a small,
bright and strongly condensed nucleus around which the textureless
nebulosity decreases smoothly outwards in all directions to an
indefinite edge where it fades into the general luminosity of the
Fig. 1. Standard classification: Hubble's
tuning-fork diagram (1925).
Sub-types are defined by the
index n = 10(1 - b/a),
if a, b are the apparent major and minor axes measured
on photographs. The most strongly elongated objects, type E7,
such as NGC 3115, depart notably from a geometrical
``elliptical'' shape, being pointed near the ends of the major axis.
spirals (S) show the characteristic spiral arms when
seen pole-on and, as a rule, a ``spindle'' shape with heavy absorption
lanes of dark matter when seen edge-on. In the normal spirals the arms
emerge tangentially from a bright central nucleus at opposite points
on its indefinite edge and vanish after about one complete turn of the
best fitting logarithmic spiral
(cf. Sect. 6). In the more
regular or classical examples only two main arms, very nearly
symmetrical with respect to the nucleus, are present. In most cases,
however, additional or secondary arms may exist and the spiral pattern
is often far from regular.
Sub-types, noted a,
b, c, are defined
by the relative importance of the nucleus (decreasing from a to
c) and the degree of unwinding
and resolution of the arms (increasing from a to
c). According to Hubble
:``the arms appear to build
up at the expense of the nuclear regions and unwind as they grow; in the
end the arms are wide open and the nuclei inconspicuous. Early in the
series the arms begin to break up into condensations, the resolution
commencing in the outer regions and working inwards until in the final
stages it reaches the nucleus itself''
p. 326). The
resolution referred to is into blue supergiants and emission
objects characteristic of a Type I population. The gradual decrease
of the axial ratio nucleus/spiral arms is best seen in edgewise
systems (see Plate VIII), while face-on systems show more clearly the
increasing resolution and irregularity from ``early'' types (Sa)
to ``late'' types (Sc) (Plate V)1.
Intermediate types: Hubble also
introduced the notion of ``lateral''
extension or width of the classification sequence in its intermediate
section, giving M81 with ``large nuclear region and thin, rather
open arms'' and M94 ``having smaller nuclear region with closely
coiled arms'' as extreme cases. This distinction has been little used in
Of more importance was the
recognition of objects intermediate
between normal and barred spirals, such as M83 and M61 which have been
classified alternatively as Sc or SBc. Their intermediate
characteristics first noticed by Hubble
p. 46) and Lundmark
have been discussed by Lindblad and Langebartel
2. They more
or less fill the gap between the two branches of the tuning-fork diagram.
spirals (SB), include the ``pin-wheel'' or ``-type'' first
described by Curtis .
In it a very bright central nucleus is
crossed diametrically by a bar at the extremities of which spiral
arms start at right angles (in ``late'' sub-types) or tangentially from
the rim of a continuous ring of which the bar is a diameter (in
``early'' sub-types). Additional or secondary arms may exist, but as a
rule the symmetry of the pattern is more regular than in normal
spirals (see Plate VI).
Sub-types, noted a, b,
c, are defined as for the
normal spirals by the relative size of the nucleus and the degree of
resolution and opening of the spiral structure. In Hubble's original
system the ring, closed in the SBa and SBb sub-types,
opens at SBc, producing the aspect sometimes described as
``S-shaped'' spirals. SBa objects observed under various angles
give rise to singular ``Saturn-like'' shapes (Plate X).
(iv) Irregulars (I),
were described originally by Hubble
 as a
class of objects ``lacking both dominating nuclei and rotational
symmetry'' and of which ``the Magellanic Clouds are the most conspicuous
p. 328). These were termed more specifically
``Magellanic nebulae'' by Lundmark
the class was broadened by Hubble to include peculiar or chaotic
objects which ``do not find a place in the sequence of classification''
since ``the remaining irregulars might be arbitrarily placed in the
regular sequence as highly peculiar objects, rather than in a separate class
... Others, such as M82, are merely nondescript''
([B], p. 47).
In fact, the symbol I has often
been used as almost equivalent to
the subscript p for peculiar; such an extension of the
notation is both confusing and unwarranted. As a result, the relation of
``irregulars'' to other sections of the classification sequence was not
clear, some of them being clearly related to late-type spirals and
others to early-type spirals.
b) Frequency of types.
From 600 bright galaxies in the Lick and
Mt. Wilson plate collections Hubble
found the following apparent relative frequencies:
Hence, the spirals constituted about 80% of the sample, with (S
+ SB) / E = 4 or 5 and, further, a ratio S / SB
= 2 or 3.
However, Shapley and Ames3
found that in the Coma-Virgo region for m < 12 spirals comprise
only 46% of the population and for m < 14, S = 48%,
E = 47%, I = 5%.
These early statistics, although not
necessarily inconsistent, were affected by selection effects arising
from clustering and
different interpretations of some nebular types not included in the
standard classification. Frequencies based on revised types are given
in Sect. 3c.
1 In accordance with
establish custom, the words "early" and "late"
are used in connection with the position in the spiral sequence but
have no temporal connotation.
2 See also in  a discussion by Lindblad
of the batted spiral characteristics in the Andromeda nebula.
3 H. Shapley and A. Ames: Harvard Bull. 1926. No. 838,
3; 1930, No. 876, 39.