gravity comes from the density of the core, which is the result of the pressure pushing down on it, which is a result of the mass.......for example, a star first starts out as a stellar cloud, a mixture of dust and gas swirling around in space.........after awhile, this shit heats up, begins to spin, forming magnetic fields, and essentially gravity comes into play......as a star gets hotter, brighter, bigger, and what not, its gravity increases as well...........