The Sun is the most prominent object in our sky, and understanding it has been one of the primary focuses since the birth of modern astronomy. Additionally, after the realization that the Sun was itself a star, solar research became a means for investigating conditions too distant to observe directly. Starting in the early twentieth century, astronomers began building a model of the Sun that would accurately predict the Sun's observable features. If the model can accurately predict what is observed, then it is reasonable to assume that it can accurately tell astronomers about what they cannot observe, both inside the Sun and its behavior at other epochs. This model is known as the standard solar model, and it has been in a state of constant evolution since its inception. The standard solar model has four basic assumptions;.
1. The first being that the sun evolves in hydrostatic equilibrium.
2. The second assumption of the standard solar model is that energy can be transferred in the star via radiation, conduction, convection, and neutrino losses.
3. The third assumption of the model is that thermonuclear reactions are the only source of energy production inside the star.
HYDROSTATIC EQUILIBRIUM.
What this means in plain English is that there are two forces on a hot fluid which is massive enough so that gravity counts, and the pressure force balances the self-gravity so that the total force on each piece of the fluid is zero. ``Hydro'' means pertaining to a fluid, and ``static'' means not moving. ``Equilibrium'' means force balance. So ``hydrostatic equilibrium'' means force balance on the pieces of a fluid such that the fluid does not move in bulk. .
The first of these forces is the pressure. The gas is composed of a very large number of particles which are moving about in random directions. Individual particles are colliding with each other very often, so the probability distribution of their velocities is according to a bell curve: the probability is highest at zero velocity, but the bell curve has a finite width in velocity, so that there is an average energy which is not zero.
