How Do We Characterize Galaxies?

Here is the basic form of an isolated galaxy

We can observe the stars in the galaxy bulge and disk, finding both individual stars, stars in binary or tertiary solar systems, and stars grouped tightly together in clusters. We cannot observe dark matter directly with telescopes, because it does not shine like stars in the visual or produce radio emissions like hydrogen gas. Why then do we think it is present in the halo? We see the effect of its gravitational attraction upon the luminous matter, and thus infer its presence. In fact, the dark matter outweighs the luminous matter by a large fraction, much like the ratio of birthday cake to cake frosting!

Diagram of a galaxy. At the center a red sphere is labeled bulge; a horizontal thin light blue disk intersects the center of the bulge; two dark blue curved arms within the disk adorn the bulge like wings on a golden snitch, both curved in the same direction (if you placed them on top of each other, they would line up); a larger sphere filled with red stars and tiny black stars contains the bulge and disk and is labeled halo.
Galaxy structure: central bulge, thin disk, and diffuse outer halo
[NMSU, N. Vogt]

When people first began to observe significant numbers of galaxies, they categorized them according to their optical morphology (appearance) on the sky. Edwin Hubble built up a chart of different types of galaxies. We call this the galaxy tuning fork because it resembles a musical tuning fork in form (a handle on the left, with two tines projecting to the right). Elliptical, round galaxies were classified according to how round or oval they appeared. Spiral galaxies were divided into two families, those with and those without a central barlike structure. The progression through the spiral families indicated the degree to which the spiral arms were wound tightly, and how large they appeared relative to the central bulge. A final, catch-all class was established for irregular galaxies, containing all of the galaxies which didn't fit into nice, neat bins.

Edwin Hubble's Classification Scheme. Visualize a huge letter Y set on its side, formed from a horizontal line with a curved letter C attached. The line lies to the left and is labeled Ellipticals, running from subtype E0 through E7. Small attached images of elliptical galaxies show that the E0s are spherical and the galaxies appear more and more elongated (ovoid, like baguettes) as we advance through the sequence to E7. The point where the line and the C curve intersect is labeled S0, and the attached lenticular galaxy image shows a central bulge and a smooth disk without spiral arm features. The top half of the letter C shows the spiral sequence from Sa through Sc. The Sa galaxy has a large bulge and tightly wound spiral arms in the disk; as we progress down the sequence the bulge components become smaller and the arms become more open and diffuse. The bottom half of the letter C show the spiral sequence from SBa through SBc. It parallels the Sa through Sc sequence, except that now the spiral arms do not arise from the center of the galaxy but instead begin at the ends of a prominent bar shape which intersects the bulge and extends out into the disk. Note that there are also Sd and SBd galaxy types, though they are not shown.
Galaxy tuning fork, showing the progression through elliptical, spiral, and irregular types.

We will be looking at pictures of individual galaxies. But it is worth remembering that galaxies are found in all sorts of environments, from the splendid isolation of the empty field to the hearts of gigantic clusters (composed of thousands of galaxies tightly clustered together). This image shows one such gigantic cluster. If you look closely at the image, how many galaxies can you see?

Telescope image showing thousands of galaxies gravitationally bound together into the Coma cluster. There are large elliptical galaxies and smaller spiral galaxies spinning wildly to form a shape not unlike a swarm of busy bees.
The Coma galaxy cluster in optical and infrared wavelengths (made up of thousands of galaxies!)

Recall that light takes a finite time to travel from place to place, so for galaxies which lie far away from Earth, an appreciable amount of time (up to billions of years) may pass before the light emitted reaches us on Earth.