Primordial gas clouds, from which stars are ordinarily formed, are not the same thing as most nebula - which are more often the result of exploding stars. Newborn stars efficiently consume their mother clouds then give them a 'kick' when they ignite. In rich gas clouds, this often leads to starburst activity - where rapid star formation in a local region occurs. This rapidly depletes the nursery cloud. The best known nearby stellar nursey is the Orion nebula.