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Hubble went on to take photographs (this was done on large glass plates) of tens to hundreds of galaxies. Just like Annie J. Cannon who classified stellar spectra, Hubble classified galaxies. This classification scheme is strictly morphological (based on physical appearance) and does NOT imply an evolutionary sequence.

Hubble defined two major classes of galaxies, called Ellipticals and Spirals. Spiral galaxies come in two varieties: Barred and Normal.

Hubble went further to try and show the connectedness of these types by making a flow-chart, sometimes called a tuning fork because of its shape. This does NOT constitute an evolutionary sequence! It's based SOLELY on morphology..

E stands for Elliptical, while S stands for Spiral:



The frequency of galaxy types. We will find when we study CLUSTERS of galaxies, the ratio of Ellipticals to Spirals increases. Do Ellipticals more likely FORM in clusters or does the cluster environment evolve Spirals into Ellipticals? (nature verses nurture). Stay tuned.