Fusion is a nuclear process
in which two light nuclei combine to form a single heavier nucleus. An
example of a fusion reaction important in thermonuclear weapons and in
future nuclear reactors is the reaction between two different hydrogen
isotopes to form an isotope of helium:
 |
This
reaction liberates an amount of energy more than a million times
greater than one gets from a typical chemical reaction. Such a large
amount of energy is released in fusion reactions because when two light
nuclei fuse, the sum of the masses of the product nuclei is less than
the sum of the masses of the initial fusing nuclei. Once again,
Einstein's equation, E=mc2, explains that the mass that is lost it converted into energy carried away by the fusion products.
Even though fusion n is an energetically favorable reaction for
light nuclei, it does not occur under standard conditions here on Earth
because of the large energy investment that is required. Because the
reacting nuclei are both positively charged, there is a large
electrostatic repulsion between them as they come together. Only when
they are squeezed very close to one another do they feel the strong
nuclear force, which can overcome the electrostatic repulsion and cause
them to fuse.
Fusion reactions have been going on for billions of years in our
universe. In fact, nuclear fusion reactions are responsible for the
energy output of most stars, including our own Sun. Scientists on Earth
have been able to produce fusion reactions for only about the last
sixty years. At first, there were small scale studies in which only a
few fusion reactions actually occurred. However, these first
experiments later lead to the development of thermonuclear fusion
weapons (hydrogen bombs).
Fusion is the process that takes place in stars like our Sun.
Whenever we feel the warmth of the Sun and see by its light, we are
observing the products of fusion. We know that all life on Earth exists
because the light generated by the Sun produces food and warms our
planet. Therefore, we can say that fusion is the basis for our life.
When
a star is formed, it initially consists of hydrogen and helium created
in the Big Bang, the process that created our universe. Hydrogen
isotopes collide in a star and fuse forming a helium nucleus. Later,
the helium nuclei collide and form heavier elements. Fusion is a
nuclear reaction in which nuclei combine to form a heavier nucleus. It
is the basic reaction which drives the Sun.
Lighter elements fuse and
form heavier elements. These reactions continue until the nuclei reach
iron (around mass sixty), the nucleus with the most binding energy.
When a nucleus reaches mass sixty, no more fusion occurs in a star
because it is energetically unfavorable to produce higher masses.
Once
a star has converted a large fraction of its core's mass to iron, it
has almost reached the end of its life.
The fusion
chain cannot continue so its fuel is reduced. Some stars keep shrinking
until they become a cooling ember made up of iron. However, if a star
is sufficiently massive, a tremendous, violent, brilliant explosion can
happen. A star will suddenly expand and produce, in a very short time,
more energy than our Sun will produce in a lifetime. When this happens,
we say that a star has become a supernova.
While a star
is in the supernova phase, many important reactions occur. The nuclei
are accelerated to much higher velocities than can occur in a fusing
star. With the added energy caused by their speed, nuclei can fuse and
produce elements higher in mass than iron.
The extra energy in the
explosion is necessary to over come the energy barrier of a higher mass
element. Elements such as lead, gold, and silver found on Earth were
once the debris of a supernova explosion. The element iron that we find
all through the Earth and in its center is directly derived from both
super novae
and dead stars.
and dead stars.
More
peaceful uses of fusion are being researched today with the hope that
soon we will be able to control fusion reactions to generate clean,
inexpensive power.
Source:
http://www.lbl.gov/abc/Basic.html
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