Galaxies are huge collections of stars, gas, and dust that are held together by gravity. They can be different shapes, such as spiral, elliptical, or irregular. Spiral galaxies are flat disks with a bulge in the center and arms spiraling outwards. Elliptical galaxies are round or oval-shaped and have less visible structure. Irregular galaxies have a mix of shapes and sizes. Galaxies are formed from clouds of gas and dust that collapse under their own gravity. As the gas and dust collapse, they heat up and ignite into new stars.