Think of a galaxy, any galaxy. Chances are your intuition guided you to think about a distinct, regular shape - such as a spiral or elliptical galaxy. However, there is a ‘junk draw’ of sorts for galaxies known as the irregular galaxies! Approximately 1 out of every 4 galaxies is irregular. The shape of these is uncommon, as they do not fall into any regular classifications. If the Hubble sequence, a system of classifying galaxies by their shapes, has no category for a particular galaxy’s shape - it is known as irregular. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull.
There are two major Hubble types of irregular galaxies:
- An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence.
- An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence.
A third classification of irregular galaxies are the dwarf irregulars; they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe.