Galaxies come in many shapes and sizes: when we understood that,it was needed a classification for them.
Edwin Hubble invented a classification of galaxies, grouping them into four classes: spirals, barred spirals, ellipticals and irregulars.
He classified spiral and barred spiral galaxies according to the size of their central bulge and the texture of their arms.
For example, a spiral galaxy is dubbed as Sa, Sb or Sc. A barred one is dubbed SBa, SBb or SBc.
The Hubble classification, often called the tuning fork diagram, is still used today to describe galaxies.
The milky way is classified as a SBc.
Here's a pic showing the diagram. Hope you enjoy it!
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