Irregular Galaxies

Irregular galaxies are those that cannot be classified as either ellipsoidal or spirals. They make up roughly 3% of all galaxies. These galaxies have little symmetry in their structure, and are made up of a steadily forming supply of small clusters of young blue stars. They have masses in the range 10Number eight in the exponent to 10Number one in the exponent Number zero in the exponent solar masses, diameters of 1 to 10 kiloparsecs, and blue magnitudes from -13 to -20 (fairly faint).

Some irregulars are believed to be low mass, low density galaxies which are less evolved than their more massive spiral neighbors. With less mass, the gravitational force contracting the galaxy is less, and it stays in a more diffuse, relaxed state for a longer time. Irregulars typically contain a lot of hydrogen gas, which has not been used up yet in stellar burning in stellar cores. Many of the small dwarf galaxies which surround the Milky Way are classified as irregulars, including the Magellanic Clouds.

If we look back in time at distant galaxies, we find that the proportion of irregular galaxies increases and the proportion of spirals and elliptical galaxies decreases. This supports the theory that galaxies start out their lives in a more diffuse form, and over time they evolve to the more defined, compact states.

M82 (Cigar) Small Magellanic Cloud Large Magellanic Cloud NGC 6822
Telescope image of M82, showing disordered intersecting wings and spears of red, white, and dark yellow light. Telescope image of the Small Magellanic Cloud, showing disordered clumps of white and violet light embedded in a diffuse faint cloud of white light. Telescope image of the Large Magellanic Cloud, showing bright disordered patches of white and violet light embedded in a diffuse cloud of white light. Telescope image of NGC 6822, showing an elongated bright white structure and patches of red and white light embedded in a diffuse cloud of white light.
Telescope image of NGC 1313, showing bright white patches and arcs of light and a dense white core, with disordered diffuse light and smaller white clumps surrounding it. Telescope image of IC 5152, showing white and greenish light (this is because of the particular filters used to image the galaxy) in a patchy elongated blob. A bright foreground star shines strongly in the image as well. Telescope image of NGC 4449, showing a diffuse white cloud of light, and numerous patches of bright red and white congregating towards the outer portions of the cloud. Telescope image of four distant irregular galaxies from the MDS survey, showing similar distributions of patchy light and disordered structure.
NGC 1313 (starburst) IC 5152 NGC 4449 MDS (distant)
Patchy, irregular galaxies in the nearby and distant Universe. [NASA/HST, AAO]