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.