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How Stars Produce Energy
The sun warms the earth and allows life to exist. It provides us with most of our energy and with
light. But how does it keep giving off light? Will it ever stop shining? The answer to the 2nd
question is that the sun has been burning for 5 billion years and should burn for another 5 billion
years. What does it use for fuel?
The fuel for the sun and other stars is hydrogen, the simplest of atoms. It is a single proton
surrounded by one electron. The sun’s mass is about 75% hydrogen and because it has a mass of
approximately a million times the earth it has enough fuel to continue to burn for another 5
billion years. The sun doesn’t burn its fuel in the same way hydrogen burns on the earth. On
earth, hydrogen and oxygen burn to form water in a chemical reaction. In a star, hydrogen goes
through a nuclear reaction, which gives off much more energy than a chemical reaction.
Nuclear reactions bond 4 hydrogen atoms to form 1
helium atom in the sun’s core, as shown in the diagram.
This process is called fusion because the 4 small atoms
are fused (joined) to form a larger atom. (Fusion is the
opposite process from fission, which is used to power
nuclear reactors on earth. Fission breaks a larger atom
apart and makes smaller ones). When the hydrogen
fuses into helium, a small amount of mass is lost – about
0.07%. This confused scientists for years until Albert
Einstein’s equation explained it: E = mc2 The
missing mass is converted into energy and can be
calculated. This energy is what makes the star shine.
In order to have hydrogen fuse, positively charged protons must be brought together to form a
larger atom. However, like charges repel, so the protons push each other apart. Fusion occurs
when you get the protons close enough together that another force, called the strong
nuclear force, overpowers the electric force. Only then will the helium atom form.
But how can the positively charged protons get close enough to fuse? Fusion occurs in the
center, or the core of the sun. In the core, the temperature is very hot, 10 million degrees. The
pressure is also huge because of the weight of the rest of the sun is pushing on it. Since atoms
move faster when they are hot, they move very fast at the high temperatures in the core. The
electric force is not strong enough to stop them. They collide and join together.
Fusion occurs only in the core of the sun. The core is at a high enough temperature and pressure
for fusion to occur. The high pressure makes the hydrogen very dense, meaning that the atoms
are very close together. In order for 2 protons to fuse, the positively charged protons must hit
other protons head-on. If the gas is very dense, the chances of having a proton hit another proton
head-on are much greater. Therefore, the core of the sun, with its high temperature and very
dense gas, is the only place fusion can occur.
The energy released by fusion results from the strength of the nuclear force that holds the helium
nucleus together. It is very, very strong. Once it over-powers the electric force, it releases huge
amounts of energy. This energy is millions of times more than is released in traditional chemical
burning.
Fusion occurs when the strong nuclear force can pull two nuclei together. In order to overcome
the normal repulsive electric forces of the protons, the nuclei must have lots of kinetic energy
(energy of motion). A very high temperature can give the nuclei the high kinetic energy needed
to fuse together.
Fusion on earth hasn’t been achieved because it must overcome the same obstacles. Hydrogen
bombs use atomic bombs with plutonium or uranium to heat and compress the atoms that are to
be fused. Attempts to get controlled fusion on earth have involved either using lasers to heat and
compress or use of magnetic bottles. Neither of these methods has yet been successful in giving
off more energy than has been put in the system to get it started.
Nuclear reactions allow the sun to give off a great deal of energy because they are much more
energetic than chemical reactions. An atomic bomb can be made out of 20 lbs. of plutonium and
have the explosive power of more than 1000 tons of TNT. This difference between chemical and
nuclear energy is hard for most people to understand. A star could not produce enough energy
with just chemical reactions.
The energy produced in the sun’s core actually takes thousands of years to be released. This is
because the sun is so huge. In the inner part of the sun, the heat comes out by radiation, while in
the outer part it comes out by convection. Because the sun is so dense, the transfer of heat by
radiation is very slow. Photons keep hitting atoms on their way to the outside. Because the
atoms are so closely packed, a photon bounces around for thousands of years before it reaches
the area where convection occurs.
Review Questions:
1. What is the main fuel that stars use to generate their energy? (same element most stars
are made of)
2. In a star this substance (from #1) doesn’t really BURN as it does on the earth. Fully
explain how it works.
3. Explain the main difference between the processes of FISSION and FUSION.
4. When helium forms in a star some mass from the initial products is “lost”. What happens
to that missing mass? Who figured it out? List the equation that calculates it.
5. How long has the sun been “burning” for? How much longer will it continue to “burn”
for?
6. Why can’t we get fusion to work efficiently on the earth?
7. Why does it take so long for energy produced in the core of the sun to be released?
8. Electrical force says that Protons (+) charges want to repel each other. How do they
eventually join together?
9. What is the approximate temperature inside the core of the sun?