Hubble Tuning Fork

After Hubble proved that galaxies outside of our own existed, he introduced a way of classifying them according to their appearance, or morphology. Hubble separated the galaxies into 3 main categories - ellipticals, lenticulars and spirals. Those galaxies which did not fit into any of these 3 classes were then identified as irregulars. Hubble illustrated his classification scheme with his famous ʻtuning fork diagramʼ, shown in the figure below.

Hubble originally arranged the different shapes of galaxy in the form of a tuning fork, because he noticed a gradual change in visual appearance between the different galaxies that he observed.  At one time people thought that the Hubble diagram
might actually be showing an evolutionary sequence with galaxies starting off on the left hand side and then gradually evolving towards the right.  This idea has long since been recognised to be false, but confusingly the terms "early type" and "late type" are still often used when describing types of galaxies.  So, astronomers often refer to an Sa galaxy as an "early type spiral" and to an Sc galaxy as a "late type" spiral.

As we move along the top prong of the tuning fork from Sa to Sc, or along the bottom from SBa to SBc, the following changes generally occur:
   
    1.  the disc to bulge ratio increases
    2.  the openness of the spiral arms increases
    3.  individual stars and pink emission nebulae (HII regions) become easier to pick out, and the overall colour of the galaxy  gets bluer as the spiral arms contain more young bright bluish stars
    4.  the hydrogen gas content of the disc increases