The Sun and Nuclear Fusion

This science article tells us about the sun; what it is, what it is made of and how it creates all that heat and energy.The sun is a star. It is not just any star. It is our star; it provides us with the energy without which life on our planet could never have evolved.

It is the only source of heat and light for maintaining life on earth. Because it is so close it feels very hot; it can give us sunburn and damage our eyes if we look straight at it.

shadow of woman apparently holding the sun whilst standing in the sea

The sun is no different to many other stars; we just happen to be near it so it looks bigger and brighter than other stars.

sunset demonstrating how close the sun is to our planet

The sun is 149,600,000 kms (92,957,130 miles) from the earth. This diagram compares its size as it would appear from different planets.

Comparison of size of sun as seen from different planets

A distance of 149,600,000 kms is tiny compared to the distance from earth to other stars outside our solar system.

 Mount Rainier, Washington stars at night© Dave Morrow

To get an idea of other stars’ vast distance from us think about this scale model:

*reduce the sun to the size of a soccer ball.
*place the ball on the half way line of a soccer pitch; drop a peppercorn, representing the earth, 25 meters away.
*Proxima Cenauri, our closest star, would be 6,500 kms (4,038 miles) away!

Proxima Centauri is ‘only’ 4.24 light years in distance from earth but is too faint to be seen with the naked eye.

This is what it looks like through the Hubble space telescope:

Proxima Centauri our nearest neighbour seen through the Hubble space telescope

What the sun is made of

The sun is a massive ball of gas; it contains 92.1% hydrogen atoms, 7.8% helium atoms and 0.1% atoms of other elements. 99.86% of the total mass of the solar system lies within the sun, with the remaining 0.14% of mass contained inside the planets.

chemical composition of the sun showing different elements

The sun’s force of gravitational attraction

As we learnt in Understanding Gravity and Mass the larger and more dense an object is, the stronger the force of gravitational attraction towards that object’s center.

The sun is far larger than the earth; 1,300,000 earths could fit inside the sun!

Sun and Earth compared in size

Despite being 74.5% less dense than the earth, the sun’s greater mass means that it has a far larger force of gravitational attraction.

At its ