Additional Physics Topic 15 - Fusion


This involves light nuclei, two isotopes of hydrogen, deuterium and tritium.




Question 1

What nucleons are found in the deuterium and tritium nuclei?



If two helium nuclei are forced together, they join together or fuse to form a helium nucleus, giving off lots of energy, more than in fission.  The process is easier to illustrate than to achieve.  The tritium and deuterium nuclei have to be slammed together by heating them to temperatures of millions of degrees Celsius before they fuse.


The reason that the temperature is so high is that nuclei, being positively charged, will tend to repel each other. The nuclei must be slammed into each other with enough kinetic energy to overcome the repulsive force and fuse.





The process has three stages:

1. Proton + Proton Deuterium + positron + electron neutrino

2. Deuterium + proton Helium 3 + photon

3. Helium 3 + Helium 3 Helium 4 + proton + proton

Some new words:

Since two protons are left over, the reaction is self sustaining.


Question 2

Answer the interactive gap-fill exercise.




The vast amounts of energy can be released in a massive explosion.  The amount of hydrogen involved in a hydrogen bomb explosion is tiny; it would fill a party balloon.  The most obvious use of fusion is the Hydrogen bomb. The amount of hydrogen fused is enough to fill a small balloon, from which a phenomenal amount of energy is released in a titanic explosion. Anything within 20 km is fried to a crisp.


However it would be a wonderful idea to harness such huge amounts of energy to generate cheap electricity. Hydrogen is plentiful. A cubic metre of hydrogen could keep a power station going for several days. And scientists at the Culham Laboratory near Oxford have been trying to do just that. There are some problems:

The huge machine, called a torus due its doughnut shape, has produced fusion energy for a couple of seconds at a time but far more electricity has had to be put in to keep the machine going. The biggest problem is keeping the plasma stable; it has a habit of going out.  Achieving controlled fusion has continued to prove challenging, and commercial fusion power stations remain a distant prospect.


Picture by David Burke, Wikimedia Commons


Question 3

Why has it been so difficult to get energy out of fusion?




Fusion in Stars

Space is not empty; there are huge amounts of gas (mostly hydrogen) and particles of dust. These form large clouds called nebulae.


Although gravity is a tiny force, it attracts the dust and gases together over millions of years. The dust and gas might come together more quickly if there is the shockwave of an exploding star (a supernova)


The dust gets squashed together and gets hot. It glows and gives out heat. It's a protostar.


If the temperature gets hot enough, about 15 million Kelvin, hydrogen nuclei start fuse together in a process called nuclear fusion.


Hydrogen Helium + energy


A lot of energy is given out in the process, and, if the star is big enough, the reaction becomes self sustaining.  Fusion is the process that fuels stars.  In the Sun, four million tonnes of hydrogen fuel is consumed every second.  This sounds a lot, but the Sun has enough fuel to keep burning for another 4500 million years, by which time we will all be long gone and forgotten.


  Do not write "fussion".  This indicates that you are not sure of the difference between fission and fusion.  You will not score any marks.



Comparing Fission and Fusion

Question 4

Answer the interactive gap-fill exercise on fission



Question 5

Answer the interactive gap-fill exercise on fusion



Question 6

Have a go at the crossword that gets you to think about nuclear physics.




Birth of a Star

Stars are usually born in clusters.  Gravity is the driving force behind the birth of a new star.  Gravity is a very weak force but has an infinite range.  Gravity is always attractive, never repulsive.  It pulls the particles in a gas cloud together, and they accelerate inwards.  The process is very slow indeed, but there is all the time in the universe for it to happen.


NASA, Wikimedia Commons


As the gas cloud collapses and heats up, it will emit significant amounts of infra-red radiation.  This is known as a protostar.  At this stage the temperature is still too low for nuclear fusion to happen.  If the mass is too low, the failed star ends up as a brown dwarf.  Some astronomers consider Jupiter to be a failed star.


As the material gets hotter, molecules are torn apart and atoms are ionised.  As the mixture gets hotter still a plasma is formed where atoms are stripped of most, if not all their electrons.  Finally an ignition temperature is reached and fusion starts.  The temperature is about 15 million oC. 



Stable Phase of a Star

Nuclear fusion releases a lot of energy.  Humans have achieved it but only in the context of an explosion that would make a thunderclap like a whisper.  The largest fusion bombs used deuterium (an isotope of hydrogen) to produce Helium.  The amount of fused gas used would fill little more than a large party balloon.  So why does a star not fly apart?


There are two opposing forces:

In the life time of a star, the two forces balance each other out and the star remains the same size.



