You and I are solar powered, at least indirectly. It is the
Sun's energy that grows plants that (one way or another) we eat and get our
energy from. Until Einstein derived E = mc2 from his
of relativity it was a complete mystery as to what the fuel source of the Sun
was. For example, it had been calculated that if the Sun was made of coal it
would use up its total fuel supply in about 6000 years. With an ever-increasing
understanding of the age of the Solar System, based mainly on geological
research, it was clear that there must be a process going on within the Sun that
we didn't understand. We now know that the Sun is about 4.5 billion years
old and has about the same length of time before it will have used up all of its
fuel. This page deals with nuclear fusion, and in doing so explains why the Sun
can produce so much energy over such a long period of time.
We are all solar powered
We have seen in previous pages in this
series that a heavy atom,
such as uranium, can "fall apart", that is, undergo
fission. When this
happens, a little of the mass of the original atom is turned into energy. It's
also possible to turn mass into energy by taking less massive atoms, such as hydrogen,
and squeezing them together to form another type of, and heavier, atom. This process
is called nuclear fusion.
Fusion can occur with many different kinds of atom. In fact,
over a third of all the different kinds of atoms, when fused, release energy. This is a point we will
return to later, but for now we will concentrate on the simplest form of nuclear
fusion; that of hydrogen.
Hydrogen is the simplest of all atoms. The first isotope of
hydrogen contains nothing but a single proton, with a single electron in
"orbit" around it. If the hydrogen atom is given energy (for example,
by heating it or speeding it up) the electron is "shaken" away and we
are left with just a proton. Strictly speaking we should now call the
"atom" an "ion", but in this page we will continue to call
it an atom in order to keep things simple (with apologies to chemists).
A proton (i.e. the nucleus of a hydrogen "atom") has a positive
electrical charge, that is, it acts like the positive end of a magnet. If we
bring two protons together they repel each other. The closer we try to push the
protons together the more energy we need to overcome the repulsion. You may have
experienced something similar yourself. If not, find two magnets and try to push either both
the negative or both the positive ends together. You will find that when the
magnets are far apart they are easy to move towards each other, but as they get
closer more and more energy is required in order to push them together.
If we apply a lot of energy (on an atomic scale) we
overcome the magnetic resistance and the two protons stick together; they have fused.
In doing so they give up a little of their mass in the form of energy. In fact, the energy released is greater than the energy that was required
to force the two protons together. We now have a source of energy: nuclear
Why is energy released during the fusion of hydrogen? We said
that when two protons are forced together they fuse. However, that's not all
that happens. What actually happens is that one of the protons changes into
another particle; it "transmutes" from being a proton into being a
neutron. Not only that, but in doing so it ejects two further particles: a
positron (a positively charged electron) and a strange, almost mass-less,
particle called a neutrino. We can show this in a schematic diagram:
We now have the nucleus of an atom that is the second isotope of
hydrogen, called deuterium (d). It contains one proton and one neutron.
The positron and neutrino go flying off with kinetic energy supplied by
converting some of the mass of the transmuted proton into kinetic energy, in
accordance with E = mc2.
When the Sun was formed about 4.5 billion years ago, it did so out of a huge cloud of gas.
Most of that gas was hydrogen, but it also contained some helium (about 30%) and
small amounts of many other elements such as carbon, oxygen, silicon, and so on. The gas
cloud contracted under its own gravity and started to spin, in doing so ejecting
most of the heavier elements, some of which became the planets, asteroids and
comets, and some of which eventually ended up as you and me. What remained was a huge ball
of mostly hydrogen and helium that we now call the Sun.
The gas in the Sun continued to contract under its own gravity until the
pressure at the core grew to enormous proportions (about 100 billion times the
pressure of air on the Earth). A law of nature states that if you squeeze anything
it heats up. As the centre of the Sun became more and more compressed the
temperature at the core reached about 15 million degrees Celsius (59
million degrees Fahrenheit). This meant that the protons (hydrogen "atoms") at
the core possessed, in atomic terms, huge amounts of energy. So much energy in
fact, that some of them could overcome any magnetic resistance and fuse into deuterium,
releasing even more energy in the process. This is the first stage in the Sun's
source of energy:
Further energy is provided by the positron. This is a form of antimatter.
