A nebula is a cloud of gas and dust in space. Astronomers think a star's explosion causes this cloud to spin. While it is spinning and moving, gravity from the gases, rocks, and dust pull itself together in the cloud. These things in the center of the nebula become very tightly packed. This causes the atoms of gas to fuse together. When this happens, enormous amounts of energy is released. The gases in the nebula then shine because of the energy that was released. If the star turns out big or small all depends on the amount of material in the cloud.
Binary Stars
Star Clusters
Sometimes the cloud might have enough material to create two stars or even more. Most nebulae seem to create more than one star most of the time. When a nebula creates two stars that orbit each other, they are called binary stars. When a nebula forms more than two stars that orbit around each other, they are called multiple stars.
Clusters of stars can form from one nebula and then they live bounded together by gravity. They are called globular (a cluster of stars that form a globe shape), or open or galactic star clusters (star clusters that don't have any specific shape).