|Thanks to the University of Texas, McDonald Observatory for this diagram|
The tuning fork diagram was first drawn by Edwin Hubble as a way of describing galaxies by how they look. Elliptical galaxies make up the branch on the left while the spirals and the barred spirals make up the "prongs" of the fork. The most round ellipticals are on the left and they get flatter towards the right.
From left to right, the spirals and barred spirals become less tightly wound with smaller middles. All of the galaxies also have more dust and gas in them as you go from left to right in the diagram.
There is a S0 galaxy at the junction of the three arms of the diagram. S0 galaxies have discs like spiral galaxies but don't have spiral arms or the same amount of dust and gas as spirals. They look a bit like flat ellipticals. This means that they are often mistaken for elliptical galaxies, and some galaxies are given two classifications - one as an elliptical and one as an S0 galaxy because astronomers aren't really sure which type it is.
It was originally thought that galaxies change from ellipticals to spirals along the tuning fork diagram but astronomers now know this isn't true. Instead, galaxies stay roughly the same shape as when they were born.
Author: Nigel Bannister
Updated by: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001