Galaxy morphological classification

Galaxy morphological classification sorts galaxies by observable structure, especially the shapes of their disks, bulges, and spiral arms. Systems include spiral, elliptical, and irregular galaxies, with finer schemes describing intermediate forms.

Spiral galaxy: A disk galaxy whose stars and gas form spiral arms around a central bulge. Spiral galaxies are a major class, subdivided by arm tightness and bulge prominence.
Hubble sequence: A scheme that arranges galaxies by morphology into elliptical, lenticular, spiral, and irregular forms. It is the best-known framework for organizing visual galaxy types.
Edwin Hubble: An American astronomer who established the distance scale of galaxies and introduced a morphological classification scheme. Hubble's scheme gave the field its enduring sequence of visual galaxy types.
Galaxy morphology–density relation: The observed tendency for dense environments to contain more elliptical and lenticular galaxies than sparse environments. It uses morphological categories to compare galaxies across environments.
Galaxy kinematics: The motions of stars, gas, and other components within a galaxy. Kinematics can reveal disk or spheroidal structure that appearance alone may obscure.
Elliptical galaxy: A galaxy with a smooth, rounded or elongated appearance and little visible disk structure. Ellipticals form the principal smooth-structure class in the Hubble sequence.
Hubble tuning fork: A diagram of the Hubble sequence that branches from elliptical galaxies into barred and unbarred spirals. Its branching layout shows how spiral arms and bars define subgroups.
The Realm of the Nebulae: Edwin Hubble's 1936 book presenting observations and ideas about galaxies and the expanding universe. The book made Hubble's classification widely known in its original context.
Galaxy merger: An interaction in which two or more galaxies combine through gravity into a remnant system. Mergers can reshape galaxies and produce disturbed morphologies.
Galaxy spectral classification: The categorization of galaxies using features in their emitted spectra. Spectra classify physical populations through light signatures rather than visible shape.

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