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
Large Magellanic Cloud (a satellite galaxy of the Milky Way) and
the Messier object
Properties of Irregular Galaxies
Irregular galaxies have
masses in the range 108 to 1010 solar masses,
diameters from 1 to 10 kpc,
magnitudes from -13 to -20. Other than that, they have few systematic
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.