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Hubble recognized that there are basically two classes of galaxies: disk-shaped galaxies and non-disk-shaped galaxies. In a disk galaxy, the structure is dominated by a highly flattened stellar component. Within this disk, other structures may be seen, such as spiral arms, bars, rings, a central bulge and extensive distributions of interstellar gas and dust. The way these structures are seen depends on the inclination of the plane of the disk to our line of sight. For a given galaxy, we say the inclination i is 0° when the disk is seen face-on, and 90° when the disk is seen edge-on. Disk planes are randomly oriented to the line of sight (meaning they are uniformly distributed in sin i), which complicates the interpretation of highly inclined cases. Even a century ago, disk-shaped galaxies were known to be more common than non-disk galaxies to the point that the latter were considered of greater interest (Keeler 1899).

Although not all disk-shaped galaxies are spiral, the typical disk galaxy is a spiral, where luminous, outwardly winding arcs of stars and often star-forming regions form a major part of the morphology. Classic nearby spirals such as M51, M81, and M101 initially fueled the misperception that spirals are generally regular systems having only a bulge in addition to the disk and spiral arms. It was thought by Hubble that barred spirals are significantly less abundant than non-barred spirals, or what he called “normal spirals”. It is now known that barred spirals are at least as abundant as non-barred spirals, and that some galaxies that appear to be non-barred in blue light can appear to be barred when imaged at infrared wavelengths (Eskridge et al. 2000).

The existence of non-disk-shaped (and therefore non-spiral) galaxies was at first somewhat controversial. Based on the limited plate material available in his day, Curtis (1918) believed that all galaxies were spiral, and any that did not appear to be spiral on his plates would be found to be spiral when observed with larger telescopes. Hubble, having access to better telescopes, disagreed with this conclusion and believed that genuine non-spiral galaxies existed. Hubble recognized such galaxies as elliptical galaxies, where the luminosity distribution is defined by a regular decline from a bright center to the faint outer regions. Ellipticals were thought to be characterized by no other features but their isophotal shapes, which ranged from round (E0) to a flattening approaching that of disk-shaped galaxies (E7). In fact, Hubble believed the sequence of E galaxy shapes blended smoothly into the domain of disk-shaped galaxies, which he split into two parallel sequences of normal and barred spirals characterized by the degree of central concentration, the degree of resolution of the spiral arms into what were likely to be star-forming regions and the degree of openness of the spiral arms. Hubble (1936) illustrated these features in his famous “tuning fork” of galaxy morphologies (Figure 1.1). Although this view is now obsolete, the tuning fork is still an effective way of binning galaxies into astrophysically meaningful classes.

In stellar astronomy, temporal terms are often used to describe certain kinds of stars. Stars of spectral classes O, B and A tend to be young and are known as “early-type” stars, while those of spectral classes K and M tend to be older and are called “late-type” stars. Those of spectral classes F and G are known as “intermediate-type” stars. Hubble found it convenient to use similar terms for galaxies, calling galaxies on the left part of the tuning fork early-type galaxies and those on the right part late-type galaxies. In general, E and S0 galaxies are said to be early-type galaxies, S0/a and Sa galaxies are called early-type spirals, Sc-Sm galaxies are called late-type spirals and Sab-Sbc galaxies are called intermediate-type spirals. Unlike for stars, no real temporal meaning was to be implied by these terms.

The value of the Hubble classification system, as well as its limitations, can only be appreciated by examining large numbers of galaxies and attempting to classify them within that system. The small number of astronomers who actually did this, such as Sandage, de Vaucouleurs, van den Bergh and Morgan, all saw the need for modifications or even alternative views, which led to major revisions of the system. This article uses images from the EFIGI project (Baillard et al. 2011; de Lapparent et al. 2011) to illustrate galaxies of different types within the framework of the CVRHS classification system (Buta et al. 2015). The classifications are from Buta (2019).

Figure 1.3. The de Vaucouleurs (1959) revised Hubble–Sandage (VRHS) system of galaxy classification, where the Hubble “tuning fork” has become a classification volume. Top: The VRHS stage sequence. Bottom left: The cross-section of families and varieties. Bottom right: Examples of families and varieties at stage Sbc (from Buta et al. 2007)

The basic outline of the HS system (Sandage 1961) is depicted in Figure 1.2 and that for the VRHS system is depicted in Figure 1.3. In the HS system, Sandage added the (r) and (s) subtypes, which are also used in the VRHS. The HS system still depicts galaxy morphology as a “tuning fork”, but is more complicated than Hubble’s original tuning fork (Figure 1.1). In the VRHS, the tuning fork is replaced with a classification volume whose long axis is the stage (E..S0..Sa, Sb..Im, etc.), and whose short axes are the family (SA, SAB, SB) and the variety [(s), (rs), (r)]. The use of “SAB” to denote galaxies having a bar of intermediate apparent strength and “(rs)” for partial inner rings (pseudorings) are major hallmarks of the VRHS and CVRHS systems. Not depicted in the volume are outer rings, large structures about twice the size of a bar in barred galaxies and nuclear rings, much smaller structures about one-tenth the size of a bar in barred galaxies. The appearance of the volume with a broad middle section and narrower ends is intended to depict how the diversity of families and varieties varies with stage. The greatest diversity is found near stage S0/a, while the diversity diminishes by stages S0 and Im either because bars become difficult to recognize or because rings are simply not present.

The CVRHS is defined by the same kind of classification volume as the VRHS, but with a more finely detailed division into various subtypes. Figure 1.4 shows a cross-section of types through any CVRHS stage from S0° to Sm. The arrangement of families and varieties is the same as in the VRHS, except that de Vaucouleurs’ vision of continuity is carried further with the use of underline notation that he had applied mainly in his 1963 paper, “Revised Classifications of 1500 Bright Galaxies”. The family sequence became SA, SAB, SAB, SAB, SB and the variety sequence became (s), (rs), (rs), (rs), (r). However, in order to recognize outer and nuclear rings in the system, the CVRHS system denotes the presence or absence of an inner feature as the “inner variety”, the presence or absence of an outer feature as the “outer variety”, and the presence or absence of a nuclear feature as the “nuclear variety”. The term “inner feature” now refers to inner rings and lenses, not just inner rings, and the same is true for outer and nuclear features (the latter also including nuclear bars). The recognition of outer pseudorings (R′) is also one of the hallmarks of the VRHS and CVRHS systems. The central subtype in Figure 1.4, (R)SAB(rs,nr), is meant to convey the generalization of VRHS varieties to include inner, outer and nuclear varieties.

Figure 1.4. A cross-section through the CVRHS system of families and varieties including the de Vaucouleurs (1963) underline notation. The central “cell” is meant to extend the concept of galactic varieties to outer and nuclear features, as well as additional inner features such as lenses

de Vaucouleurs (1963) also introduced the underline notation for stages, as in SA(s)ab, which means “closer to Sb than to Sa”. Although this notation can be applied directly as for families and inner varieties, it has appeared mainly in classifications based on averages of multiphase (or repeat) efforts designed to check for consistency (e.g. Buta et al. 2015). This usage of underlines for stages, families and varieties considerably increases the number of “cells” in the CVRHS as compared to the VRHS, and one may well question whether some of these cells might not have any entrants. Buta (2019) showed that for the EFIGI sample, the cells of the CVRHS system are roughly evenly occupied among late S0s and early-type spirals, but not among later-type spirals. This means the classification volume lacks the symmetry depicted in Figure 1.3.

In the following sections, the different types of galaxies are described in more detail.