Other galaxies, much like our Milky Way are actually enormous groups of stars. Many of the stars in these other galaxies are much like our Sun, with planets that orbit them which comprise a solar system. Galaxies vary drastically in size, they have between 10 billion and 1 trillion stars. The galaxies in turn lump together into groups called clusters, these clusters vary from a few systems to several thousand systems. Our cluster is made up of about 30 galaxies, which include our own Milky Way and the Andromeda nebula. The galaxies that are far beyond the Milky Way are sometimes called the extra galactic galaxies. Scientists have estimated that there are between 50 and 100 billion other galaxies in the universe.
Main Shapes of Galaxies
Almost all galaxies have a nucleus, which is the bright central part of the galaxy. The galaxy nuclei are made of millions of stars and scientists have a strong belief that there may even be enormous black holes in the center of some galactic nuclei. The galaxies are classified by their shapes and are divided into two main groups, elliptical shaped (indicated by the symbol E) and spiral galaxies (indicated by the symbol S). Another type of galaxy is a bar shaped spiral galaxy (indicated by the symbol S.B.). Some do not fit into any category so they are called irregular shaped galaxies and can be distinguished by the symbol Ir.
A spiral galaxy is characterized by a large flat disc with a bulging center, which is called the nucleus. The largest amount of stars is located in the nucleus and then the concentration thins out as you move away from it. The stars are situated on a pinwheel-like structure, called the spiral arm, that wind out in a spiral-shape from the nucleus to the edge. They are regions of dust, gas, and stars where star formation is occurring. In between the spiral arms reside stars, the stars in these areas give the galaxy its characteristic shape. Spiral galaxies are comprised mostly of gas and dust. The youngest and most active stars are situated in the nucleus and the spiral arms.
In elliptical galaxies the stars are pretty evenly spaced across the entire stars system. However, the number of stars gradually decreases from inside to the outside of the ellipse. Elliptical galaxies have little or no dust or gas which indicates that they have completed their star formation. They are so far away that it is impossible, even with the strongest telescopes, to observe the individual stars of such a constellation. You can only see the common light generated by all stars as a hazy spot.
Irregular Bar Spiral Shaped
Sometimes a galaxy is deformed by the influence of gravity from within the galaxy itself or from a neighboring galaxy. Some spiral galaxies have a "bar" running through the galaxy's nucleus. Usually the spiral arms "connect" at the end of this bar. In this bar region, new stars are forming from the dust and gas particles in that nebula.
Irregular Shapes and Deformations
Sometimes a galaxy does not fit into either the spiral or elliptical shape category. These galaxies are classified as irregular shaped and created because they are too small to form the structures seen in a spiral or elliptical galaxy. Stars and gas clouds in this type of galaxy are scattered in very random patches. Another way these can be formed is when two galaxies collide and virtually rip each other apart. Some known peculiar galaxies are going through a very active period right now with rapid star formation and a lot of supernova explosions.