A little background on galaxies

Galaxies are huge collections of stars, gas, and dust. Most of the galaxies that we can see are very far away (outside our own Milky Way Galaxy) - hundreds of millions, if not billions, of light years). There are many different kinds of galaxies, and no one knows for sure why there is such diversity. Some galaxies have very little gas and look spherical, while others have quite a bit of gas and look like frisbees.

Historically, galaxies have been classified by their shapes. Edwin Hubble (for whom the Hubble Space Telescope is named) developed a classification system. The parameters in this classification system are the ratio of the size of the bulge to the size of the disk (if there is one) and how tightly wound the spiral arms are. As it turns out, these parameters are usually related to other more physical properties of the galaxy, such as color. The classification system is usually represented as the Hubble Tuning Fork Diagram.

Please note that not all astronomers favor this classification system. The SDSS observing speciallists, who watch the sky scroll by as the photometric camera (find this link) takes data, have commented that many, if not most, of the galaxies that they see cannot be put into one of Hubble's categories. Hubble called the galaxies on the left side of this diagram "early" and those on the right side "late" because he believed that galaxies evolved from spherical to spiral. Today we know that this is not the case - in fact, when two spiral galaxies of similar mass collide, the result is usually elliptical.

At the left end of the tuning fork diagram are the elliptical galaxies. They are so called because they look round or football-shaped. The number after the E (E0-E7) indicates how elongated the galaxy is - E0 are perfectly round and E7 are very elongated. They are usually found in large aggregations of galaxies, known as clusters. Because they don't have much gas, they cannot form new stars, so they usually appear reddish (huh?). The individual stars orbit the center of the galaxy randomly, and the galaxy as a whole rotates very slowly, if at all.

To the right of the ellipticals are the S0 galaxies. These look like flying saucers. They are assigned an S instead of an E because they have a clear stellar disk. However, their physical properties are closer to those of ellipticals than proper spirals, and are often grouped with the former