It has exactly the same properties as an electron (in terms of
mass etc.), but has an
opposite (and equally strong) electrical charge. The Sun's core contains many "free" electrons in what is called a "plasma" (a
sort of high energy gas). The positron soon meets an electron and when it does
so the two annihilate each other producing a high-energy photon, i.e.
"light". Our star is shining, but not yet in the visible part of the
The second major stage is the formation of helium-3 (He-3), again by a process
of fusion. There are four isotopes of helium, He-3, -4, -5, and -6. He-5 and -6
have short half-lives (in the case of He-5 only 6 x
10-20 seconds!) and we will not be concerned with them here.
Each helium atom, by definition, has two protons.
We now have an "atom" of deuterium (a proton and neutron)
in a sea of highly energetic hydrogen "atoms" (protons). Sooner or
later (but usually sooner!) the two types of atoms will collide and fuse. When they do so
they combine to make the atomic nucleus of helium-3, and eject yet another
Lastly, the third stage is the production of helium-4. In this process, two
He-3 "atoms" come together and fuse, releasing two protons. The two
protons fly off in separate directions and go back to being hydrogen
"atoms", from which they can take part in the whole process again. The process
looks like this:
Throughout each stage some mass was converted into energy and a
total of 6 high-energy photons was produced. However, we still
haven't seen any visible light. The visible light from the Sun is due to the
ejected particles jostling with other atoms and so transferring some energy to
them. This causes the latter atoms to emit photons with a wide range of
frequencies, including those in the visible part of the spectrum. At last, we
have visible sunlight!
All three stages taken together is sometimes called the
The Total Energy
The total energy released in all three stages is around 4 x
10-12 J (25 MeV). This is less than is produced
during in a single uranium-235 fission process. However, if we take the total
number of particles into account in each process, we find that we get
around 7 times more energy per particle in hydrogen fusion than we get from uranium-235 fission. In other words, it is a much
more efficient process. Even
so, only around 0.7% of the mass of the Sun's protons eventually ends up as
The energies we have been talking about, while large on an atomic
scale, are still very small in everyday terms. However, there is something we must take into account about the Sun, and that is it's enormous! It has
the same volume as 1.3 million planet Earths. Some numbers about the rate
at which nuclear fusion takes place in the Sun will be instructive:
There are around 8.5 x
1037 fusion cycles per second at the Sun's core.
This leads to a total energy output from the Sun of around
1026 Joules per second.
The Sun converts 4 million tonnes (4.4
million tons) of mass into energy every second.
Each square centimetre of the Sun's surface is as bright as
a 6000W light bulb.
The amount of mass converted into energy at the Sun's core is
stunning. However, the Sun has a mass of around 2 x
1030 kg and has enough hydrogen to continue its proton-proton
chain for around another 4 billion years. After that, as we shall see, the Sun
will use another process to keep shining for a "little" longer.
Lastly, it's worth mentioning that the
"sunlight" produced at the core doesn't just fly off into space. As has been
seen, the proton-proton chain doesn't even produce light in the visible part of
the spectrum. Instead, the high-energy protons and resultant kinetic energy
produced induces other atoms in the Sun to vibrate and, in turn, release photons
of many different frequencies, including those in the visible part of the
spectrum. These photons are re-absorbed and re-emitted by adjacent atoms, each,
on average, slightly closer to the surface of the Sun. Finally, after an average
of about 100 to 200
thousand years an atom at the surface of the Sun absorbs and then
re-emits a photon, which flies off into space. Then, if heading in our
direction, it takes around only 8.5
minutes for the photon to reach the Earth. All of the daylight that we use to see by
started on its journey a very long time ago.
