Galaxy morphological classification
A system for classifying galaxies by visual appearance and structure.
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance, shape, structure, and distribution of light. Several schemes exist, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. Classification and morphology are now largely done using computational methods and physical morphology.
- schemes
- Hubble sequence, de Vaucouleurs system, Yerkes (Morgan) scheme
- hubble_classes
- Elliptical (E), Spiral (S), Lenticular (S0), Irregular
- de_vaucouleurs_axes
- Stage (spiralness), Family (barredness), Variety (ringedness)
- numerical_hubble_stage_T
- −6 to +10
Lore & Background
The Hubble sequence, invented by Edwin Hubble in 1926, divides galaxies into elliptical, spiral, and lenticular classes based on visual appearance on photographic plates. Ellipticals are denoted by 'E' and an integer for ellipticity; spirals by 'S' (or 'SB' for barred); lenticulars by 'S0'. The sequence is often represented as a two-pronged tuning fork, with ellipticals on the left and barred/unbarred spirals forming the prongs. In June 2019, citizen scientists through Galaxy Zoo suggested the usual Hubble classification, especially regarding spiral arms and galactic nuclei, may need reassessment.
Reader's Guide
Galaxy morphological classification remains fundamental to astronomy, providing a framework for understanding galaxy formation and evolution. The Hubble sequence, despite its age, is still the most commonly used system in both professional research and amateur astronomy. However, the de Vaucouleurs system offers a more nuanced classification for spirals, and quantitative parameters like concentration, asymmetry, and the Gini coefficient now allow computational classification. The 2019 Galaxy Zoo findings highlight ongoing debate about the Hubble sequence's adequacy, particularly for spiral galaxies. The use of numerical stages (T) enables quantitative studies, linking morphology to stellar mass distribution. Overall, these classification schemes have evolved from visual inspection to computational methods, but the core Hubble categories remain a cornerstone of extragalactic astronomy.
Did You Know?
- The Hubble sequence is often called the 'Hubble tuning-fork' due to its traditional shape.
- De Vaucouleurs argued that rings and lenses are important structural components of spiral galaxies.
- The Yerkes scheme uses the spectra of stars in the galaxy to classify it.
- The numerical Hubble stage T runs from −6 to +10, with negative numbers for ellipticals and lenticulars.
Frequently Asked Questions
Who is Galaxy morphological classification?
It is a family of systems that sort galaxies into categories based on their visual shape, internal structure, and light distribution. The best-known version is the Hubble sequence, which groups galaxies into elliptical, spiral, lenticular, and irregular types.
What are Galaxy morphological classification's powers/role?
It gives astronomers a shared vocabulary for describing and comparing galaxy shapes across vast surveys. The de Vaucouleurs expansion adds three descriptive axes—spiralness, barredness, and ringedness—to capture structural nuance beyond the original four broad classes.
Why is Galaxy morphological classification important?
It supplies the foundational framework for comparing galaxy populations, tracking evolutionary stages, and linking structure to physical properties like mass and star-formation history. Without a common taxonomy, large surveys and theoretical models would lack a consistent way to group and contrast different galaxies.
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