When stars are born in nebulae, lighting up in fusion, they will race through life. Some will be lucky to reach a few billion or trillion years of age. The heaviest and densest of these will die in massive supernova explosions. Others, like our Sun, will burn more slowly, and probably expire less dramatically. Upon a star's death, matter will spill out into space in the form of new elements, creating new star-forming nebulae, continuing its circle of life.
Tiny imperfections. Little knots, wrinkles, and flaws will begin attracting nearby particles of matter. The clumps will grow, becoming more massive, and attracting more particles.
As these compacted clumps of hydrogen and helium grow in density, they'll also heat up. Eventually, the clumps heat and compact to form a "plasma," sort of a hot, swirling soup made up of free-floating atomic nuclei. This will form into giant balls called "protostars."
Gravity squeezes the center of the protostars tighter and tighter. Their temperature rockets until they reach a flash point. And they "light up" as free-floating nuclei, slamming together with such intensity that they fuse into a new element. This process of "nuclear fusion" releases a tremendous amount of energy, presenting the Universe with a new complexity that is critical to the formation of galaxies, larger clusters, and superclusters.