Relative sizes of galaxies
Galaxies are mostly empty space. Compared to their sizes, stars are VERY far apart.
For example, the nearest star to our Sun is 4.2 ly away. That's about 30 million times larger than the radius of Sun itself.
To make this more understandable, suppose that you build a scale model of Sun and its nearest companion, Proxima Centauri. If Sun is a tennis ball in High Point, then Proxima Centauri would be about 1140 miles away. That's about the distance from High Point to Dallas-Ft. Worth, Texas. A tennis ball in High Point and another tennis ball in Dallas, TX. That's what the distance between stars is like.
As another example, consider the image below. There is a single red dot inside the black circle. It is probably very hard to see.
But suppose that this red dot has a diameter of one pixel. The image resolution is 72 pixels per inch. Proxima Centauri would be 29 million pixels, or 400,000 inches away. That's 6.3 miles. Thus, showing Sun and Proxima Centauri on the same computer screen, with each star being 1 pixel, would require a computer screen 6.3 miles wide.
Stars in galaxies are therefore very spread apart. On the other hand, how far apart are galaxies?
Let's consider the Milky Way and Andromeda galaxies. The diameter of Milky Way is about 100,000 light years across the disk. Andromeda is about 2.5 million light years away. Thus, its distance is only about 25 times the size of our galaxy. That's relatively close!
In the picture below, the distance between the circles is approximately 25 times their diameter. So, it gives you a rough idea of the proximity of the Milky Way and Andromeda galaxies.
Therefore, compared to their sizes, galaxies are relatively close.
Types of Galaxies
Science can be thought of as pattern matching. It's only natural, I think, for astronomers to group and name similar types of galaxies.
Edwin Hubble, who figured out what galaxies really were, also developed a classification scheme for galaxies. It's called the tuning fork. You can read great information about this classification scheme at the Space Telescope Science Institute web site.
It's sufficient for you to remember that there are basically three general categories: (1) elliptical; (2) spiral; and (3) those that aren't elliptical or spiral. This last category includes lenticular, dwarf, and irregular galaxies that have resulted from colliding galaxies, for example. Our companion galaxies, the Large Magellanic Cloud and Small Magellanic Cloud, are irregular galaxies.
You should know these general types of galaxies, and you should be able to identify the galaxy type by viewing a picture.
The picture below is of a cluster of galaxies, the most massive of which is a giant elliptical galaxy. Ellipticals have little gas and dust and is composed mainly of older stars. They are also very dense in terms of the number of stars throughout the galaxy. You an read more about this elliptical galaxy at the Hubble Site.
Here's a beautiful picture of a spiral galaxy, M101. (Hubble Site link)
When looking at a side view, such as the Sombrero Galaxy (Hubble site link), you can really identify the major parts of a spiral galaxy: the disk, the hucleus, the nuclear bulge, and the galactic halo. The gas in the bulge of this galaxy is amazing.
The following image is an artist's illustration of a side view of a spiral galaxy. This image comes from our textbook.
You should be able to identify the different regions of a spiral galaxy.