Why Galaxies have Spiral Arms?! - The majestic spiral arms of galaxies are a source of wonder, but what exactly causes them to form? Is it the swirling dance of stars as they orbit the galactic center, or something more mysterious? Does the speed at which different parts of the galaxy rotate play a role? And why don't these arms simply wind up and disappear over time? | The Brain Maze

The grand spiral galaxy NGC 5643... - Hubble Space Telescope