Types of galaxies
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Principal schemes of classification
Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by the American astronomer Edwin Hubble in 1926. In Hubble’s scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes: ellipticals, spirals, and irregulars. Hubble subdivided these three classes into finer groups.
In The Hubble Atlas of Galaxies (1961), the American astronomer Allan R. Sandage drew on Hubble’s notes and his own research on galaxy morphology to revise the Hubble classification scheme. Some of the features of this revised scheme are subject to argument because of the findings of very recent research, but its general features, especially the coding of types, remain viable. A description of the classes as defined by Sandage is given here, along with observations concerning needed refinements of some of the details.
Elliptical galaxies
These systems exhibit certain characteristic properties. They have complete rotational symmetry; i.e., they are figures of revolution with two equal principal axes. They have a third smaller axis that is the presumed axis of rotation. The surface brightness of ellipticals at optical wavelengths decreases monotonically outward from a maximum value at the centre, following a common mathematical law of the form: I = I0( r/a +1 )−2, where I is the intensity of the light, I0 is the central intensity, r is the radius, and a is a scale factor. The isophotal contours exhibited by an elliptical system are similar ellipses with a common orientation, each centred on its nucleus. No galaxy of this type is flatter than b/a = 0.3, with b and a the minor and major axes of the elliptical image, respectively. Ellipticals contain neither interstellar dust nor bright stars of spectral types O and B. Many, however, contain evidence of the presence of low-density gas in their nuclear regions. Ellipticals are red in colour, and their spectra indicate that their light comes mostly from old stars, especially evolved red giants.
Subclasses of elliptical galaxies are defined by their apparent shape, which is of course not necessarily their three-dimensional shape. The designation is En, where n is an integer defined by n = 10( a− b)/a. A perfectly circular image will be an E0 galaxy, while a flatter object might be an E7 galaxy. (As explained above, elliptical galaxies are never flatter than this, so there are no E8, E9, or E10 galaxies.)
Although the above-cited criteria are generally accepted, current high-quality measurements have shown that some significant deviations exist. Most elliptical galaxies do not, for instance, exactly fit the intensity law formulated by Hubble; deviations are evident in their innermost parts and in their faint outer parts. Furthermore, many elliptical galaxies have slowly varying ellipticity, with the images being more circular in the central regions than in the outer parts. The major axes sometimes do not line up either; their position angles vary in the outer parts. Finally, astronomers have found that a few ellipticals do in fact have small numbers of luminous O and B stars as well as dust lanes.
Spiral galaxies
Spirals are characterized by circular symmetry, a bright nucleus surrounded by a thin outer disk, and a superimposed spiral structure. They are divided into two parallel classes: normal spirals and barred spirals. The normal spirals have arms that emanate from the nucleus, while barred spirals have a bright linear feature called a bar that straddles the nucleus, with the arms unwinding from the ends of the bar. The nucleus of a spiral galaxy is a sharp-peaked area of smooth texture, which can be quite small or, in some cases, can make up the bulk of the galaxy. Both the arms and the disk of a spiral system are blue in colour, whereas its central areas are red like an elliptical galaxy. The normal spirals are designated S and the barred varieties SB. Each of these classes is subclassified into three types according to the size of the nucleus and the degree to which the spiral arms are coiled. The three types are denoted with the lowercase letters a, b, and c. There also exist galaxies that are intermediate between ellipticals and spirals. Such systems have the disk shape characteristic of the latter but no spiral arms. These intermediate forms bear the designation S0.
S0 galaxies
These systems exhibit some of the properties of both the ellipticals and the spirals and seem to be a bridge between these two more common galaxy types. Hubble introduced the S0 class long after his original classification scheme had been universally adopted, largely because he noticed the dearth of highly flattened objects that otherwise had the properties of elliptical galaxies. Sandage’s elaboration of the S0 class yielded the characteristics described here.
S0 galaxies have a bright nucleus that is surrounded by a smooth, featureless bulge and a faint outer envelope. They are thin; statistical studies of the ratio of the apparent axes (seen projected onto the sky) indicate that they have intrinsic ratios of minor to major axes in the range 0.1 to 0.3. Their structure does not generally follow the luminosity law of elliptical galaxies but has a form more like that for spiral galaxies. Some S0 systems have a hint of structure in the envelope, either faintly discernible armlike discontinuities or narrow absorption lanes produced by interstellar dust. Several S0 galaxies are otherwise peculiar, and it is difficult to classify them with certainty. They can be thought of as peculiar irregular galaxies (i.e., Irr II galaxies) or simply as some of the 1 or 2 percent of galaxies that do not fit easily into the Hubble scheme. Among these are such galaxies as NGC 4753, which has irregular dust lanes across its image, and NGC 128, which has a double, almost rectangular bulge around a central nucleus. Another type of peculiar S0 is found in NGC 2685. This nebula in the constellation Ursa Major has an apparently edge-on disk galaxy at its centre, with surrounding hoops of gas, dust, and stars arranged in a plane that is at right angles to the apparent plane of the central object.
