Irregular Galaxies

Introduction

Irregular galaxies can have any shape. There are two types of irregular galaxy: Type I's and Type II’s

Type I Irregulars

These are single galaxies that have a strange shape. They have discs and bulges like spiral galaxies, but they don’t have spiral arms and their bulges are not in the middle. This is a type I irregular galaxy called NGC 55 (©AAO). You can see its bright bulge on the right side of the middle. You can also see dark patches of cold dust and bright patches of hot dust.

Type II Irregulars

Type II irregular galaxies are made when two galaxies collide.

Astronomers have used powerful computers to show what this might look like. You can see what they found by clicking here (Space Movie Archive).

The simulation also shows regions of star formation where the gas is squeezed and heated. The red regions are areas where lots of stars are being made, and blue areas are quieter, with few stars being made. (This research was carried out by Chris Mihos and Lars Hernquist of University College, Santa Cruz). The collision shown would last for about 1.5 billion years.

This picture on the left is of the middle of the Antennae galaxy. It was taken by the AAO. This is one of the best known type II irregular galaxies and is made up of two separate galaxies that are colliding.

Image courtesy of the Isaac Newton Group of Telescopes, La Palma

Another well known example is the Whirlpool galaxy (SEDS). This picture (right) shows the two galaxies that make up the Whirlpool.

Our galaxy, the Milky Way, has two irregular galaxies close by. These are called the Large and Small Magellanic Clouds. They can be seen easily from the southern hemisphere, looking like bright patches in the night sky.

		Galaxies Introduction
		Formation of Galaxies
		The Milky Way
		Spiral Galaxies
		Elliptical Galaxies
		Tuning Fork Diagram

Author: Nigel Bannister

Updated by: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001