Why do some nebulae form into stars while others remain diffuse?

[Nova]

Since Nebulae are in "cloud" like structures, why does it not form itself into a sphere based on the highest density of which the largest amount of mass is held? resembling an extremely large gas planet.

 

[russ_watters]

Welcome to PF!

Some nebulae are simply too diffuse for the gravitation forces to be high enough to pull them together...and ones that are dense enough eventually collapse into stars.

 

[davenn]

Welcome to PF!

Some nebulae are simply too diffuse for the gravitation forces to be high enough to pull them together...and ones that are dense enough eventually collapse into stars.

yup
as with the hot young stars of the Trapesium in the Orion Nebula ( M42)

Dave