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Types of Galaxies Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 18, 2011

Irregular Galaxies

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.

Spiral Galaxies

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.

Galaxies and Quasars Types Of Galaxies Spiral Galaxies

Figure 15-2. Hubble classification of spiral galaxies. Type S0 is shaped like an oblate sphere. Type Sa has tight spiral arms; types Sb and Sc have more open spiral arms.

Galaxies and Quasars Types Of Galaxies Spiral Galaxies

Figure 15-3. Hubble classification of barred spiral galaxies. Type S0 is shaped like an oblate sphere. Type SBa has tight spiral arms and a short bar; types SBb and SBc have more open spiral arms and longer bars.

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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