Introduction
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 cosmos/universe in its entirety.
Hubble's classification system, "the Tuning Fork"
Hubble found two groups of galaxies that had distinct shapes: elliptic galaxies and another group called spiral galaxies, after their resemblance to spirals. The spirals were divided into two subgroups, depending on how their core was shaped. Some spiral galaxies has 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 couldn't be translated into a fourth group, since they do not have a characteristic shape. 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 of the horizontally laying tuning fork, while E7, the flattest, are at the right. They have a somewhat consistent density throughout the galaxy.
Note that elliptic galaxies are actually fairly round in shape, but they are classified as to how tilted they appear to be on the sky. Messier 87 is an example of an elliptical galaxy.
Spiral galaxies: At the right end of the "handle" of the tuning fork, there are S0 and SB0 galaxies. They 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 coiled the spiral arms are, where class c is the most tightly coiled. There seems to be a relation between how large a galactic core is, and the size of the spiral arms.
The milky way galaxy is thought to be a barred galaxy.This is an example of what a barred core looks like, it is the barred core of the milky way galaxy.
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 are the the Antennae galaxies.
Next: Supermassive black holes
Back to the galaxies chapter: Galaxies
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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".
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