 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 starburst galaxy ) . When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability , it begins to collapse under its own gravitational force . 
 The formation of a star <a href='uninflect' title='1'><mark>begins</mark></a> 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 starburst galaxy ) . When a region <a href='uninflect' title='2'><mark>reaches</mark></a> a sufficient density of matter to satisfy the criteria for Jeans instability , it <a href='uninflect' title='3'><mark>begins</mark></a> to collapse under its own gravitational force . 
The formation of a star begin 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 , da collision of different molecular clouds , or tdacollision of galaxies ( as in a starburst galaxy ) . When a region reach a sufficient density of matter to satisfy the criteria fa Jeans instability , it begin to collapse under its own gravitational force .