2. Revisions and additions. About 1935 Hubble undertook a systematic morphological study of the approximately 1000 brighter galaxies listed in the Shapley-Ames Catalogue, north of - 30° declination, with a view to refining his original classification scheme. This work begun with the 100-inch reflector and later continued with the 200-inch reflector was practically completed at the time of Hubble's death in 1953; his notes collected and organized by Sandage will be published in the near future with a collection of about 175 photographs illustrating the morphological features characterizing the various galaxy types. Through the courtesy of Dr. A. R. Sandage the main revisions introduced by Hubble to his system can be briefly described here.

a) The most important addition was the introduction of the S0 and SB0 types regarded as transition stages between the ellipticals and spirals at the branching off point of the tuning-fork.

S0 objects have the smooth appearance of ellipticals, but a luminosity distribution more like that of spirals, although no spiral arms are visible. They are characterized by a sharp, bright nucleus in the center of a more or less uniform disc or ``lens'' having a rather sharp outer rim and surrounded by a faint, diffuse ``envelope'' with indefinite boundaries; the diameter of the lens in which dark crescents or rings of obscuring matter are often observed is usually about one-third of that of the envelope (see Plate III).

There is apparently a continuous transition from ``late'' ellipticals, such as NGC 4270, NGC 4958, etc. and ``early'' S0's such as NGC 3630, NGC 4564, etc., then through various stages of the S0 sequence to the ``earliest'' regular spirals.

HUBBLE distinguished two groups of S0 objects:

S0 (1): smooth lens and envelope; early examples are NGC 1201, NGC 1332, and late examples NGC 3065, NGC 4684.

S0 (2): Some structure in the envelope in the form of a dark zone and ring; examples are NGC 4459 and NGC 4111, the latter seen edgewise.

In edgewise objects the presence of rings manifests itself by the appearance of ``ansae'' simulating Saturn's ring, such as in NGC 4215, NGC 7332, etc. (Plate IX). The transition stage S0/a between S0 and Sa shows incipient spiral structure in the envelope.

SB0 objects are characterized by a bar through the central lens, sometimes broad and hazy, sometimes narrow and sharp; the envelope may form faint outer rings, sometimes conspicuous, sometimes vague or imperceptible (Plate IV). Hubble distinguished three groups of SB0 objects:

SB0 (1): a bright lens, with broad, hazy bar and no ring, surrounded by a larger, fainter envelope, e.g. as in NGC 3384, NGC 4262, etc., some have circular envelopes, e.g. NGC 4203.

SB0 (2): a broad, weak bar across a primary ring, with faint outer secondary rings, e.g. NGC 2859.

SBO (3): well developed bar and ring pattern, with the bar stronger than ring, e.g. NGC 4643, NGC 5101.

b) Many of the objects originally classified as SBa were then reclassified as SB0 and the SBa sub-division had to be redefined as follows:

SBa: smooth bar and lens, with poorly developed, closely coiled arms in envelope and either massive and structureless or filamentary and partially resolved.

Hubble further distinguished two or three groups of barred spirals, one in which the arms extend from the rim of a ring crossed by a bar, e.g. NGC 2217, NGC 5566, NGC 5701 (SBa), NGC 4999, NGC 5950 (SBb), and one in which the arms start at the ends of the bar without ring, e.g. NGC 2798, NGC 4290, NGC 7743 (SBa), NGC 1300, NGC 5430, NGC 6951 (SBb). In another group still the ring is formed of closely coiled filamentary arms, e.g. NGC 3185, NGC 4037, NGC 4385, NGC 4389. This distinction remains well marked at the SBb stage, showing well developed, partially resolved arms going through more than one revolution, but there is no spiral structure in the lens; primary rings consist of spiral arms, secondary rings are seldom found. It was not followed up into the SBc sub-division, characterized by wide open, well resolved spiral arms with absorption lanes and spiral structure in the lens, but no ring pattern.

Sandage has also recognized two types of normal spirals, one in which the arms start at the rim of a ring structure, the other in which they start from a central nucleus. Some objects of the ringed type had previously been described by Shapley and Paraskevopoulos ¹ as ``plate spirals'' and an investigation of ring structures was made by Randers [57].

There are further differences in the multiplicity of the spiral pattern, some objects showing only two main regular arms, others having a great many tightly coiled whorls. Reynolds had already pointed out ² that some spirals have ``massive'' arms, e.g. M31, M33, while others have ``filamentary'' arms, e.g. M81, M101. The importance of this distinction was acknowledged by Hubble [B] but could not be incorporated in the classification.

c) An extension of the normal spiral sequence beyond the stage Sc was proposed by Shapley who used the notation Sd for objects such as NGC 7793 (20), showing a very small, bright nucleus and many knotty irregular spiral arms. This notation could also, and perhaps more appropriately, be applied to highly disorganized and complex spirals of low surface brightness, such as NGC 4395-4401 (see Plate VII).

