One of the earliest theories states that massive stars form as gas falls into a sphere uniformly from all directions, forming a spherical ball of gas (a protostar) that slowly contracts to become denser and hotter.
This theory states that massive stars form the same way that smaller stars do: material falls into a disk, which spins due to the conservation of angular momentum. The infalling material eventually spirals into the star, while strong jets are expelled from the poles.
This theory states that massive stars form when smaller stars collide with each other, probably in a star cluster. Eventually one star emerges from all the collisions as a massive star.