First, I would like to start off by telling you what an irregular galaxy is. An irregular
galaxy is a galaxy that possesses no particular shape, and does not have any sort of structure to
it. Iirregualr galaxies are fairly easy to distinguish from one another, first you check whether the galaxy is elliptical or spiral, if not it is safe to say that it is an irregular galaxy
These galaxies have many bright stars and regions of hot, luminous gas as well. Though
these galaxies don fit into elliptical or spiral classes, some do have what appears to be patches of structure that suggest fragments of a spiral arm.
Due to this, as discover by Edwin Hubble, some irregular galaxies appear to be an extension of the spiral galaxy classification. After looking closer at these irregular galaxies, Hubble decided to reclassify them into other subclasses. A good example of this would be an irregular galaxy of Hubble type Irr I called NGC 1156, this galaxy is inconspicuous and somewhat messy-looking compared with spiral galaxies, is now termed as an SO galaxy.. Another reason these galaxies are similar is that they possess a bulge, though, the bulge is much smaller and the disk shows no structure. Generally these types of galaxies have a low mass, and are very rich in gas which makes up ten to twenty percent of their mass.
Another class of irregular galaxies, which was discovered during the 1950 s by a Swiss-American astronomer named Fritz Zwicky, are blue compact galaxies. These galaxies have much smaller dimensoins than irregular galaxies and , unlike the Magellinc type, has masses of the order of several hundred million solar masses. Blue compact galaxies have not always had a high rate of star formation, but being that they have a large quantity of ionised gas there is a lot of activity taking place within the galaxy. But these two different galaxies do share one similarity, they both are not highly evolved. This is shown when examining both of their chemical compisitions, for example, both of these galaxieshave a low heavy element abyndance compared to our galaxy.
Another type of irregular galaxy is the NGC 1332, it es anSO galaxy.the shape is not perfectly eliptical, as in the E6 galaxy, the E6 galaxy appears to be a lot flatter.
When studyuing iirregualr galaxies we observe a finished product and it is therefore difficult to reconstruct it s evolution. However in blue compact galaxies we can observe it s intermediate stages.
A good example of this is galaxy Zw 40, it is about seven megaarsecs away. It has a mass of about 4 x 10(to the seventh power). It is forty times brighter than the two extensions, this is due to the stellar populations, the nucleus is the centerof an evormous amount of stellar activity. The gas in the nucleus is ionised by the ultraviolet flux from it;s stars. This nucleus therefor appears like a large H II region of about 500 parsecs in size.
These irregular galaxies all have one thing in common , they all have extremely bright stars and have no real shape.