Stars eventually leave the spiral arm while new stars enter the arm or form within it, resulting in the formation of density waves through the galaxy. The higher density within the arms also means that star-formation will occur more often within a galaxy’s spiral arms, which further increases the overall density. This is because stars and nebulae possess a higher density, and so they have a lower speed than the arms themselves. It begins with the fact that the spiral arms of a galaxy rotate faster than the stars and nebulae contained within them. The density wave model of spiral formation can be rather complicated and hard to visualize. While the exact origin of the spiral shape remains a mystery, the most likely explanation based on current evidence is that density waves caused by star-formation are behind the spiral shape. However, this theory had a major problem, the most notable being that over time, spiral galaxies would wind up as their arms closed inwards, causing the entire spiral structure to cease to exist. Like how the planets in a solar system will orbit a star at different speeds depending on their distance, astronomers believed that stars within a galaxy would do the same depending on their distance from the central bulge. In the early 20th century, it was generally believed that the spiral shape formed due to differences in a galaxy’s rotational velocity. Since spiral galaxies have such complex shapes, astronomers have had a difficult time explaining their origin. How Do Spiral Galaxies Form? Image of a spiral galaxy taken by Hubble.
While the most common feature of every spiral galaxy are the vast spiral arms, there are several other noticeable features that spiral galaxies possess, such as a central bulge, a flat rotating disk of stars, a supermassive black hole at the galaxy’s center, and a galactic halo of material. Regular spiral galaxies are those that possess multiple spiral arms, while a barred spiral galaxy will generally have two large arms. Spiral galaxies can actually be divided into two subgroups: regular spirals and barred spirals. Spirals also tend to have the most visually stunning and complex shapes, making them a sight to behold. Spiral galaxies are perhaps the most recognizable type of galaxy, and this is likely due to the fact that the Milky Way, along with most of the closest galaxies, are all classified as spiral galaxies. Spiral Galaxies Hubble image of a barred spiral galaxy. Each of these types of galaxies have their own unique features and history, adding more beauty to an already stunning universe. In total, there are four main groups of galaxies: spiral, elliptical, irregular, and peculiar. Galaxies come in a wide array of shapes and sizes.
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