Does the Hubble sequence show how galaxies evolve?

No. Hubble's terms early-type and late-type are historical labels, not an evolutionary order. Galaxies do not simply move along the tuning fork; the sequence is a morphological classification, not an evolutionary track.

What is the most common type of galaxy?

By number, dwarf galaxies are by far the most common, though they are faint. Among bright, easily catalogued galaxies, spirals and ellipticals dominate, with their proportions depending strongly on environment.

Galaxy Morphology and Classification

Galaxies are organized into a small number of morphological types, spirals, ellipticals, lenticulars, and irregulars, whose shapes correlate with their stellar content, gas, and dynamics.

Definition

Galaxy morphology is the classification of galaxies by their visual structure, sorting them into types such as spirals with disks and arms, smooth ellipticals, intermediate lenticulars, and irregular or dwarf systems, categories that correlate with stellar age, gas content, and kinematics.

Scope

This area covers the systems used to classify galaxies by appearance, principally the Hubble sequence and its refinements; the structural and dynamical distinctions among spiral and disk galaxies, ellipticals and lenticulars, and dwarf and irregular galaxies; and the physical meaning behind the morphological categories.

Sub-topics

Core questions

What classification schemes describe the observed variety of galaxies?
How do the major morphological types differ in structure, kinematics, and gas content?
What physical properties underlie the morphological categories?
How do environment and mass shape a galaxy's morphology?

Key theories

The Hubble sequence: Hubble arranged galaxies along a tuning-fork diagram from ellipticals through lenticulars to barred and unbarred spirals, a scheme that, despite not being an evolutionary track, remains the organizing framework for galaxy types.
Morphology-physics correlations: Morphological type correlates with measurable properties, ellipticals being gas-poor and pressure-supported while spirals are gas-rich and rotation-supported, linking appearance to dynamics and star formation.
Extended classification systems: De Vaucouleurs refined the Hubble scheme into a three-dimensional system capturing bars, rings, and intermediate stages, providing the detailed vocabulary used in galaxy catalogs.

Clinical relevance

Morphological classification is the entry point to extragalactic astronomy, providing the categories through which scaling relations, star-formation histories, and the influence of environment on galaxies are studied across cosmic time.

History

Hubble's 1926 and 1936 classification turned the newly recognized external galaxies into an ordered sequence. De Vaucouleurs and Sandage refined it through mid-century, and large digital imaging surveys, including citizen-science classification efforts, have since extended morphological study to millions of galaxies.

Key figures

Edwin Hubble
Gerard de Vaucouleurs
Allan Sandage

Seminal works

hubble1936
devaucouleurs1959
sparke2007

Frequently asked questions

Does the Hubble sequence show how galaxies evolve?: No. Hubble's terms early-type and late-type are historical labels, not an evolutionary order. Galaxies do not simply move along the tuning fork; the sequence is a morphological classification, not an evolutionary track.
What is the most common type of galaxy?: By number, dwarf galaxies are by far the most common, though they are faint. Among bright, easily catalogued galaxies, spirals and ellipticals dominate, with their proportions depending strongly on environment.