Approximately 3% of galaxies observed cannot be classified as either ellipsoidal or spirals. These galaxies have little symmetry in their structure and are termed irregular galaxies. An example is Sextans A, shown in the image on the right. This irregular galaxy is a member of the Local Group, at a distance of about 10 million light years (Ref). The blue regions are clusters of young stars; the brighter stars are members of our own Milky Way galaxy in the foreground. Other examples of irregular galaxies are the Large Magellanic Cloud (a satellite galaxy of the Milky Way) and the Messier object M82.

Properties of Irregular Galaxies

Irregular galaxies have masses in the range 108 to 1010 solar masses, diameters from 1 to 10 kpc, and blue magnitudes from -13 to -20. Other than that, they have few systematic features.

Peculiar Galaxies

Hubble's original classification just lumped all galaxies that are not spirals or elliptical into the irregular category, but it is more common today to make further distinction between more "normal" irregular galaxies and peculiar galaxies, which are galaxies that look unusual in some respect (M82, for instance). For example, some of these are objects that have been tidally distorted by interaction with another galaxy that we shall discuss in conjunction with colliding galaxies. Others are active galaxies with some evidence of violent internal processes taking place. Still others may be more normal galaxies, but given an unusual appearance because of obscuring dust. We shall discuss these special kinds of irregular galaxies separately later.

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