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How are irregular galaxies different from spiral galaxies?
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Galaxy
The Whirlpool Galaxy (left), also known as M51, an Sc galaxy accompanied by a small irregular companion galaxy, NGC 5195 (right).
Irregular galaxies differ from spiral galaxies in several key structural and compositional ways:
- Symmetry and Structure: Spiral galaxies are characterized by circular symmetry, a distinct central nucleus, and a thin outer disk with a superimposed spiral structure. In contrast, most irregular galaxies lack noticeable symmetry or an obvious central nucleus, appearing as grainy, chaotic assemblages of luminous areas.12
- Stellar Populations: Spiral galaxies contain a mix of populations, with Population I stars concentrated in the disk and Population II stars found in the nucleus and halo. Irregular galaxies are dominated by a thick disk of Population I stars, with very few Population II stars; in some cases, the spheroidal component of old stars found in spirals is entirely absent in irregulars.314
- Color: While the arms and disks of spiral galaxies are blue, irregular galaxies are generally bluer overall than the spiral components of spiral galaxies.21
- Classification: Spiral galaxies are systematically classified (S or SB) based on nucleus size and arm coiling. Irregular galaxies are less orderly, though some may feature a central bar.12