An irregular galaxy is a galaxy with no rotational symmetry. Irregular galaxies get their odd shapes in many ways. 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.
A peculiar galaxy is an irregular galaxy that has an abnormal shape (neither elliptical, spiral, nor lenticular) and/or has another unusual characteristic, like jets of gas spewing from the nucleus, unusual amounts of dust, low surface brightness, etc. They are probably formed as galaxies collide or have companion galaxies that influence them. Peculiar galaxies are generally smaller and optically dimmer than regular galaxies. They have the designation p or pec at the end of their name. In 1966, the astronomer Halton Arp published a catalogue of 338 of these galaxies in his “Atlas of Peculiar Galaxies.” Arp suggested that peculiar galaxies create stars in intense bursts. Ring galaxies are a type of peculiar galaxy
Some irregular galaxies
Large Magellanic Cloud (LMC)\
The Large Magellanic Cloud (LMC) is an irregular-shaped galaxy in the Local Group. The irregular shape may be the result of a disturbance, perhaps a collision of two galaxies. The Large Magellanic Cloud (LMC) is near the constellation Dorado, and is 163,000 light-years away.
Small Magellanic Cloud (SMC)
The Small Magellanic Cloud (SMC) is an irregular-shaped galaxy in the Local Group. The irregular shape may be the result of a disturbance, perhaps a collision of two galaxies. The Small Magellanic Cloud (SMC) is near the constellation Tucana, and is under 200,000 light-years away.
The Cartwheel Galaxy has a ring-like structure that is the result of a head-on collision of two galaxies! It started out as a regular spiral galaxy that was hit by a smaller galaxy. The ring-like band of stars formed much like ripples form in water when a rock is tossed into it. The Cartwheel Galaxy is about 500 million light-years from Earth in the constellation Sculptor.