ABSTRACT
A solarlike star starts its existence with the contraction of a gas cloud as a result of gravitational attraction between its constituents. As we have seen in the preceding chapter, the initial energy content of the primordial cloud is very small compared to the gravitational potential energy released during the contraction. When the star contracts, the released gravitational energy heats the matter. The increase in temperature forms a temperature gradient, and the star begins radiating. When the protostellar material is neutral its opacity is very low, and the energy released can easily flow out ward. This energy release causes a very rapid contraction which is almost like a free-fall motion. If we assume that the density in the primordial cloud is uniform, then we have for m(r) (the mass enclosed in a sphere of radius r): m(r) = 4^r3p. The gravitational acceleration at a point r, g(r), is given by:
This equation means that the inward acceleration increases with the ra dius, and the acceleration in the outer zones is stronger than in the inner ones. With the increase in density, the number of collisions between the particles increases. Their velocities, originally directed toward the centre, now become random — in other words, thermal velocities. The tempera ture rises. With increasing temperature and density, pressure forms whose gradient slows down the contraction. According to the virial theorem, the kinetic energy of the particles in an ideal gas within the classical limit is minus half of the potential energy.