Fusion is possible in atoms other than hydrogen and helium. In
fact, fusion, one way or another, is possible with almost any kind of atom. In
nature, the heaviest atoms that we could encounter are of uranium, but
scientists have been able to "build" heavier atoms in atomic
accelerators, such as the one at CERN. This is
done by firing atoms at each other until they "stick" (fuse) together. Most
of the resultant atoms have very short half-lives. Another feature of such
fusion is that it takes a lot of energy while little, or even none, is given
It turns out that the atom that gives up the most of its mass in
the form of energy during fusion is the humble hydrogen atom, i.e. the lightest
atom of them all. As we progressively fuse heavier and heavier atoms we get
progressively less and less energy back out. In fact, we get at least some
energy back out all the way up to the element containing 26 protons. That
element is iron (Fe). For iron, and all heavier atoms, we need to put more
energy in than we get back out in order to achieve fusion. This is called an
As we have seen, the Sun's fusion is exothermic, that is, more
energy is "produced" than was entered into the system. We have also
seen that the Sun is slowly converting its hydrogen into helium-4. Once the Sun
has used up all of its hydrogen in this process its core will contract because
there is now not enough radiation ("light" and so on) being produced
to withstand the gravitational pressure from the outer layers. Although the core
contracts, the very outer layers of the Sun will expand, possibly out to the
orbit of Mars (the Earth will be turned into a wisp of gas in the process). The
Sun will then be what is a called a "red giant". The contraction of
the core raises its pressure, and by doing so also raises its temperature. This
results in there being enough energy for the He-4 to begin to fuse into (mostly)
carbon and oxygen, and so produces enough radiation energy ("light")
to stabilise the star. Once the He-4 has been used up no further fusion will
take place at the core, and the Sun will contract again into a state called a
"white dwarf". At that point, the Sun will no longer be
"alive" as such, and will just continue to cool over many millions of
years, eventually becoming a cold "black dwarf":
Apart from the fact that we live near it, there is nothing particularly
special about the Sun. It's just an average star. Some stars are much heavier
than the Sun, and these go through further fusion chains, producing even heavier
elements. A detailed description of all the phases in a very massive star's life
is beyond the scope of this page, but we will look briefly at what happens to a
star with a mass greater than about four times that of the Sun.
Such massive stars start by converting hydrogen into helium,
then, when the hydrogen is used up, helium into carbon and oxygen, just as in
the Sun. However, the core of such a star is so massive that when the helium has
been used up it gravitationally contracts still further, and produces yet
heavier elements via the fusion process. These contractions continue until the
star starts to fuse its remaining elements into iron. As we have noted, fusion
into iron is endothermic; it takes more energy to do than we get back out. For
the star there is now no radiation (of any significance) being produced to stop
the outer layers from gravitationally collapsing onto the core. Within a few
seconds of the star's core starting to fuse into iron, the outer layers start
accelerating inwards at tremendous speeds. As they do so they become ever more
dense as more and more material is compressed together and accelerated. The
highly compressed material eventually reaches near-light speed (in only a matter
of seconds), before it crashes down onto the dense core of the star. When this
happens the core is compressed slightly, then bounces back with such enormous
force that it blows the outer layers back out into space. The star has,
effectively, blown itself apart in what is called a supernova explosion. The
energy released in a single supernova is more than many billions of atom
bombs being detonated all at once.
The picture below shows the remnants of a supernova. This was a
"nearby" (163,000 light years away) supernova that was recorded in
1987, and given the name SN1897A. The ring of material that is expanding away
from the central core is actually a bubble of gas, but we are looking at it edge
The energy released in the explosion is such that many different types of atoms
in the expanding gas shell are fused together to form all the elements heavier
than iron; such as copper, gold and uranium. Next time you see some gold try to
remember that it was created billions of years ago by a process of fusion during the last moments of a
massive star's active life!
Generally, what is left of the star is a rapidly expanding bubble of gas
rushing off into space, and a super-dense central core composed almost entirely
of neutrons. The pressure of the outer layers crashing down onto the central
core during the initial implosion causes the atomic protons and free electrons
in the core to fuse into neutrons. The star is now called a neutron star. From
this point on, the bare core that the star has become starts to cool, emitting
energy into space as it does so.
For even more massive stars the end result is a black hole. These stars have
so much mass that as the outer layers hit the core they compress it into such a
state that even the resulting neutrons are compressed into other particles,
possibly, for a very short time, the building blocks of neutrons and protons, called
quarks. This results in such an enormous mass being squeezed down into such a
very small size that, under the pressure of its own gravity, the core continues
to collapse until it becomes infinitely small. The gravitational pull of such an
object is so strong that not even light can move fast enough to escape it, and
we have a "black hole".
