Why do stars form in nebulae rather than in empty space?

Nebulae contain the raw material for star formation: hydrogen, helium, dust, and heavier elements. In the densest parts of these clouds, gravity can pull matter together until a protostar forms. The process often happens in giant molecular clouds, such as the Orion Nebula region, where turbulence and nearby massive stars can trigger collapse. Once fusion begins, the new star clears away much of the surrounding material. Without a sufficiently dense cloud, there is simply not enough matter to build a star.