To a close time, i settled my interpretation of expansion of the SUn... When the H=fuel in the core is over, the helium atoms is now having more density. Hence, the pressure increases on the outer H-fuel which ultimately will fuse faster and faster making a great deal of energy casuing expansion. Recently, i came to another conclusion which is as H is converted to He, g decrease due to the decrease of mass, and with the fusion of the outer Hydrogen, radiation force dominated over the decreasing G.. It is also written in many sites that the gravitational collapse cause a great amount of heat.. WHY?????.. IN general , what is the cause of this expansion!!??? Thanks to whoever replies!!