Spiral galaxies are a type of galaxy that was first discovered by Edwin Hubble in 1936 based on their appearance in optical light. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an Original Essay A spiral galaxy is shaped like a pinwheel and consists primarily of a flat, rotating disk containing stars, gas, and dust. The central point of the galaxy, known as the bulge, is a large cluster of stars and is also thought to contain a supermassive black hole. These galaxies have arms that extend from the center and rotate around it. These arms are made up of ongoing star formation that is brighter than the surrounding disk. The exact formation and mechanics of spiral arms puzzle scientists. If the arms were permanent features of the galaxy, they would disappear and become tightly wound in a billion years. Scientists think they could be the result of density waves traveling through the outer disk. About 77% of all galaxies are spiral galaxies, including the one we live in, the Milky Way. Spiral galaxies vary in shape. About 60% of spiral galaxies have multiple arms, about 10% have two, and about 30% of spiral galaxies do not have well-defined arms, their features having faded over time. These galaxies range from a billion to a trillion times the mass of the sun. The disk of the galaxy can vary from 10 to 300 thousand light years in diameter. The largest spiral galaxy is 5 times larger than the Milky Way and has a diameter of 522,000 light years. Spiral galaxies are divided into two classes, normal spirals and barred spirals. Barred spirals are when the spiral arms come out of a bar that passes through the galactic center of the galaxy. When the universe was still young, several galaxies collided and interacted with each other frequently, so the spiral shapes of these ancient spiral galaxies were quickly disrupted. Please note: this is just an example. Get a custom paper from our expert writers now. Get a Custom Essay The oldest spiral galaxy is nearly 10.7 billion years old. Because of the association between the amount of time and the distance needed for light to travel, scientists are only able to see the galaxy 3 billion years after the Big Bang formed the universe. After spiral galaxies burn through their gas and dust and as star formation slows, they lose their spiral shape and move on to the next stage of evolution, elliptical galaxies.