How are stars formed?
Like so many things in the universe, stars begin very small — mere particles in vast clouds of dust and gas. Far from active stars, these nebulae remain cold and monotonous for ages. Then, like some sleepy little town in a biker movie, everything stirs up when a newcomer speeds through. This disturbance might take the form of a streaking comet or the shockwave from a distant supernova. As the resulting force moves though the cloud, particles collide and begin to form clumps. Individually, a clump attains more mass and therefore a stronger gravitational pull, attracting even more particles from the surrounding cloud.
As more matter falls into the clump, its center grows denser and hotter. Over the course of a million years, the clump grows into a small, dense body called a protostar. It continues to draw in even more gas and grows even hotter.
