Spiral galaxies are a type of galaxy that was discovered by Edwin Hubble first in 1936 based on their appearance in optical light. A spiral galaxy is a pinwheel shape and mostly consists of a flat, rotating disk containing stars, gas, and dust. The central point in 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 stretch out from the center and revolve around it. These arms are made up of an ongoing star formation that is brighter than the surrounding disc. The exact formation and mechanics of the spiral arms puzzle scientists. If the arms were permanent features of the galaxy then the arms would disappear and wind up tightly in a billion years. Scientists think they could be a 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 have faded over time. These galaxies range from a billion to a trillion times the mass of the sun. The disk in the galaxy can vary from 10 to 300 thousand light years across. The largest spiral galaxy is 5 times the size of the Milky Way and is 522,000 light years across. Spiral galaxies are divided into two classes, normal spirals and barred spirals. Barred spirals, are when the spiral arms come out from a bar passing through the galactic center of the galaxy. When the universe was still young, different galaxies collided and interacted with each other frequently, so the spiral shapes of these ancient spiral galaxies would be disrupted quickly. The oldest spiral galaxy is nearly 10.7 billion years old. Because of the association between the amount of time and distance it takes for light to travel, scientists are able to see the galaxy only 3 billion years after the Big Bang formed the universe. After spiral galaxies burn through their gas and dust and as the star formation slows, they will lose their spiral shape and move on to the next stage of evolution, elliptical galaxies.