Galaxy Types

The early-type (elliptical) galaxy M87.
Credit: AAO
The early-type (S0) galaxy NGC4526.
Credit: DSS
The late-type (spiral) galaxy M100.
Credit: AAO
The late-type (irregular) galaxy NGC1313.
Credit: AAO

For ease of reference, the Hubble classification scheme is often used to subdivide galaxies into two morphological ‘types’:

  1. Early-type galaxies: elliptical and S0 galaxies.
  2. Late-type galaxies: spiral and irregular galaxies.
    Although we now know that the Hubble classification does not form an evolutionary sequence, these names are derived from this initial assumption.

A comprehensive survey of nearby galaxies reveals the following percentages for the different morphologies:

  • 10% elliptical
  • 20% S0 (lenticular)
  • 60% spiral
  • 10% irregular or peculiar

Extrapolating to the Universe as a whole, it is estimated that 70% of all galaxies are late-type galaxies and only 30% are early-type. However, this is in contrast to what is found in galaxy clusters where early-type galaxies dominate – an example of the observed morphology density relation where the dominant galaxy type varies with environment.

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