In Stellar constitution and evolution direct observation deal only with an outer portion of a star, and this is a very small fraction of the whole. With the help of direct observation we may determine the size of a star, its mass, its luminosity or total output of radiation.
To proceed from these data, to derive information about the interior of a star, it is necessary to apply mathematical principles to the general physical properties of the matter of which the stars are made.
The interior of the stars are so hot that no chemical compounds can exist there. They are broken up into their constituent atoms, and the atoms in turn are almost completely ionized: in other words, all but the inner most electrons are stripped off from the atoms.
Though this is the case, the atom nuclei retain their identity. The mean molecular weight increases towards the surface as the temperature and the ionization decreases. It has been seen that in the cooler stars simple compound can exist in the surface layers.
A star is held together by its own gravitation. Opposed to this are the pressure of the gaseous matter and the pressure of radiation. The pressure of gas tends to disperse the material of the star in all directions outwards.
The radiations in the interior of a star consist largely of extremely short wave length: the radiation is being continually emitted by some atoms and absorbed by other which in turn emit again.
A luminosity of a star according to Stellar Evolution
At any point inside the star there is an outward flow of heat. The heat is carried partly by conduction and partly by radiation. At the high temperature the heat carried by conduction is negligible.
The outward passage of the radiation is impeded by the capacity of the gaseous material of the star. The luminosity of a star is determined by the radiation passing through the surface of the star.
Navigate reading to: