Blog #20: The Hubble Tuning Fork

In 1926, Edwin Hubble, for whom the Hubble Space Telescope is named after, saw that the different galaxies seen in the night sky had different shapes and structures. Therefore, he created a classification system for galaxies that split these galaxies into 3 categories, namely: (1) Elliptical galaxies, (2) Spiral galaxies, and (3) Irregular galaxies. Each of these three classifications had further sub-divisions, which ended up creating a system of galaxy classification known as the Hubble Tuning Fork. The Hubble Tuning Fork shows how the different categories of galaxies and their respective subdivisions are all related to each other in terms of shape and structure. In order to understand the Tuning Fork, let's first discuss the subdivisions of the Elliptical, Spiral, and Irregular galaxies.

Elliptical Galaxies: 

Elliptical galaxies get their name for their uniform, elliptical shape. The elliptical galaxies are classified by the eccentricity of the ellipse, where E0 is an elliptical galaxy with no eccentricity, and is therefore perfectly round, to E7, where the eccentricity is very high. The "E" in the E0 - E7 range stands for "elliptical", specifying only elliptical galaxies. Look at the diagram below to get a sense of the categories of Elliptical galaxies:









Spiral Galaxies:

Spiral galaxies get their name for their characteristic bands, or "arms" of gas and dust that swirl around the center of the galaxy. Spiral galaxies come in two subcategories: (1) without a central bar $S$ (top photo), (2) with a central bar $SB$ (bottom photo).

These two categories are further divided by the tightness of their spiral arms, where spiral galaxies with very tight spirals are in category "a",  galaxies with medium spirals in category "b", and galaxies with loose spirals are in category "c". See below for the differences between these galaxies:

There is also another category of spiral galaxies, where there are no spirals at all! This category, S0, is characterized by the fact that there is a bulge, and a distinct disk, unlike elliptical galaxies, but the disk doesn't have any arms. An example of a S0 galaxy is the Sombrero Galaxy:

Irregular Galaxies: 

Irregular galaxies are those that do not belong to either spiral or elliptical galaxies, and can take on a range of shapes, from shapeless blobs, to distorted spirals.


Hubble's Tuning Fork: 

All these different types of galaxies can be rearranged to show how their shapes relate to each other in the form of a tuning fork. Here's a tuning fork I made: