The Hubble Sequence: A Key Tool for Galaxy Classification

Image description: A composition of galaxy photographs taken with the Hubble Space Telescope and the Sloan Digital Sky Survey. The galaxies are arranged according to the Hubble sequence classification (E: elliptical galaxies; S0: lenticular galaxies; Sa_b_c_d: spiral galaxies, and Pec: irregular galaxies). Credits: HST, SDSS, GEPI.

Image description: Spiral galaxies are further subdivided based on the structure of their arms. Sa galaxies have loosely defined spiral arms and a larger central bulge, while Sc galaxies have more distinct arms and a smaller bulge. Elliptical galaxies are classified from "E0" to "E7" depending on their degree of flattening. "E0" represents a nearly spherical shape, while "E7" is significantly flattened.

Galaxy Types in the Hubble Sequence

The Hubble sequence is a galaxy classification system proposed by astronomer Edwin Hubble in 1926. It organizes galaxies based on their shape and morphological characteristics. This classification has been pivotal in understanding galaxy evolution and the universe.

The Hubble sequence divides galaxies into four main categories:

• Elliptical galaxies (E): About 10–15% of observed galaxies are elliptical. These galaxies have an elliptical shape and are often poor in gas and dust. They primarily contain old stars with little new star formation.
• Lenticular galaxies (S0): About 5–10% of galaxies are lenticular. These galaxies have a spiral-like shape, with a central bulge surrounded by spiral-like arms. Spirals are further classified based on the prominence of their arms, ranging from "S0" (less prominent spirals) to "Sb" (more defined spirals).
• Spiral galaxies (S): About 70–80% of galaxies are spiral. These galaxies are a blend between elliptical and spiral galaxies. They have a disk structure like spirals but lack well-defined spiral arms. They are often flattened and poor in gas and dust.
• Irregular galaxies (Irr): About 10–15% of galaxies are irregular. These galaxies lack any defined structure like ellipticals or spirals. Their irregular shapes often result from galactic mergers or other dynamic processes.

The Hubble sequence is sometimes interpreted as an evolutionary progression. For instance, spiral galaxies can become elliptical after major mergers. However, this is not a strict rule, as galaxy formation depends on complex factors like gravitational interactions, local environments, and gas reserves.

Examples

• Elliptical galaxies: M87 (NGC 4486), NGC 4636 in the Virgo Cluster, NGC 4881 in the Coma Cluster.
• Lenticular galaxies: NGC 5866 (Needle Galaxy) in the constellation Draco, Centaurus A (NGC 5128) in the constellation Centaurus, NGC 2787 in the constellation Ursa Major.
• Spiral galaxies: The Milky Way, Andromeda (M31) in the Local Group, constellation Andromeda, M51 (Whirlpool Galaxy) in the constellation Canes Venatici.
• Irregular galaxies: Large Magellanic Cloud (LMC) in the Local Group, IC 10 in the constellation Cassiopeia, NGC 1427A in the Fornax Cluster.