Types of GalaxiesAuthor: Pyaraka Vijay
PRINCIPAL SCHEMES OF CLASSIFICATION
Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by 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. His basic definitions are as follows:
Galaxies of this class have smoothly varying brightnesses, with the degree of brightness steadily decreasing outward from the centre. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are nearly all of the same colour: they are somewhat redder than the Sun.
These galaxies are conspicuous for their spiral-shaped arms, which emanate from or near the nucleus and gradually wind outward to the edge. There are usually two opposing arms arranged symmetrically around the centre. 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. The arms are embedded in a thin disk of stars. Both the arms and the disk of a spiral system are blue in colour, whereas its central areas are red like an elliptical galaxy.
Most representatives of this class consist of grainy, highly irregular assemblages of luminous areas. They have no noticeable symmetry nor obvious central nucleus, and they are generally bluer in colour than are the arms and disks of spiral galaxies. An extremely small number of them, however, are red and have a smooth, though nonsymmetrical, shape.
Hubble subdivided these three classes into finer groups according to subtle differences in shape, as described in detail below. Other classification schemes similar to Hubble's follow this pattern but subdivide the galaxies differently. A notable example of one such system is that of Gerard de Vaucouleurs. This scheme, which has evolved considerably since its inception in 1959, includes a large number of codes for indicating different kinds of morphological characteristics visible in the images of galaxies. The major Hubble galaxy classes form the framework of de Vaucouleurs's scheme, and its subdivision includes different families, varieties, and stages, as shown in Table 1.
Examples of the de Vaucouleurs classification scheme are for galaxy M33, the Triangulum Nebula, which is classified as SA(s)cd, and the nearby small galaxy NGC 6822, classified as IB(s)m.
An entirely different kind of classification scheme is the luminosity classification developed in 1960 by Sidney van den Bergh. Based on morphological considerations, luminosity classes are assigned to individual galaxies within the Hubble classes. Those that are the most luminous are given a luminosity class of I, and the intrinsically faintest members of a class are assigned a V or VI, recalling the general approach of the luminosity class scheme used for stellar spectra . Thus a very luminous galaxy with open, resolved arms would be an Sc I galaxy, while a somewhat intrinsically fainter object with the same basic structure would be an Sc II or Sc III galaxy. To assign a luminosity class, a galaxy's image has to be compared with a set of standard images of galaxies for which distances are known and for which luminosity classes have been established by van den Bergh.
Classification schemes based on criteria other than optical appearance have been proposed. There is, for example, the Morgan scheme (proposed by W.W. Morgan), which combines information on the spectrum of a galaxy with its general shape. Here, a class is coded with a letter that indicates the spectral type of the galaxy in the blue (either as measured or as determined from the galaxy's bulge morphology, which correlates with the spectral type): e.g., a, af, f, fg, g, gk, k, for increasing dominance by cooler stars. The code then includes a capital letter to indicate general morphology--e.g., E, S, or I--in accordance with Hubble's general classes. This is followed by a number that indicates the overall optical shape of the image, with 0 representing a circular image and a 10 (never actually realized) standing for a linear, infinitely thin image. An example is the galaxy M31, the Andromeda Nebula, which is classified as kS5 in the Morgan system.
Systems that separate galaxies according to the character of their radio structure and the strength of their radio emissions also have been devised. For example, radio galaxies can be classified according to the following scheme:
g: galaxies with normal radio fluxes.
R: galaxies with strong radio emission. Many have distorted morphology, with evidence of explosive events or interactions with companions.
cD: galaxies with abnormally large, distended shapes, always found in the central areas of galaxy clusters and hypothesized to consist of merged galaxies.
S: Seyfert galaxies, originally recognized by the American astronomer Carl K. Seyfert from optical spectra. These objects have very bright nuclei with strong emission lines of hydrogen and other common elements, showing velocities of hundreds or thousands of kilometres per second. Most are radio sources.
N: galaxies with small, very bright nuclei and strong radio emission, probably similar to Seyfert galaxies but more distant.
Q: quasars, small, extremely luminous objects, many of which are strong radio sources. Quasars apparently are related to Seyfert and N galaxies but have such bright nuclei that the underlying galaxy can be detected only with great difficulty.
Although such schemes are sometimes used for special purposes, including, for example, certain kinds of statistical studies, the general scheme of Hubble in its updated form is the one most commonly used and so will be described in detail in the following section.