Galaxies are humongous systems of stars and interstellar matter. Galaxies are located in the billions all throughout the universe and contain very many stars; these numbers range from the millions to the trillions. Galaxies like our own, the Milky Way, can be 100,000 light years in diameter. Imagine how large that is! A light year is 9,365,829,120,000 kilometers. A galaxy can be, as said, 100,000 times that figure! There are four different types of galaxies the universe contains—or the only four types that have been discovered: spiral, irregular, lenticular and elliptical.
Spiral galaxies, like our own Milky Way, have two main parts to them: the main, flat disk in the center, with the younger generations of stars arranged in a spiral pattern emerging from the central disk. Our galaxy, the Milky Way, has several of these arms spiraling outward from the central disk: arms such as the Orion Arm, in which we are located, the Sagittarius Arm and more.
Lenticular galaxies in effect spiral galaxies without any arms emerging from the center. They appear to be simply flat disks in the sky. The reason why these galaxies have not become spiral is because all of the interstellar matter they had in the beginning was used up.
Therefore, Lenticular Galaxies are mostly if not fully composed of the older generations of stars. Elliptical Galaxies usually appear to be either strangely circular or highly eccentric in shape. The galaxies often seem to be just large masses of stars without a common center because the stars in these types of galaxies do not rotate together as a group. Elliptical Galaxies, as Lenticular ones, have little interstellar matter to create new stars with and are, again, composed of mostly older generation stars.
Irregular Galaxies, as their name suggests, have no particular shapes as the spiral or Lenticular Galaxies do. This is due to the many neighboring celestial objects that exert a gravitational pull on these galaxies making them irregular. For example, it’s as if you were to take a nice round piece of dough and started pulling on all sides of it, top and bottom as well; you would receive a total irregular mess.
Some galaxies float through space by themselves and are lonely wanderers. But many galaxies occur in large groups in which they create large gravitational fields that alter the galaxies’ appearances. The numbers of galaxies in a cluster could range from a few to a dozen to several thousand. Galaxies are majestic objects that will forever mystify the imaginations of human beings everywhere.