A parallel extension of the barred spiral sequence beyond the stage SBc was introduced by de Vaucouleurs ³ through the recognition of spiral structure in the Magellanic Clouds and objects of similar type, such as NGC 1313 (32), NGC 4027, NGC 4618, etc., which may be noted as SBd or SBm.

Still ``later'' stages extending the spiral sequence into the irregular types may be represented by objects such as IC 2574 or NGC 2366, IC 4662, etc. These objects are characterized by low surface brightness, high degree of resolution and sometimes outstanding emission nebulosities similar to 30 Doradus in the Large Cloud. They always show an abundance of blue supergiants and strong emission lines in their spectrum.

An important characteristic of the group of irregulars related to the Magellanic type I(m) is their small diameter and low luminosity which marks them as dwarf galaxies. Typical objects of this group are NGC 6822, IC 1613, the Sextans system, the Wolf-Lundmark-Melotte nebula, etc.⁴ (Plate VII).

In the poorest and smallest of them emission objects may be few or absent, but when their distance is small enough they are always well resolved into blue supergiants and giants.

d) The existence of dwarf ellipticals (dE) of very low surface brightness was first brought to notice in 1938 through the discovery of the Fornax and Sculptor systems by Shapley ⁵. These are close enough to be resolved on blue plates at m 18; except for their very low density they seem to share all other population characteristics of normal ellipticals. Because of their very low surface brightness such systems are difficult to detect and might be much more abundant in space than their belated discovery suggests; nevertheless a special search for them on Harvard plates failed to disclose other examples. A few more ``Sculptor type'' systems have, however, been found since 1949 with the 48-inch Palomar Schmidt ⁶.

Dwarf galaxies of a possibly related type have been investigated in 1952 by Reaves [58] on plates of the Virgo cluster taken by C. D. Shane with the Lick 20-inch astrograph. These objects of which the brightest example in the Virgo cluster is IC 3475 have the same smooth, circular or little elongated appearance as the dwarf ellipticals and likewise show little central condensation. Hubble and Reaves suggested that they may be related to a group of dwarf spirals of low surface brightness exemplified by NGC 3299 because their color was at first thought to be bluish; however, recent observations indicate them to be reddish and this strengthens the similarity with the Sculptor-type systems.

e) After all such additional types or variants have been weaned out, there remains a hard core of ``irregular'' or ``peculiar'' objects which do not seem to fit in any of the recognized types. One group of such objects consists of strongly interacting or colliding systems such as NGC 1275, NGC 4038-39, NGC 5128, etc.⁷ which can often be identified by their distorted structure, abnormal spectrum and strong radio-emission; these systems are discussed in the article by B. Y. Mills. Double or multiple systems showing moderate interaction in the form of connecting links, distorted spiral arms or irregular filamentary extensions are discussed by F. Zwicky. Only isolated galaxies or weakly interacting systems are considered in the present article.

Even among them, however, there remains a small number of objects not clearly assignable to any of the previous types; one possible group, exemplified by NGC 3034, NGC 3077, etc. is characterized by an early-type spectrum (A, F) contrasting with a reddish color (C 10.8), irregular absorption patches and filaments and a smooth, unresolved nebulous structure indicating an absence of blue supergiants and of discrete emission nebulosities (see Plate XI). Another possible group, exemplified by NGC 5253 ⁸ is characterized by fairly strong emission lines and a complex structure unlike that of the Magellanic irregulars or late-type spirals. Still other puzzling objects such as Mayall's nebula ⁹, also show strong emission lines but have a fairly smooth structure ¹⁰.

Fig.2. Revised classification: a plane projection of the classification volume. Compare with Fig.3. The ordinary spirals SA are in the upper half of the figure, the barred spirals SB in the lower half. The ring types (r) are to the left, the spiral types (s) to the right. Ellipticals and lenticulars are near the center, magellanic irregulars near the rim. The main stages of the classification sequence from E to Im through S0 ^-, S0, S0 ⁺, Sa, Sb, Sc, Sd, Sm are illustrated, approximately on the same scale, along each of the four main morphological series SA (r), SA (s), SB (s), SB (r). A few mixed or ``intermediate'' types SAB and S (rs) are shown along the horizontal and vertical diameters respectively. Structures predominantly of Type I population are dashed, of Type II dotted. This classification scheme, as used in the Mount Stromlo survey of southern galaxies, is superseded by the slightly revised and improved system illustrated in Fig. 3 and Plates I to XI.