Hubble Classification System – GCSE Astronomy Edexcel Revision

The Hubble Classification System is an approach developed by American astronomer Edwin Hubble for categorizing galaxies based on their visual appearances.
Hubble further grouped these galaxies into three broad classifications: elliptical, spiral, and irregular galaxies.

Elliptical galaxies are named for their nearly featureless, ellipsoid shapes.
They are given the designation ‘E’, followed by a number representing their level of ellipticity (from 0 for nearly circular, to 7 for highly elliptical).
Stars in elliptical galaxies follow randomly oriented elliptical orbits, leading to the spherical or ellipsoidal shape.
These galaxies tend to consist primarily of older, less luminous stars with little to no gas or active star formation.

Spiral galaxies exhibit a central bulge surrounded by arms spiralling outwards. They are classified as normal spirals (S) or barred spirals (SB).
The spiraling arms of these galaxies are usually bluish, indicating the presence of young stars, interstellar gas, and dust.
They are subdivided based on the tightness of the spiral arms and the size of the central bulge. The subdivisions are a, b, and c, where ‘Sa’ or ‘SBa’ have tightly wound arms and large bulges, and ‘Sc’ or ‘SBc’ have loosely wound arms and small bulges.

Barred spiral galaxies have a bar-shaped structure of stars projecting from the central bulge, extending outward before turning into spiral arms.
The Milky Way, our home galaxy, is an example of a barred spiral galaxy.

Irregular galaxies, as the name implies, do not fit into the categories of elliptical or spiral and lack a regular structure or symmetry.
These galaxies can be of two types: Irregular I galaxies that have some structure and evidence of spiralling, and Irregular II galaxies that lack an organized structure altogether.

Besides the main classifications, Hubble also included lenticular galaxies ‘S0’ and ‘SB0’, which are disc-like but lack distinct spiral arms or a bar.
At the end of the sequence, Hubble included ‘S’ for spiral galaxies and ‘SB’ for barred-spiral galaxies with irregular appearance due to highly distorted arms.

Hubble’s classification system indicates an evolutionary sequence for galaxies from ellipticals to spirals, also known as the ‘Hubble tuning fork’ due to its shape, though this idea is now largely rejected.
Despite its shortcomings, the classification system is widely used for its simplicity and ability to provide a basic framework for studying galaxy morphology.
Understanding the types and structures of galaxies aids astronomers in studying the formation, history, and evolution of galaxies and the Universe as a whole.