Fusion


For centuries, humankind has looked at the stars, and for just as many years
humankind has tried to explain the existence of those very same stars. Were
they holes in an enormous canvas that covered the earth? Were they fire-flies
that could only be seen when the Apollo had parked his chariot for the night?
There seemed to be as many explanations for the stars as there were stars
themselves. Then one day an individual named Galileo Galilei made an astounding
discovery: the stars were replicas of our own sun, only so far away that they
seemed as large as pin pricks to the naked eye. This in turn gave rise to many
more questions. What keeps the stars burning? Have they always been glowing, or
are they born like humans, and thus will they die? The answers to all these
questions can be summed up in two words; stellar fusion. Therefore one can begin
to understand the stars by understanding what fusion is, how it affects the life
of a star, and what happens to a star when fusion can no longer occur. The first
question one must ask is, "What is fusion?" One simple way of explaining it is
taking two balls of clay and mashing them into one, creating a new, larger
particle from the two. Now replace those balls of clay with sub-atomic
particles, and when they meld, release an enormous amount of energy. This is
fusion. There is currently three known variations of fusion: the proton-proton
reaction (Figure 1.1), the carbon cycle (Figure 1.2), and the triple-alpha
process (Figure 1.3). In the proton-proton reaction, a proton (the positively
charged nucleus of a hydrogen atom) is forced so close to another proton (within
a tenth of a trillionth of an inch) that a short range nuclear force known as
the strong force takes over and forces the two protons to bond together (1). One
proton then decays into a neutron (a particle with the same mass as a proton,
but with no charge), a positron (a positively charged particle with almost no
mass), and a neutrino (a particle with almost no mass, and no charge). The
neutrino and positron then radiate off, releasing heat energy. The remaining
particle is known as a deuteron, or the nucleus of the hydrogen isotope
deuterium. This deuteron is then fused with another proton, creating a helium
isotope (2). Then two helium isotopes fuse, creating a helium nucleus and
releasing two protons, which facilitate the chain reaction (3). This final
split is so violent that one-half of the total fusion energy is carried away by
the two free protons. The second fusion variation, the carbon cycle, starts
with a carbon nucleus being fused with a lone proton (1). This creates a
nitrogen isotope. One proton then decays into it\'s primaries -- a neutron,
positron and neutrino. The positron and neutrino separate from the nuclei as
another proton fuses with the cluster. This creates a nitrogen nucleus which is
then fused with yet another proton, forming an oxygen isotope (2). One proton
then decays again as still another proton is forced into the nucleus (3). This
final fusion splits into a nitrogen and a carbon nucleus; the nitrogen carries
away the majority of the fusion heat, while the carbon goes back into the cycle.
The triple-alpha process, the last known variety, is perhaps one of the simplest
fusion reactions to understand. In this process, two helium nuclei fuse
together to form a beryllium nucleus (four protons and four neutrons) (1).
Almost immediately after this, another helium nucleus is forced into the cluster,
creating a carbon nucleus of six protons and six neutrons (2). In this reaction,
all of the heat given off is short-wavelength gamma rays, one of the most
penetrating forms of radiation. Each variety of fusion occurs depending on the
size and age of the star. This will affect core temperature, causing the
corresponding variety of stellar fusion. Now that fusion has been explained,
one can learn how it occurs in the different star types. All stellar bodies
start off as protostars, or concentrations of combusting gases found within
large clouds of dust and various gases. These protostars, under their own
gravity, collapse inward until itís core has been heated and compressed enough
to begin proton-proton fusion reactions. After that starts, a starís mass will
determine how long and through what kind of reactions it will go through.
Generally, there are three classes of stars which can form: dwarfs, sun-class
stars, and giants. Dwarfs begin as protostars of low size and mass (most
protostars fall under this category). These stars, which