Spiral galaxies are a type of galaxy that are disc-like in shape but have their disc of stars spun into a set of spiral arms where there is a higher concentration of nebulae and large, bright stars. It is believed that stars will regularly "pile up" inside these arms like a traffic jam, and then leave the arm to travel to the next one. Like elliptical galaxies, they vary in size from millions of stars to trillions. Well-known spiral galaxies in our universe include M33, the Andromeda Galaxy, and the Milky Way.