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 it’s core has been heated and compressed enough
to begin proton-proton fusion reactions. After that starts, a star’s mass will
determine how long and through what kind of reactions it will go through.

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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 have on average less
than one-third the mass of our sun, go through very basic existances. One
variety is the red dwarf, which has at least one-third the mass of the sun.

Because of it’s low mass, the red dwarf is predicted to last thousands of
billions of years. The gravitational pressure of the star will cause the
proton-proton reaction to occur in it’s core, but after all the hydrogen has
been fused into helium, the star lacks the pressure to begin the triple-alpha
process. It is predicted that it will then contract into and inert, compressed
ball of gas known as a black dwarf. Another variety of dwarf is the brown dwarf,
which is so light (less than one-tenth the mass of the sun) that it lacks the
pressure to even begin the proton-proton reaction, and becomes a black dwarf
within just a few hundred million years, it’s nuclear fuels unexpended. Sun-
class stars are massive enough to move past the hurdle that the dwarves
encounter and continue on the fusion chain. With a mass of two to five times
that of the sun, the core of these stars rise to several million degrees Kelvin,
bringing the surface temperature to approximately 6,000 degrees. After ten
billion years, the inert helium in the core has compressed and the released heat
ignites a hydrogen shell around the core. The energy given off by the
combustion causes the stars size to double. The star continues to grow into a
super-giant, raising the core temperature so high that in what’s known as a
helium flash, the helium core fuses into carbon. The series of these reactions
causes varying shells of helium, hydrogen, and fusing hydrogen until the lack of
pressure to fuse carbon ends the fusion in the core, it’s gaseous surroundings
dissipating, leaving a highly compressed and hot ball of carbon known as a white
dwarf. Giants, the largest of all stars, have the shortest and most complex
lives of any of the stars. These bright blue monstrosities begin from
protostars which are hundreds of times the size of our sun. Within only a
hundred million years, the proton-proton reaction at the core ends. The star is
now six times the sun’s size, and almost four times as hot. Once the core has
changed to helium, the heat from it’s compression causes the star to double in
size. The star now makes it’s final journey into oblivion. Most stars end
their lives by lacking pressure to continue fusion and calmly fade into inert
masses. This is not the case with giant class stars. After a mere 9 or 10
million years, all of the hydrogen atoms in the core have fused into helium
(Figure 2.1). This causes a temporary pause to the fusion in the core, allowing
gravity to compress it. This compression raises the core temperature to 170
million degrees Kelvin (from 40 million degrees during the proton-proton
reaction phase). This energy is transferred to the hydrogen envelope
surrounding the core, expanding it to a thousand times the diameter of our sun.

After this, most of the events of importance that occur happen in the core.

With one million years to go, the collapse of the star raises the core
temperature enough to halt the collapse and fuse it’s core into carbon and
oxygen while fusing the outer shell into helium (Figure 2.2). It remains this
way for almost a million years. With a thousand years to go, most of the helium
in the core is gone. This again pauses fusion, and collapse continues. The
periods of collapse and fusion get increasingly shorter as time goes on. Once
the collapse raises the temperature to 700 million degrees Kelvin, the
carbon/oxygen core begins to fuse into neon and magnesium, creating layers
around the core that continue to fuse hydrogen into helium, and helium into
carbon (Figure 2.3). With a mere seven years to go, the core temperature of 1.5
billion degrees, the neon atoms in the core begin to fuse into more oxygen and
magnesium, giving the star an onion-like appearance, each layer being denser
toward the center (Figure 2.4). With one year to go, the core temperature
reaches two billion degrees, fusing the oxygen core into sulfur and silicon
(Figure 2.5). Only a few days to go, and the core temperature soars to three
billion degrees, fusing the core into tightly compressed iron, which has a mass
of almost 1.44 solar masses (the mass of our sun is one solar mass) (Figure 2.6).

Since iron cannot fuse into anything further, the core continues to collapse
under it’s own gravity. With a tenth of a second to go, the iron core is
collapsing at approximately 45,000 miles a second, packing the earth-sized core
into a sphere only ten miles across. The iron atoms become so compressed that
the nuclei melt together, creating enough heat to fill the core with neutrinos.

The core has now reached maximum crunch, meaning it can no longer contract
(Figure 2.7). The repulsive force in the core becomes so strong that it
overpowers the gravitational force, and the core recoils and projects matter in
a shock wave that bursts through all the outside layers. Almost one hundred
percent of the energy is released as neutrinos, the first outwardly noticeable
sign of the death of the star. The shock wave dissipates all of the surrounding
layers, leaving a small dense sphere composed of neutrons which is known as a
neutron star. This final explosion can be seen for thousands of years. Most
remain neutrino stars , but if the core had more than three solar masses, it’s
gravity continues to collapse it, condensing the star into a singularity, or
point of infinite mass and density. The gravity of this singularity is so great
that even light cannot escape. This is what is known as a black hole. Through
examining the above circumstances, one can now understand what solar fusion is,
and how a star is directly connected to it. And yet one must take the
information with a grain of salt. Scientists have only determined these facts
from the information they now have. Everyday new things are discovered that may
discredit all we believe to be fact. One can only hope that one day we as a
people can learn enough to prove once and for all the exact nature of the

Stellar Fusion
The Cosmic Ballet
Dylan Richards Chemistry 30 Mr. Hartley October 20, 1996
Time – Life Editors, Voyage Through the Universe – Stars. Time – Life Books Inc.,