Irregular Galaxies

Galaxies that are classified as irregular are those that simply fit no other organizational category for galaxies such as lenticular, spiral, or elliptical. Since the universe is an orderly place by definition, irregular galaxies become so because they are being acted on by some other force, usually a much bigger galaxy. We have already noted the irregular companion of M51, designated NGC 5195 and M82, the Cigar galaxy, which is the only irregular galaxy in the Messier catalogue. Two other irregulars of consequence that are not in the Messier catalogue (because he could not see them) are the Large and Small Magellanic Clouds deep in the southern sky. These two galaxies are gravitationally disrupting each other as well as by the Milky Way as well. The two Magellanic Clouds have populations of stars, clusters and nebulae that are remarkably similar to the Milky Way and other star forming spirals. This tells us that irregular galaxies are relatively young and that the only difference between them and young star-forming spirals is the gravitational interactions that are destroying their structures. In some cases, remnants of that structure can still be viewed. Careful study of the Small Magellanic Cloud reveals that it may at one time have been a barred spiral. Both galaxies are very bright. The Large Magellanic Cloud shines with a total light of magnitude +0.1, the Small Magellanic Cloud at +2.2. The LMC spans about 11 degrees by 9 degrees while the SMC covers about 5 by 3 degrees. The LMC is about 170,000 light years away while the SMC is about 210,000 light years distant. They are the second and third nearest galaxies to the Milky Way. Only the Sagittarius Dwarf Elliptical galaxy is closer than the LMC.

M82 is the relatively bright companion of M81. It shines at magnitude +8.4. Both galaxies can be easily viewed in the same wide field eyepiece field of view. Under a dark sky a medium-sized telescope will easily show you what has been done to M82. The galaxy is undergoing tremendous bursts of star-forming activity and several dark lanes can be viewed crossing the galaxy in varying directions. The galaxy clearly at one point had a disk shape. The enormous movement of gases within the galaxy, rushing into star-formation areas, creates large amounts of radio noise. In fact it is the strongest such source of noise in Ursa Major. Irregular galaxies therefore represent the manifestation of enormous amounts of gravitational energy at play, ripping galaxies asunder and setting off astonishing bursts of star-forming activity.

When spring comes around and the great Virgo supercluster comes into view, galaxy season kicks into peak time. Six months later, the early autumn skies offer our best look at our closest large intergalactic neighbors. Let's break out the telescopes and look beyond our own island universe into the deepest parts of the cosmos.

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