Types of Galaxies Help (page 2)
Types Of Galaxies
When telescopes became powerful enough to resolve nebulae into definite shapes, one type of nebula presented a conundrum. Many of the spiral nebulae , which looked like whirlpools of glowing gas, had spectral lines whose wavelengths were much longer than they ought to be. This phenomenon, called red shift , suggests that an object is receding. Red shifts were seen commonly for spiral nebulae, but blue shifts—foreshortening of the waves when an object is approaching—were almost never seen, and when they were observed, they were minimal. Some astronomers thought that the spiral nebulae actually were huge congregations of stars at immense distances and that our own Milky Way was just one such congregation. Until individual stars could be resolved within the spiral nebulae, however, this idea remained an unproved hypothesis.
Today we know that the spiral nebulae do consist of stars, and we call them spiral galaxies . Spirals are not the only type of galaxy. Other objects, previously thought to be emission nebulae or globular star clusters within our Milky Way, turned out to be irregular galaxies or elliptical galaxies . Some of these are billions (units of 1 billion or 10 9 ) of light-years away from us. Many contain hundreds of billions of individual stars.
Some of the brightest galaxies, containing the greatest numbers of stars, are ellipsoidal or spherical in shape. These galaxies are classified according to their eccentricity, the extent to which they are elongated from a perfectly spherical shape. (Actually, the sphere or ellipsoid represents the median boundary, the two-dimensional region such that half the stars are inside and half are outside.) Eccentricity zero (E0) represents a perfect sphere; E1 and E2 are egg-shaped. The E3 and E4 elliptical galaxies resemble elongated eggs. When we get to E5, the median boundary is football-shaped. Elliptical galaxies of E6 and E7 classification are even more elongated. Figure 15-1 shows approximate representations for E0 through E7 elliptical galaxies. This scheme was devised by astronomer Edwin Hubble in the 1930s.
Elliptical galaxies contain comparatively little gas and dust. It is believed that this is so because most of the interstellar material has developed into stars. This suggests that elliptical galaxies are old. There are numerous red giants in these galaxies, and this is consistent with the theory that they are old. Some of these must be supergiants, with diameters hundreds of times that of our Sun, and thousands of times brighter. Despite the vast distances separating other galaxies from ours, some of the stars in elliptical galaxies resolve into points of light in large telescopes. Were it not for the red shift, these giant galaxies could be mistaken for globular star clusters within our own galaxy.
Irregular galaxies have no defined shape or apparent structure. Our Milky Way has two small irregular galaxies near it. These are the Magellanic Clouds , named after the famous explorer who sailed around the world. They can be seen with the unaided eye, but only from the Earth’s southern hemisphere. They look like faintly glowing clouds on a moonless night. With a good telescope, it is easy to tell that they are made up of stars.
Some irregular galaxies show signs of coordinated motions among their stars, such as slow rotation around a central axis. In some cases, it is difficult to tell whether such apparent organization is real or an artifact of the expectation phenomenon : Sometimes we think we see something only because we expect to see it. In other instances, there is clear evidence of rotation. If the rotation is significant enough, the galaxy can be classified as a spiral.
The most stunning galaxies, from the standpoint of the visual observer, are the spirals. Their variety is almost infinite. Some spirals appear broadside to us, some appear at a slant, and still others present themselves edgewise. The spiral arms can have many different shapes.
There are two major types of spiral galaxies: the normal spiral and the barred spiral . There are subclassifications within these two major categories. These are rather subjective and must be judged based on what we see. It is easy to classify spirals when they present themselves nearly broadside to us but difficult when they present themselves edgewise or nearly edgewise. Normal spirals are classified S0, Sa, Sb, and Sc (Fig. 15-2) depending on how tightly their bands of stars are wound around the nucleus. The barred spirals are classified as S0, SBa, SBb, and SBc (Fig. 15-3). The S0 galaxies are shaped like oblate (flattened) spheres, or like donuts with golf balls stuck in their centers. Both the normal spirals and the barred spirals branch off from this common root type. These classifications, like the classifications for the elliptical galaxies, were devised by Hubble.
In a normal spiral galaxy, the arms extend from the bright nucleus and are coiled in a more or less uniform fashion throughout the disk. Some spirals have prominent arms, whereas others have almost invisible arms. In a barred spiral, the central region is rod-shaped. The spiral arms trail off from the ends of the rod, in some cases prominently and in other cases almost invisibly. The barred spirals are especially interesting because the rod-shaped region appears to rotate with constant angular speed at all points along its length, as if it were solid. According to one theory, the nuclei of such galaxies are undergoing catastrophic explosions, and the bars are streamers of gas and dust ejected from the core at high speeds.
The appearance of a spiral galaxy gives the illusion that it is a uniformly rotating system. This is basically true, but the motions of the individual stars within a spiral galaxy are varied, and the overall pattern of motion is quite complicated. The fact that these galaxies rotate is verified by spectral examination of different regions when the disk of the spiral presents itself nearly edgewise to us. On one side of the galaxy, the light is shifted toward the blue end of the spectrum compared with the light from the nucleus. On the other side of the galaxy, the light is shifted toward the red end compared with the light from the nucleus. This indicates that the two sides of the galaxy have different radial speeds, and this can be explained only by rotation around the center. These determinations must be made independently of the overall motion of the galaxy with respect to us; in general, most galaxies are moving away.
Practice problems of this concept can be found at: Galaxies and Quasars Practice Problems
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