 The formation of a star begins with gravitational instability within a molecular cloud , caused by regions of higher density - often triggered by compression of clouds by radiation from massive stars , expanding bubbles in the interstellar medium , the collision of different molecular clouds , or the collision of galaxies ( as in a <unk> galaxy ) . When a region reaches a sufficient density of matter to satisfy the criteria for <unk> instability , it begins to collapse under its own gravitational force . 
