Hubble's Classification of Galaxies
As astronomers used a special kind of stars - cepheid variables - to calculate the distance to distant "spiral nebulae", they realized in the beginning of the 20th century, that the Milky Way isn't the only galaxy in the universe, but that there are immeasurable quantities of other galaxies in various shapes.
Now astronomers needed a way to classify and organize them into groups. The man behind this classification system was Edwin Hubble. Some people named Hubble's system the Tuning Fork after its graphical resemblance to a real one.
In 1936 the system was fully developed.
Hubble's work would lay the foundation for a new subset of astronomy: Cosmology - the science which studies the galaxies, and the universe in its entirety.
Hubble's "Tuning Fork"
Hubble found three main groups of galaxies that had distinct shapes: elliptic galaxies, lenticular galaxies and another group called spiral galaxies, after their resemblance to spirals. The spirals are divided into two subgroups, depending on how their core is shaped. Some spiral galaxies have a normal, round core, but spiral galaxies of the second type have a core that is barred in shape. These galaxies are called barred spiral galaxies. The Milky Way is thought to be one of those.
Furthermore, the rest of the galaxies were translated into a fourth group. Hubble called them irregulars.
Elliptic galaxies: These galaxies are labeled from E0 to E7, depending on how elliptic they appear to be. E0 type galaxies are located at the far left, while E7, the most elliptic, are at the right on the "handle". The ellipticals have a somewhat consistent density throughout the galaxy.
Messier 87 is an example of an elliptical galaxy.
Elliptical galaxies are the most massive type of galaxies in the universe. Their mass has usually been acquired through merging of smaller galaxies. The stars within do not orbit in a plane like in a spiral galaxy: the orbits are random. Ellipticals contain mostly old stars, with little to no dust in the galaxy.
Lenticular galaxies: At the right end of the "handle" of the tuning fork are S0 and SB0 galaxies. Their shape resembles the spiral galaxies in the sense that they have a core which is surrounded by a disc of stars.
Because they have used up most of the gas in the disc, there is very little stellar formation going on. Most of the gas is located near the core, but the galaxy may have fractions of spiral arms.
Spiral galaxies: The lenticular galaxies break up the diagram into two pieces, with regular spiral galaxies on one side - Sa, Sb and Sc classes and the barred galaxies on the other side of the tuning fork - SBa, SBb and SBc classes. The letter a, b or c indicates how tightly wound the spiral arms are AND how large the galactic bulge is, where class c has the smallest bulge, and the arms are the least coiled. Sometimes Hubble's classification system is extended by adding a d type, which has even more loosely wound spiral arms and a smaller core.
The Milky Way galaxy is thought to be a barred, SBb galaxy. Here is an illustration of what the Milky Way's barred core may like.
Irregular galaxies: Irregular galaxies are, as the name indicates, irregular in shape. Most likely, this shape is the result of some kind of galactic interaction, where the irregular galaxy has either passed close to another galaxy, or may be on the verge of a collision with another galaxy. A beautiful irregular galaxy is the Antennae galaxy.
Next: Formation of Galaxies.
Illustration 1: This is an illustration of Hubble's classification of galaxies. The shape of the diagram somewhat resembles a tuning fork, hence it is sometimes called "the tuning fork".
This illustration is available upon request, as a print (4000x3000 pixels, 300 dpi), and as a PSD-document so that it can be customized according to your own desire.