284 The Poetry of Physics and The Physics of Poetry
and burning to describe thermonuclear fusion but these terms are being
used metaphorically because technically the terms ignite and burn are
used to refer to oxidation or fire but they are also a handy way to
describe fusion.) The force of gravity pulling the star in is just balanced
by the outward force of the pressure generated by the dissipation of
energy produced by the thermonuclear fusion of hydrogen through the
radiation of light.
This balance is extremely stable. If the gravitational force begins
to overpower the outward force the star begins to collapse and its
temperature and density increase. The rate of nuclear burning increases
as a result and more energy is generated and dissipated. The outward
force, therefore, increases preventing the star from collapsing any
further. If, on the other hand, the inward force begins to overpower the
gravitational force the star expands and cools. The rate of nuclear fusion
slows down, less energy is dissipated and the outward force decreases by
itself. This continues until the outward force just equals the gravitational
pull inwards. The star, therefore, remains in stable equilibrium as long as
its supply of hydrogen to be converted into helium lasts.
The lifetime of the star in this state depends on its supply of nuclear
fuel and the rate at which it uses the fuel. The larger stars, despite their
greater fuel supplies, have shorter lifetimes because of the rapid rate
at which they burn nuclear fuel. The absolute brightness and the
temperature of a star are determined by its mass. The brightness of a star
is not determined solely by its temperature but depends on the radius of
the star as well. The larger the surface area of a star the more light it will
radiate. Temperature and brightness are independent variables. If a two-
dimensional plot of the temperature and the brightness of different stars
is made it is found that the majority of stars lie along the same line called
the main sequence. These stars are in the stable hydrogen-burning phase
of their existence. Their position on the main sequence depends solely on
their masses. Those stars not on the main sequence are either in the stage
of their existence before they undergo nuclear ignition or else they are in
their death throes that occur once their supply of hydrogen fuel has been
exhausted.
The rate of energy production by a main sequence star due to
thermonuclear fusion is quite enormous. The Sun radiates 4 × 10^26 Joules
per second, which requires the destruction of 4 billion kilograms of
matter each second. Since 1% of the hydrogen’s mass is lost when it is
converted into helium the Sun burns 4 × 10^11 kilograms of hydrogen fuel