The characteristic feature of extra-galactic nebulae is rotational
symmetry about dominating non-stellar nuclei. About 97 per cent of
these nebulae are regular in the sense that they show this feature
conspicuously. The regular nebulae fall into a progressive sequence
ranging from globular masses of unresolved nebulosity to widely open
spirals whose arms are swarming with stars. The sequence comprises two
sections, elliptical nebulae and spirals, which merge into each other.
Although deliberate effort was made to find a descriptive
classification which should be entirely independent of theoretical
considerations, the results are almost identical with the path of
development derived by Jeans[8] from purely theoretical investigations.
The agreement is very suggestive in view of the wide field covered by
the data, and Jeans’s theory might have been used both to interpret
the observations and to guide research. It should be borne in mind,
however, that the basis of the classification is descriptive and
entirely independent of any theory.
_Elliptical nebulae._—These give images ranging from circular
through flattening ellipses to a limiting lenticular figure in which
the ratio of the axes is about 1 to 3 or 4. They show no evidence
of resolution,[9] and the only claim to structure is that the
luminosity fades smoothly from bright nuclei to indefinite edges.
Diameters are functions of the nuclear brightness and the exposure
times.
[Illustration: PLATE XII Elliptical and Irregular Nebulae
E0 NGC 3379, E2 NGC 221 (M 32), E5 NGC 4621 (M 59), E7 NGC 3115,
NGC 3034 (M 82), NGC 4449]
The only criterion available for further classification appears to
be the degree of elongation. Elliptical nebulae have accordingly been
designated by the symbol “E,” followed by a single figure, numerically
equal to the ellipticity (a – b)/a with the decimal point omitted.
The complete series is E0, E1, ..., E7, the last representing
a definite limiting figure which marks the junction with the
spirals.
The frequency distribution of ellipticities shows more round or nearly
round images than can be accounted for by the random orientation of
disk-shaped objects alone. It is presumed, therefore, that the images
represent nebulae ranging from globular to lenticular, oriented at
random. No simple method has yet been established for differentiating
the actual from the projected figure of an individual object, although
refined investigation furnishes a criterion in the relation between
nuclear brightness and maximum diameters. For the present, however,
it must be realized that any list of nebulae having a given apparent
ellipticity will include a number of tilted objects having greater
actual ellipticities. The statistical average will be too low, except
for E7, and the error will increase with decreasing ellipticity.
_Normal spirals_.—All regular nebulae with ellipticities greater than
about E7 are spirals, and no spirals are known with ellipticities
less than this limit. At this point in the sequence, however,
ellipticity becomes insensitive as a criterion and is replaced by
conspicuous structural features which now become available for
classification. Of these, practically speaking, there are three which
fix the position of an object in the sequence of forms: (1) relative
size of the unresolved nuclear region; (2) extent to which the arms are
unwound; (3) degree of resolution in the arms. The form most nearly
related to the elliptical nebulae has a large nuclear region similar to
E7, around which are closely coiled arms of unresolved nebulosity.
Then follow objects in which 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 inward until in the final stages it reaches
the nucleus itself. In the larger spirals where critical observations
are possible, these condensations are found to be actual stars and
groups of stars.
The structural transition is so smooth and continuous that the
selection of division points for further classification is rather
arbitrary. The ends of the series are unmistakable, however, and, in
a general way, it is possible to differentiate a middle group. These
three groups are designated by the non-committal letters “a,” “b,” and
“c” attached to the spiral symbols “S,” and, with reference to their
position in the sequence, are called “early,” “intermediate,” and
“late” types.[10] A more precise subdivision, on a decimal scale for
example, is not justified in the present state of our knowledge.
In the early types, the group Sa, most of the nebulosity is in the
nuclear region and the arms are closely coiled and unresolved. N.G.C.
3368 and 4274 are among the latest of this group.
The intermediate group, Sb, includes objects having relatively large
nuclear regions and thin rather open arms, as in M 81, or a smaller
nuclear region with closely coiled arms, as in M 94. These two nebulae
represent the lateral extension of the sequence in the intermediate
section. The extension along the sequence is approximately represented
by N.G.C. 4826, among the earliest of the Sb, and N.G.C. 3556 and
7331, which are among the latest. The resolution in the arms is seldom
conspicuous, although in M 31, a typical Sb, it is very pronounced in
the outer portions.
[Illustration: PLATE XIII Normal and Barred Spirals
Sa NGC 4594, SBa NGC 2859, Sb NGC 2841, SBb NGC 5850, Sc NGC 5457
(M 101), SBc NGC 7479]
The characteristics of the late types, the group Sc, are more
definite—an inconspicuous nucleus and highly resolved arms. Individual
stars cannot be seen in the smaller nebulae of this group, but knots
are conspicuous, which, in larger objects, are known to be groups and
clusters of stars. The extent to which the arms are opened varies from
M 33 to M 101, both typical Sc nebulae.
_Barred spirals_.—In the normal spiral the arms emerge from two
opposite points on the periphery of the nuclear region. There is,
however, a smaller group, containing about 20 per cent of all spirals,
in which a bar of nebulosity extends diametrically across the nucleus.
In these spirals, the arms spring abruptly from the ends of this
bar. These nebulae also form a sequence, which parallels that of the
normal spirals, the arms apparently unwind, the nuclei dwindle, the
condensations form and work inward.
H. D. Curtis[11] first called attention to these nebulae when he
described several in the intermediate stages of the series and called
them φ-type spirals. The bar, however, never extends beyond the inner
spiral arms, and the structure, especially in the early portion of the
sequence, is more accurately represented by the Greek letter θ. From a
dynamical point of view, the distinction has considerable significance.
Since Greek letters are inconvenient for cataloguing purposes, the
English term, “barred spiral,” is proposed, which can be contracted to
the symbol “SB.”
The SB series, like that of the normal spirals, is divided into three
roughly equal sections, distinguished by the appended letters “a,”
“b,” and “c.” The criteria on which the division is based are similar
in general to those used in the classification of the normal spirals.
In the earliest forms, SBa, the arms are not differentiated, and
the pattern is that of a circle crossed by a bar, or, as has been
mentioned, that of the Greek letter θ. When the bar is oriented nearly
in the line of sight, it appears foreshortened as a bright and definite
minor axis of the elongated nebular image. Such curious forms as the
images of N.G.C. 1023 and 3384 are explained in this manner. The latest
group, SBc, is represented by the S-shaped spirals such as N.G.C. 7479.