Question 7

How does a star stay the same size?




Dying Star

The centre of a star is where the fusion takes place.  The outer regions are not hot enough and there is still hydrogen in a shell about the core.  There is no convective circulation of gases into the core.


As fusion dies down, the expansive pressure reduces and gravity pulls the gases in.  They heat up and the pressure on the helium in the core rises.  Helium nuclei fuse to form heavier elements.  Hydrogen fusion increases in shells outside the core.  Therefore there is more helium and the core expands.


Meanwhile while the outer shell where there is hydrogen fusion moves outwards, and the star swells.  The star becomes a giant.  The core and the hydrogen fusion shell are relatively small, while the majority of the space of a red giant is taken up with a low density envelope.



When the Sun has reached this stage, all the inner planets would have been engulfed and fried to a crisp.  Jupiter and Saturn will lose their gas layers to reveal their rocky cores.


In the Sun, elements like carbon and oxygen will be formed in the core.  In more massive stars the conditions in the core will be sufficiently extreme further fusion will take place so that, for example, silicon nuclei will fuse to form iron (the most stable nuclide).


Question 8

Which elements would you expect to find in the Sun, other than carbon and oxygen?




Death of a Star

Eventually gravity will overcome the expansive force.  In small stars, there is convection so that all the hydrogen fuses.  However the temperature never gets hot enough for helium fusion to happen.  The star collapses under gravity to become a white dwarf.  The volume is about the same as that of the Earth.  Eventually it cools to a black dwarf, a forlorn lump in space.  These have never been seen as the Universe is simply not old enough.


For Sun like stars, death is more spectacular.  The star is expanding in the outer layers and contracting at the core.  The radiation pressure acting outwards pushes the outer layers away from the core to form a planetary nebula, a ring of gas that glows brightly because of the intense radiation from the core.





Novae and Supernovae

With more massive stars, where the core has a mass of more than 1.4 solar masses,  the limit is no longer observed and electrons start to combine with protons to form neutrons, releasing neutrinos and causing the core to collapse in on itself.  The collapse takes less than 1 second, and the density rises to extreme values. We have a neutron star.  The outer layers also collapse in to collide with the dense core, which can no longer be compressed.  The material bounces back as a shockwave, which takes as little as 45 minutes to propagate through the material of the star.  The material is torn away in a titanic explosion called a supernova.



Photo NASA


Often there is a cloud of gas, a nebula, which propagates away from the site of the explosion.  This can be seen in the picture below:


Photo NASA


Iron is the largest element that can be caused by fusion.  It is made in the largest of stars.  It requires the extreme conditions of a supernova to make elements that have nucleon numbers greater than 56.  So copper, with a nucleon number of 64, will have been made in a supernova explosion, as have many of the other rare elements in your computer.



Neutron Stars

The resulting neutron star is very small compared to the original star.  A sun-like star would give a neutron star of diameter 30 km.  The material in the Earth would be occupy a ball about 200 m across that would fit on top of your school (and squash it flat).


Some neutron stars rotate rapidly and give of beams of radiation with a regular period.  They are called pulsars.  The frequency of rotation can be as high as 30 Hz. 




Such a star will be spinning at a rate of 900 times a minute.  When a pulsar was detected using a radio telescope in 1967, the astronomers thought initially that the regularity of the pulse suggested a civilisation.  They dubbed the pulsar LGM-1 (Little Green Men - 1).  The radio waves come from the interaction of charged dust particles with the magnetic field.



Black Holes

In very massive stars gravity is so great that the core keeps on collapsing, shrinking away, according to the theory, so that it is no more than a point in space.  This point is called a singularity, and the laws of Physics no longer apply.  The gravity field in a black hole is so strong that even light cannot escape.


The star is surrounded by an event horizon, inside which nothing can be seen.  It is the boundary at which light cannot escape.  Abandon hope all ye who enter here. 


You cannot see a black hole.  But you can tell where it is, as there's a black hole as jets of high energy particles are ejected


Picture from NASA, Wikimedia Commons


A passing star is gobbled up by a black hole:


Picture from NASA, Wikimedia Commons


Astronomers believe that at the centre of most galaxies is a super-massive black hole, a black hole of mass of 108 times the mass of the Sun.



Question 9

What is meant by a black hole?




The picture below summarises the life of a sun-like star:


And the picture here summarises the life of a star that is much bigger than the Sun:



The fate of the star following a supernova explosion depends on its size.  The largest stars end up as black holes.


Question 10

Complete this diagram of the life cycle of a star