Elliptical
oval shaped, not quite a circle or a sphereGalaxy
a group of stars that are held together by gravityStar
a ball of hot, dense material held together by its own gravityIrregular
having an atypical shape.Irregular Galaxies
Irregular galaxies are the most unusual of galaxies. They don't seem to fit into either the spiral or elliptical galaxy categories. They don't have nice spiral arms, but they do have dark patches of gas and dust. Some irregular galaxies clearly look like two galaxies ran into each other! When it's not clear what group the galaxy should go in, astronomers usually call the galaxy irregular.
Irregular galaxies do seem to have things in common with each other, though. They all have some of the gas and dust that gets made into new baby stars as well as some older red stars.
Astronomers have measured the age of irregular galaxies and their ages are older than spirals, but younger than ellipticals. They are like teenage galaxies.
Many astronomers think that irregular galaxies are the 'in-between' for spiral and elliptical galaxies. Based on the ages, it's likely that galaxies start as young spiral galaxies. Once they've been around for a while, they turn into irregular galaxies by colliding into other galaxies. At this point they are in their 'teenage years'. Even later, after using up all their dust and gas to make new stars, they become old elliptical galaxies.
Additional images via Wikimedia Commons. Cartwheel Galaxy by ESA/Hubble & NASA.
Irregular galaxies often form when one galaxy collides with another. Here, a smaller galaxy passed through the middle of a large spiral galaxy, forming this irregular Cartwheel Galaxy.
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