Galaxy classification
Galaxies come in a range of different shapes and sizes. There is a system by which we can describe what a galaxy looks like. It's called the Hubble "tuning fork" diagram and it classifies galaxies into characteristic shapes.
On the left of the diagram are the Elliptical galaxies, named by E0 to E7. The E stands for elliptical, obviously, and the number indicates how egg-shaped the ellipse is - 0 means a ball shape, and 7 looks rather like a throwing discus.
 |
 |
| An E0 elliptical galaxy | An E7 elliptical galaxy |
After these simple types of galaxies the diagram splits into two. On the upper branch are the S0 galaxies first - called lenticular galaxies because they are shaped like a lens in a magnfiying glass. The description is made up of the "S", meaning lenticular, the "0", meaning no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc.
 |
 |
| A lenticular galaxy with no absorbing dust - an S01 |
A lenticular galaxy with a lot of absorbing dust - an S03 |
Continuing along that branch the next 3 types are all have spiral arms, and they are grouped by how tightly those arms are wound and how large the central bulge is - the two happen to be closely related. The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are: from "a" to "d".
 |
 |
| An Sa galaxy - note the tightly-wound, smooth arms, and the central bright disc |
An Sb galaxy - better defined spiral arms |
 |
 |
| An Sc galaxy - much more loosely wound spiral arms, both of which are clearly defined | An Sd galaxy - very loose arms, with much of the luminosity in the arms, not the disc |
The lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants all have a line of stars through the centre - a bar. The first two pictures shown on the diagram are the SB0 galaxies. The B stands for barred, and a subscript number indicates how heavily defined the bar is.
Last, but not least (our own galaxy fits in here), are the barred spirals. These are named SBx, where x is the lower case letter as in the unbarred cases above.
 |
 |
 |
| An SBa galaxy - NGC 1291. Note the bright centre and tight spirals | An SBb galaxy - NGC 1300. Better defined arms which are more loosely wound | An SBc galaxy - NGC 7741. Even looser arms, and a much dimmer central portion of the galaxy |
To learn how these different types of galaxies formed, go to the Galactic formation page.