Type Sc spirals, with the smallest bulges, often have loose, poorly defined spiral structure. Type Sb galaxies, with smaller bulges, typically have more open spiral arms. Type Sa spiral galaxies, with large central bulges, tend to have tightly wrapped, almost circular, spiral arms. The size of the bulge is quite well correlated with the tightness of the spiral pattern (although the correspondence is not perfect). In Hubble's scheme, a spiral galaxy is denoted by the letter S and classified as type "a," "b," or "c" according to the size of its central bulgeType Sa galaxies having the largest bulges, Type Sc the smallest. As we progress from type Sa to Sb to Sc, the bulges become smaller while the spiral arms tend to become less tightly wound. However, within this general description, spiral galaxies exhibit a wide variety of shapes, as illustrated in Figure 24.2.įigure 24.2 Variation in shape among spiral galaxies. 23.3) The stellar density (that is, the number of stars per unit volume) is greatest in the galactic nucleus, at the center of the bulge. All galaxies of this type contain a flattened galactic disk in which spiral arms are found, a central galactic bulge, and an extended halo of faint, old stars. We saw several examples of spiral galaxies in Chapter 23for example, our own Milky Way Galaxy and our neighbor Andromeda. Many modifications and refinements have been incorporated over the years, but the basic Hubble classification scheme is still widely used today. Working with the then recently completed 2.5-m optical telescope on Mount Wilson in California in 1924, he classified the galaxies he saw into four basic types spirals, barred spirals, ellipticals, and irregularssolely on the basis of appearance. The American astronomer Edwin Hubble was the first to categorize galaxies in a comprehensive way. Even when we take into account their different orientations in space, galaxies do not all look the same. Others, however, are definitely not spiralsno disks or spiral arms can be seen. Although it is difficult to tell from the photograph, some of the blobs of light in Figure 24.1 are actually spiral galaxies like the Milky Way Galaxy and Andromeda. They have fuzzy edges, and many are quite elongatednot at all like the sharp, pointlike images normally associated with stars. Seen through even a small telescope, images of galaxies look distinctly nonstellar. (The blue spiked object at top right is a nearby star virtually every other object visible is a galaxy.) (b) A recent Hubble image of part of the cluster. Called the Coma Cluster, this group of galaxies lies more than 100 million pc from Earth. We begin our study of these enormous accumulations of matter simply by considering their appearance on the sky.įigure 24.1 (a) A collection of many galaxies, each consisting of hundreds of billions of stars. ![]() Over the years, astronomers have accumulated similar images of many millions of galaxies. Almost every patch or point of light in this figure is a separate galaxyseveral hundred can be seen in just this one photograph. ![]() Figure 24.1 shows a vast expanse of space lying about 100 million pc from Earth.
0 Comments
Leave a Reply. |