The Crab Nebula.
The supernova that created the Crab Nebula was seen on Earth in AD1054.
What remains is an expanding gas cloud and a central neutron star.
The process of hydrogen fusion that is the source of energy for
stars is the same as is used in H- (hydrogen) bombs. Briefly, in an H-bomb, a
small amount of hydrogen (actually the isotopes deuterium and tritium) is
compressed together to the point at which it fuses into helium, releasing enormous
amounts of energy. It takes a huge amount of pressure to compress hydrogen to
the point at which it fuses, and in order to make such a bomb light enough to be
delivered by an aircraft or missile, the hydrogen compressing "agent"
is actually an atom bomb surrounding the hydrogen.
Nuclear (atomic and hydrogen) weapons often have their "yields"
(explosive power) measured as the equivalent of the energy released in a quantity
of the explosive TNT, measured in tons. An atom bomb's power is typically measured
in kilotons (thousands of tons). The two atomic bombs dropped on Japan and the
end of World War II were in the 20 kiloton range. The more powerful H-bombs, however,
have their yields typically measured in megatons (millions of tons).
On detonation, an H-bomb produces He-3 and He-4 in the same way
as the Sun. While these two elements are harmless in themselves, the atoms move
at such high speeds that they can damage anything around them. They can also
strike other, heavier, atoms, breaking them down into harmful radioactive
elements. And, of course, there is the radiation caused by the triggering atom
bomb. For these reasons, nuclear weapons tests were eventually carried out
under ground, where most of the radiation could be contained. Even so, each
detonation caused environmental damage and still released a quantity of
radiation (from the atom bomb component).
As described in another page in this
series, nuclear power
stations use nuclear fission (the breaking apart of heavy elements) as their
source of power. This has a number of disadvantages (but not any more than
conventional power stations, depending on your point of view). For example,
using fission as a power source produces radiation, and, while it is mostly
contained, there is the problem of both decommissioning old nuclear power
stations, and the question of what to do with the contaminated waste products.
Allied to that, as with conventional power stations, there is only so much of
the raw materials available from which to produce the power. Great advances have
been have made in using other, renewable sources of power, such as energy
derived from wind, wave and solar sources. However, because of the limited total
renewable energy incident on the Earth, these sources of power will never be
enough to meet all of the world's rapidly growing demands, and they are often
expensive ways of producing useable power in the first place. All in
all, it would be good if we could find a cheap, safe and abundant source of
energy. So why not use controlled fusion?
The answer is simple: we don't know how to,
at least not yet. Controllable,
large scale hydrogen fusion is one of the holy grails of modern physics. There
are three points to note about controlled hydrogen fusion:
We have an almost infinite supply of the raw materials, in
We can easily, and safely, extract the hydrogen in sea
water. Using the fusion process there is enough energy in about a gallon
(about 4 litres) of water to supply America's energy requirements for a
whole day. The whole world would only need a few of buckets of
water a day.
It's completely safe:
The end products of hydrogen fusion are helium-3 and
helium-4. Helium in itself is non-reactive and is therefore very safe (we
even fill children's balloons with it). It is very easy to slow the helium
down in a contained and safe way (in a sort of piston, for example).
At the moment, it is very, very expensive:
Controlled hydrogen fusion has been achieved in very large
machines such as particle accelerators. Hydrogen is put into the system and
accelerated or heated thereby fusing
into helium, but in each case the system used is experimental and very
The goal is to be able to find a way of making the process
practical and cheap. This is thought to be so important that every major
industrial nation on the planet has access to accelerators, with the hope of
finding a way of making the fusion process work efficiently. There isn't even
any guarantee that it will work, but if it does we will have solved all of our
energy problems both now, and a long way into the future. To this end the
industrialised nations have agreed to build a single experimental fusion reactor
which is hoped to be completed by about 2015. The task is so costly and complex
that it needs a large international effort and many thousands of scientists. The
design consists of a ball of very hot plasma (a high energy gas)
contained in a magnetic field, the energy of which is enough to fuse atoms
If we do ever solve
the problem of controlled nuclear fusion, at its root will be the the conversion
of mass into energy, in accordance with this little equation:
E = mc2