A star can be considered as a shpherical ball of hot gas of radius R.Inside the star the density of the gas is `rho_r` at a radius r and mass of the gas within this region is `M_r`.The correct differential equation for variation of mass with respect to radius `((dM_r)/(dt))` is (refer to the adjacent figure)

A star in its prime age is said to be under equilibrium due to gravitational pull and outward radiation pressure `(rho)`.Consider the shell of thickness dr . If the pressure on this shell is dp then the correct equation is (G is universal gravitational constant)

Answer

`(dp)/(dr)=-(GM_r)/r^2`