The Hubble Sequence: The Secret Code of Galactic Shapes

Classification of galaxies in the Hubble sequence — Composite image of galaxies photographed with the Hubble Space Telescope and the Sloan Digital Sky Survey. They are arranged in the order of the Hubble sequence classification (E: elliptical galaxies; S0: lenticular galaxies; Sa_b_c_d: spiral galaxies and Pec: irregular galaxies). Credits: HST. SDSS. GEPI.

Types of Galaxies 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 played a crucial role in understanding the evolution of galaxies and the universe.

The Hubble sequence divides galaxies into four main categories:

Elliptical galaxies (E): About 10 to 15% of observed galaxies are elliptical. These galaxies have an elliptical shape and are often poor in gas and dust. They mainly contain old stars and little new star formation.
Lenticular galaxies (S0): About 5 to 10% of galaxies are lenticular. These galaxies have a disk shape with a central bulge, but no well-defined spiral arms. They are intermediate between elliptical and spiral galaxies.
Spiral galaxies (S): About 70 to 80% of galaxies are spiral. This type is a mix between elliptical and spiral galaxies. They have a disk like spirals but do not have well-defined spiral arms. They have a flatter structure and are often poor in gas and dust.
Irregular galaxies (Irr): About 10 to 15% of galaxies are irregular. These galaxies have no defined structure like ellipticals or spirals. Their shape is irregular and they are often the result of galactic mergers or other disruptive dynamic processes.

The Hubble sequence is sometimes interpreted as an evolutionary progression. For example, spiral galaxies can become elliptical after major mergers. However, this is not a strict rule, and galaxy formation depends on complex factors such as 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 (Spindle 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, Whirlpool Galaxy (M51) in the constellation Canes Venatici.
Barred spiral galaxies: NGC 1300 in the constellation Eridanus, NGC 1365 in the constellation Fornax, NGC 1097 in the constellation Fornax.
Irregular galaxies: Large Magellanic Cloud (LMC) in the Local Group, IC 10 in the constellation Cassiopeia, NGC 1427A in the Fornax Cluster.

Comparative Table of Galaxies

Comparison of galaxy types according to the Hubble sequence
Type	General Shape	Main Characteristics	Example
Elliptical E0-E7	Spherical to ellipsoidal	Little gas and dust, population of old stars	M87
Lenticular S0	Disk with central bulge	Transition between elliptical and spiral, little star formation	NGC 2787
Spiral Sa-Sc	Disk with spiral arms	Increasing star formation rate, more or less tight spiral arms	Andromeda (M31)
Barred spiral SBa-SBc	Disk with central bar and spiral arms	Star formation in arms and bar, more concentrated gas distribution	NGC 1300
Irregular Irr	Asymmetric, undefined shape	Rich in gas and dust, intense star formation, often result of collisions or disturbances	NGC 1427A