Galaxies are large groups of stars, dust, and gas. Galaxies contain planets, star systems, clusters, and interstellar clouds. Among these objects is a scattered interstellar medium of gas, dust and cosmic rays. There are supermassive black holes located at the centers of most galaxies. Supermassive black holes are the largest type of black hole. Galaxies that have fewer than a billion stars are considered “small galaxies.” Galaxies are classified based on their shape. There are three types of galaxies: elliptical, spiral and irregular. Elliptical galaxies have very bright centers and little dust and gas. They arise from the collision of many smaller galaxies. Their shape can vary from spherical to flat. They can have tens of millions to over a trillion stars. The smallest elliptical galaxy (called a “dwarf elliptical galaxy”) is about a tenth the size of the Milky Way galaxy! They are also among the largest single-star systems in the Universe. They contain mostly old stars. Due to the small amount of free-flowing gas, few new stars form. They are spotted less frequently because they are weak and deader. The largest galaxies are called giant ellipses. Spiral galaxies have spiral arms and a bulge at the center. There is a supermassive hole located in the very central bulge. The spiral arms are made of gas, dust and new stars. They are rich in gas and dust. They are the most common type of galaxy. Spiral galaxies have tightly coiled arms or loosely coiled spirals. They can be used to classify spirals as it is a real difference between galaxies. The arms of the spiral are believed to be density waves. As stars move through a spiral arm, gravitational force changes the speed... center of paper... that half of the stars found in the galaxy are more than four point five billion years old. The Milky Way is as old as the universe itself. There are probably more than one hundred billion galaxies. At a distance of approximately 13.1 billion light-years, z8_GND_5296 is the most distant galaxy ever discovered. The galaxy appears to astronomers as it did just seven hundred years after the big bang. Ultraviolet and X-ray telescopes are used to observe galactic phenomena. An ultraviolet glow was observed when a star in a distant galaxy was torn apart by the tidal forces of a black hole. X-rays can map the distribution of hot gas in galaxy clusters. No one knows the exact number of galaxies in existence or whether the universe has an end. One can travel forever at the speed of light without reaching a limit since the universe is borderless and expanding.