What is the name of the galaxy classification system created by Edwin Hubble?

The galaxy classification system created by Edwin Hubble is called the Hubble sequence. It organizes galaxies by their visible shapes and is often illustrated as a tuning-fork diagram.

The sequence begins with elliptical galaxies, labeled E0 through E7 according to how elongated they appear. It then branches into spiral galaxies, including ordinary spirals labeled Sa through Sc and barred spirals labeled SBa through SBc. Lenticular galaxies, labeled S0, occupy an intermediate position between ellipticals and spirals because they have disks but lack prominent spiral arms.

The classification was developed from optical appearance, not from a complete theory of how galaxies evolve. The tuning-fork illustration can therefore be misleading: galaxies do not necessarily progress from ellipticals through spirals in a simple evolutionary line. Modern astronomy recognizes that mergers, gas content, star formation, bars, and environment all influence galaxy structure.

Another common mix-up is confusing the Hubble sequence with the Hertzsprung–Russell diagram. The latter classifies stars by luminosity and temperature, while the Hubble sequence classifies galaxies by morphology. The Tully–Fisher relation, by contrast, is a distance-measurement relationship involving spiral-galaxy rotation speeds and luminosities.