Clusters, Galaxies, Black Holes and Stars 283
binaries is still another mechanism for dissipating angular momentum
since more angular momentum is contained in the motion of the two stars
about each other than in the rotation of the star about their respective
internal axes.
Protostars
The initial stages of the collapse of the protostars from its original
dimensions of light years (10^13 km) to the size of planetary orbits
(10^8 km) takes place quite rapidly within a matter of a few years. The
contraction then slows down. The gravitational potential energy of
the protostar is converted into the kinetic energy of the gas molecules as
the contraction proceeds. Half of this energy is converted into the
internal energy of the gas raising its temperature and the remainder is
radiated away as light. The temperature of the center of the protostar
when its dimension are that of a planetary orbit are 100,000 K while its
surface temperature is about 2500 K. Convection currents of hot gas flow
from the center to the edge of the star. As the star contracts more it
becomes too dense for the circulation of hot gases. At this point the
energy is transferred to the surface by means of radiation rather than
convection.
The temperature and density of the star continues to increase until the
central region reaches a temperature of 10 million K at which point
thermonuclear fusion takes place in which hydrogen is converted into
helium. The age of the star when nuclear ignition takes place depends
upon its mass. The higher the mass of the protostar the sooner it reaches
the temperature and density necessary for nuclear ignition. The Sun, an
average size star, spent 27 million years in its pre-nuclear ignition phase,
10 million years in the convection stage and 17 million years in the
radiation stage.
Main Sequence Stars
Once nuclear ignition has taken place the star quickly establishes an
equilibrium state in which it remains for the majority of its life. Once the
star exhausts its supply of hydrogen gas that has been converted into
helium it begins to evolve again. During the long stable period of its
existence while it is burning hydrogen, the temperature and luminosity of
the star remain fixed. (Note to the reader: I am using terms like ignition