IRREGULAR GALAXY

                                                                               IRREGULAR GALAXY                                                                                                                                                                                                              An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. 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.

Collectively they are thought to make up about a quarter of all galaxies. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull. Irregular galaxies may contain abundant amounts of gas and dust. This is not necessarily true for Dwarf Irregulars.

The Magellanic Cloud galaxies were once classified as irregular galaxies. The Large Magellanic Cloud has since been re-classified as type SBm  a type of barred spiral galaxy, the barred Magellanic spiral type. The Small Magellanic Cloud remains classified as an Irregular galaxy of type Im under current Galaxy morphological classification, although it does contain a bar structure. Therefore, newer classification schemes place the SMC outside the irregular class as well.

Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright. The Large and Small Magellanic Clouds are examples of irregular galaxies. They are two small galaxies which orbit around our own Milky Way Galaxy. About 20% of all galaxies are irregulars.

Environment and Galaxies

The type of galaxy one finds at a given location depends on the density of galaxies. This suggests that the interaction between galaxies over the evolution of the universe plays a central role in the evolution of individual galaxies. In the low-density regions of our universe, where galaxies are not bound gravitationally to one another, the dominant type of galaxy is the spiral galaxy. Of the field galaxies, 80% are spiral galaxies, 10% are elliptical galaxies, and less than 10% are lenticular galaxies. In the highest-density regions of our universe, where galaxies form gravitationally-bound clusters, the dominant type of galaxy is the lenticular galaxy, composing up to 50% of the cluster, followed by the elliptical galaxy, composing up to 40% of the cluster. Spiral galaxies compose only 10% of the core of a dense galaxy cluster.

                                            Galaxy Notation

The class of a galaxy is given by a letter and number notation. An elliptical galaxy is noted as En, where n is a digit ranging from 0 to 7 that gives the ellipticity of the galaxy: b/a = 1 - n/10, where a is the semimajor axis and b is the semiminor axis of the ellipse. A galaxy that appears purely isotropic is therefore an E0 elliptical galaxy, while a galaxy with b = 0.3 a is an E7 elliptical galaxy. The lenticular galaxy has a single notation: S0. Spiral galaxies have four symbols: Sa, Sb, Sc, and Sd, with the notation progressing from Sa galaxies, which have tightly-wound spiral arms and a spheroid of stars that is relatively bright compared to the disk stars, to the Sd galaxies, which have loosely-wound and clumpy spiral arms and a relatively dim spheroidal stellar component. The final type, the irregulars, are in general noted by Irr, although those that resemble the Magellanic clouds are noted by Sm or Im. The progression of galaxies from gas poor to gas rich is E0→E7→S0→Sa→Sd→Irr.

The galaxy that is our home is called the Milky Way Galaxy, or just the Galaxy. It measures about 100,000 light years across. Our local star, the Sun, is one of at least 200 billion stars in the Galaxy, and lies in one of the Galaxy’s spiral arms. We also call the faint band of light that arches across the night sky the Milky Way. This band is a just a section of our Galaxy.