1. BHS Astronomy: Galaxies – Chapter 17 May 2016

2. A Galaxy is an immense swarm of hundreds of millions to hundreds of billions of stars and vast clouds of interstellar gas. Each star moves along its own orbit, held within the galaxy by the combined gravitational force of the rest of the matter within the galaxy. Each galaxy (apart from those undergoing collisions) is therefore an independent and isolated star system.

3. The study of other galaxies began in the eighteenth century, when the French as­tronomer Charles Messier (pronounced mess-yay) accidentally discovered many gal­axies during his searches for new comets. He noticed a large number of faint, diffuse patches of light, and to avoid confusing them with comets, he assigned them numbers and made a catalog of their positions. Although many of Messier's objects have since been identified as star clusters or glowing gas clouds in the Milky Way, several dozen are galaxies. These and the other objects in Messier's catalog are still known by their Messier, or M number, such as M31, the Andromeda galaxy.

4. Even early nineteenth-century observers noticed that not all galaxies look the same. However, it was Edwin Hubble, an American astronomer working first as a graduate student at the University of Chicago and later at Mount Wilson in California in the 1920s, who demonstrated that galaxies could conveniently be divided on the basis of their shape into three main types: spirals, ellipticals, and irregulars.

5. Most people think of Spiral Galaxies as “what a galaxy looks like”…

6. Perhaps because they have seen satellite images of hurricanes
Perhaps because they have seen satellite images of hurricanes. Nature likes Spirals…

7. The second type shows no signs of spiral structure
The second type shows no signs of spiral structure. These galaxies have a smooth and featureless appearance and a generally elliptical shape, as can be seen in figure Accordingly, astronomers call them elliptical galaxies. These galaxies have no disk or arms—the stars are distributed around the center in all three dimensions, some nearly spherical and others more squashed or football-shaped. They are NOT solid objects!

8. Galaxies of the third major type show neither arms nor a smooth uniform appear­ance. In fact, they generally have stars and gas clouds scattered in random patches. For this reason, they are called irregular galaxies (fig. 17.5).

9. Spiral, elliptical, and irregular galaxies are sometimes denoted by S, E, and Irr, re­spectively. However, in addition to these three main types, astronomers recognize two additional galaxy types closely related to spiral systems. The first of these are barred spiral galaxies, which have arms that emerge from the ends of an elongated central region, or bar, rather than from the core of the galaxy, as shown in figure It is this bar that gives them their name, and they are denoted SB galaxies to distinguish them from normal spirals (they chose not to use “BS”…)

10. The second of these that Hubble identified is a kind of "spiral without a spiral:' These are galaxies with disks with no evidence of arms (fig. 17.7), which he called SO (pronounced "S"-zero) galaxies.
FIGURE 17.7
Two SO galaxies, NGC 3384 and NGC These galaxies look similar to el­lipticals, but some of the stars are distrib­uted in a disk, as occurs in spiral galaxies.

11. Hubble refined his classification system one step further, noticing that both spiral and elliptical galaxies can be subdivided into additional classes that, when properly ordered, show a smooth transition from spherical E type of galaxies to flatter E galax­ies, to spirals, and to irregulars. This sequence of types was illustrated by Hubble in the "tuning fork" diagram, illustrated in figure 17.8

12. Other ways…