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. The Hubble arrangement rates elliptical galaxies on the idea of their ellipticity, starting from E0, being nearly spherical, up to E7, which is very elongated. However, because they are relatively small and dim, we don’t see as many dwarf galaxies from Earth. Furthermore, many elliptical galaxies have slowly varying ellipticity, with the images being more circular in the central regions than in the outer parts. As in the case of Sb galaxies, there are several recognizable subtypes among the Sc systems. There are four main categories of galaxies: elliptical, spiral, barred spiral, and irregular. Dwarf galaxies are the most common type in the universe. In any of these cases, the spiral arms may be set at different pitch angles. Occasionally there is a ringlike feature external to the bar. 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 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: The central bright region at the core of a galaxy is called the “galactic bulge”. Types and Classification of Galaxies. These types of galaxies are further divided into subcategories while at the same time other types of galaxies exist based on their size and other unique features. (As explained above, elliptical galaxies are never flatter than this, so there are no E8, E9, or E10 galaxies.). These intermediate forms bear the designation S0. 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. The most popular types of galaxies, according to their morphology, are elliptical, spiral and irregular. Hubble's system of classification for galaxies. Peculiar Galaxy: A Peculiar galaxy, as its name suggests, is a galaxy of strange shape, size and has an unknown composition. Elliptical galaxies A description of the classes as defined by Sandage is given here, along with observations concerning needed refinements of some of the details. The major axes sometimes do not line up either; their position angles vary in the outer parts. Hubble and Sandage noted further deviations from the standard shape established for Sb galaxies. 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. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies. The normal spirals are designated S and the barred varieties SB. 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. 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. This correlation is part of the justification for the luminosity classification discussed below (see Other classification schemes). The most common type of galaxy found throughout the universe is the spiral galaxy. The most common types of galaxies that exist in the universe are the spiral galaxies. It also has been found that some of the variations noted here for Sc galaxies are related to total luminosity. Photographs of spiral galaxies, illustrating the different types, are shown in Figure 3, along with elliptical galaxies for comparison. The Sombrero Galaxy (M104), which is classified as an Sa/Sb galaxy, in an optical image taken by the Hubble Space Telescope. Only a small percentage of all discovered galaxies are categorized as peculiar galaxies. How We Group Galaxies. 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. Some spiral galaxies also have a bar that runs through the center, which is a transfer conduit for gas, dust, and stars. This is the most familiar type of Sb galaxy and is best exemplified by the giant Andromeda Galaxy. Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by the American astronomer Edwin Hubble in 1926.


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