Spiral galaxies

Spirals make up about two-thirds of all bright galaxies. They are subdivided into classes Sa, Sb and Sc. The two important features of the classification are (1) the openness or tightness of the winding of the spiral pattern, and (2) the relative importance of the central bulge and the disk of the galaxy. Sa galaxies have the largest bulges and the most tightly wound arms. Sc galaxies have the smallest bulges and the most open arms. We think that the Milky Way is between Sb and Sc. Different types of spirals are shown in Fig. 17.3.

Some spirals have a bright bar running through their center, out to the point where the arms appear to start. These are called barred spirals. Some examples are shown in Fig. 17.4. The barred spirals are also subclassified into SBa, SBb and SBc, according to the same criteria as Sa, Sb and Sc. In general, the spiral pattern in barred spirals is quite well defined.

When the spiral pattern is well defined, we call the galaxy a grand design spiral. They have a continuous pattern running throughout the galaxy. Other spirals have a less organized appearance. These are called flocculent spirals. A comparison is shown in Fig. 17.5.

It has been realized that spirals of the same type can have different luminosities. This point is important in trying to determine the distances

Various types of spiral galaxies. (a) NGC 7217, an Sa galaxy. (b) The Andromeda Galaxy (M3I), type Sb, one of our nearest neighbors. It is at a distance of about 700 kpc, and is more than 20 kpc across. Notice the two companions, including M32.We think that it is very similar to our own Milky Way.

Various types of spiral galaxies. (a) NGC 7217, an Sa galaxy. (b) The Andromeda Galaxy (M3I), type Sb, one of our nearest neighbors. It is at a distance of about 700 kpc, and is more than 20 kpc across. Notice the two companions, including M32.We think that it is very similar to our own Milky Way.

(Continued) (h) M83 (the Southern Pinwheel) in Hydra, is 3 Mpc away, and is type Sc. (i) MI04, (Sab) the Sombrero Galaxy, looks like a Mexican hat viewed almost edge-on.This is an edge-on spiral with a prominent bulge.The dark lane is the collective extinction of dusty molecular clouds in the disk. (j) The edge-on spiral NGC 4565, in Coma Berenices, is type Sb. [(a) Image from the OSU Bright Spiral Galaxy Survey; (b)-(f), (h), (j), (k) NOAO; (g),(i) ESO]

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