Sa galaxies
These normal spirals have narrow, tightly wound arms, which usually are visible because of the presence of interstellar dust and, in many cases, bright stars. Most of them have a large amorphous bulge in the centre, but there are some that violate this criterion, having a small nucleus around which is arranged an amorphous disk with superimposed faint arms. NGC 1302 is an example of the normal type of Sa galaxy, while NGC 4866 is representative of one with a small nucleus and arms consisting of thin dust lanes on a smooth disk.
Sb galaxies
This intermediate type of spiral typically has a medium-sized nucleus. Its arms are more widely spread than those of the Sa variety and appear less smooth. They contain stars, star clouds, and interstellar gas and dust. Sb galaxies show wide dispersions in details in terms of their shape. Hubble and Sandage observed, for example, that in certain Sb galaxies the arms emerge at the nucleus, which is often quite small. Other members of this subclass have arms that begin tangent to a bright, nearly circular ring, while still others reveal a small, bright spiral pattern inset into the nuclear bulge. In any of these cases, the spiral arms may be set at different pitch angles. (A pitch angle is defined as the angle between an arm and a circle centred on the nucleus and intersecting the arm.)
Hubble and Sandage noted further deviations from the standard shape established for Sb galaxies. A few systems exhibit a chaotic dust pattern superimposed upon the tightly wound spiral arms. Some have smooth, thick arms of low surface brightness, frequently bounded on their inner edges with dust lanes. Finally, there are those with a large, smooth nuclear bulge from which the arms emanate, flowing outward tangent to the bulge and forming short arm segments. This is the most familiar type of Sb galaxy and is best exemplified by the giant Andromeda Galaxy.
Many of these variations in shape remain unexplained. Theoretical models of spiral galaxies based on a number of different premises can reproduce the basic Sb galaxy shape, but many of the deviations noted above are somewhat mysterious in origin and must await more detailed and realistic modeling of galactic dynamics.
Sc galaxies
These galaxies characteristically have a very small nucleus and multiple spiral arms that are open, with relatively large pitch angles. The arms, moreover, are lumpy, containing as they do numerous irregularly distributed star clouds, stellar associations, star clusters, and gas clouds known as emission nebulae.
As in the case of Sb galaxies, there are several recognizable subtypes among the Sc systems. Sandage has cited six subdivisions: (1) galaxies, such as the Whirlpool Galaxy (M51), that have thin branched arms that wind outward from a tiny nucleus, usually extending out about 180° before branching into multiple segments, (2) systems with multiple arms that start tangent to a bright ring centred on the nucleus, (3) those with arms that are poorly defined and that span the entire image of the galaxy, (4) those with a spiral pattern that cannot easily be traced and that are multiple and punctuated with chaotic dust lanes, (5) those with thick, loose arms that are not well defined—e.g., the nearby galaxy M33 (the Triangulum Nebula)—and (6) transition types, which are almost so lacking in order that they could be considered irregular galaxies.
Some classification schemes, such as that of the French-born American astronomer Gerard de Vaucouleurs, give the last of the above-cited subtypes a class of its own, type Sd. It also has been found that some of the variations noted here for Sc galaxies are related to total luminosity. Galaxies of the fifth subtype, in particular, tend to be intrinsically faint, while those of the first subtype are among the most luminous spirals known. This correlation is part of the justification for the luminosity classification discussed below (see Other classification schemes).
SB galaxies
The luminosities, dimensions, spectra, and distributions of the barred spirals tend to be indistinguishable from those of normal spirals. The subclasses of SB systems exist in parallel sequence to those of the latter.
There are SB0 galaxies that feature a large nuclear bulge surrounded by a disklike envelope across which runs a luminous featureless bar. Some SB0 systems have short bars, while others have bars that extend across the entire visible image. Occasionally there is a ringlike feature external to the bar. SBa galaxies have bright, fairly large nuclear bulges and tightly wound, smooth spiral arms that emerge from the ends of the bar or from a circular ring external to the bar. SBb systems have a smooth bar as well as relatively smooth and continuous arms. In some galaxies of this type, the arms start at or near the ends of the bar, with conspicuous dust lanes along the inside of the bar that can be traced right up to the nucleus. Others have arms that start tangent to a ring external to the bar. In SBc galaxies, both the arms and the bar are highly resolved into star clouds and stellar associations. The arms are open in form and can start either at the ends of the bar or tangent to a ring.