He developed the Hubble Sequence, which classifies galaxies into four main types:

* Elliptical Galaxies: Smooth, oval-shaped galaxies with little to no structure. They are denoted by the letter "E" followed by a number indicating their ellipticity (e.g., E0, E1, E7).

* Spiral Galaxies: Disk-shaped galaxies with arms that spiral outwards. They are denoted by the letter "S" followed by a lowercase letter indicating the tightness of their spiral arms (e.g., Sa, Sb, Sc).

* Barred Spiral Galaxies: Similar to spiral galaxies, but with a bar-like structure running through the center. They are denoted by the letter "SB" followed by a lowercase letter indicating the tightness of their spiral arms (e.g., SBa, SBb, SBc).

* Irregular Galaxies: Galaxies that don't fit into the other categories, often exhibiting a chaotic or irregular shape. They are denoted by the letter "Irr".

This classification scheme, still widely used today, was groundbreaking for its time and provided a foundation for understanding the diversity and evolution of galaxies.