Astrophysics (Index)About

galaxy classification

(Hubble sequence, Hubble classification)
(morphological classification of galaxies)

The commonly-used galaxy classification indicates a galaxy's morphology (galaxy morphology), i.e., shape and overall structure, based upon its visual appearance. Edwin Hubble developed the basic classification still used, the Hubble types, known collectively as the Hubble sequence or Hubble classification (though current usage includes refinements subsequent to Hubble's work, e.g., by Allan Sandage and Gérard de Vaucouleurs):

(The term disk galaxy for a disk-like shape includes the above spiral and lenticular galaxies.) Hubble laid out the types in a Y-like diagram in the shape of a horizontal tuning fork and you do see references to the classification-structure as a tuning fork. Among the improvements over time have been suffixes and some prefixes, to specify more detail, often describing galaxies formerly classified as "irregular":

Parenthesis around suffix letters appear to be a means of distinguishing them, e.g., the parenthesis in "(s)m" makes clear that there isn't some specific meaning to the pair, "sm". Dwarf galaxies have their own terms: dE, dS, and dIrr for dwarf elliptical, spiral, and irregular, and dSph for dwarf spheroidal galaxy. Some example galaxies/types:

Another classification system is the Morgan classification, and one of its classifications, cD, is still commonly seen for central elliptical galaxies. There also exists some classification specific to radio galaxies.

Further reading:

Referenced by pages:
elliptical galaxy
lenticular galaxy (S0)
Large Magellanic Cloud (LMC)
Morgan classification
N galaxy
NGC 1600
Spindle Galaxy (NGC 5866)
NGC 6946
PCA analysis
peculiar galaxy (p)
Small Magellanic Cloud (SMC)
spiral galaxy
3C 295
3C 48
Ursa Major II Dwarf
van den Bergh galaxy classification