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An irregular galaxy is a galaxy with no rotational symmetry. Irregular galaxies get their odd shapes in many ways, the shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. One way irregular galaxies are formed is when galaxies collide or come close to one another, and their gravitational forces interact. Another source of irregular galaxies may be very young galaxies that have not yet reached a symmetrical state. Also, in some irregular galaxies, like M82, young stars eject energetic bubbles gas, giving the galaxy a blobby look.
Three Major Types of Irregular Galaxy
1. Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence.
2. 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.
3. dI-galaxy (dIrrs) it is a dwarf irregular galaxy. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as 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. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys.
1. Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence.
- Subtypes with some spiral structure are called Sm galaxies
- Subtypes without spiral structure are called Im galaxies.
2. 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.
3. dI-galaxy (dIrrs) it is a dwarf irregular galaxy. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as 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. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys.