The Hubble tuning fork classification
Edwin Hubble introduced his galaxy classification scheme in 1926, arranging galaxy shapes into a diagram shaped like a tuning fork. Elliptical galaxies sit on the handle, labelled E0 through E7 by how flattened they appear. The fork then splits into two prongs of spiral galaxies: ordinary spirals (Sa, Sb, Sc) and barred spirals (SBa, SBb, SBc).
At the junction sit lenticular galaxies, type S0, which have a central bulge and disk like spirals but lack distinct spiral arms. Irregular galaxies, with no organized structure, fall outside the fork.
What the sequence does and does not mean
Hubble called ellipticals early-type and spirals late-type, and although those labels persist, they do not describe an evolutionary path; galaxies do not simply progress along the fork from one shape to another. Ellipticals tend to host older stars and little ongoing star formation, while spirals contain gas, dust, and active star-forming regions in their arms.
The scheme remains the foundational morphological taxonomy in extragalactic astronomy, even though modern surveys add finer subtypes and recognize that mergers and environment strongly shape a galaxy's form.