Normal Galaxies

There are a number of different types of galaxies in the known universe many of which being categorised simply by their appearance as viewed from Earth. It was Edwin Hubble who began the labelling process, mainly because he also was the first to confirm the existence of galaxies outside of our own. For the most part, Hubble is credited with naming and categorising the Normal Galaxies.

A Look at Normal Galaxies

The term “Normal Galaxies” is used mostly in comparison to a second group of galaxy types, which are labelled “Active Galaxies.” The normal galaxies are defined mostly by their shape as viewed from our planet, and share many of the same characteristics.

In some cases the term “viewed from Earth” is very important, as a galaxies shaped like an American football as viewed from Earth would look simply round when viewed from the point- which begs the question of how many “round” galaxies are actually “football shaped”, but we can’t see it? There are four types of normal galaxies: spiral galaxies, barred spiral galaxies, elliptical galaxies, and irregular galaxies.

Spiral Galaxies

The spiral galaxy is sometimes referred to as a “pinwheel” or “whirlpool” galaxy. At the centre of a spiral galaxy is the nucleolus, a ball-shaped grouping of stars. Coming out from this nucleolus are a number of arms that form a spiral shape. Each arm contains stars as well, and in addition to the stars there is a lot of dust and cloudy areas, where active star formation is taking place. These galaxies were subdivided, based on how tight the arms of the spiral were; Sa being the tightest and Sc being the loosest.

Barred Spiral Galaxies

This galaxy type is mostly just like a spiral galaxy. There is a nucleolus and there are arms as well as clouds and dust. The arms don’t emanate directly from the nucleolus, however, but from a bar shape. This bar shape contains the nucleolus at its centre. These galaxies were subdivided in a fashion similar to the spiral galaxies; SBa, SBb and SBc.

Elliptical Galaxies

These galaxy types deviate quite a bit from the prior two. An elliptical galaxy range from an elongated egg shape to mostly spherical, and have little to no dust, which means there is no active star formation going on. These galaxies were subdivided from E0 to E7, with E0 being the most spherical and E7 being the most like a stretched egg. The elliptical galaxies can be the largest (Giant galaxy) to the smallest (Dwarf galaxy) galaxies in the universe.

Irregular Galaxies

This is more or less a catch all category for galaxies that have no discernable shape. They may appear similar to a Spiral, Barred Spiral or Elliptical galaxy in some fashion, or may have no real shape at all. It is thought that the pressure of the galactic tide has caused these galaxies to get “bent out of shape.”

Final Thoughts

There are billions of galaxies in the known universe, as surmised by the workings of our scientists. Therefore, it is not out of the question that more types of galaxies are yet to be discovered.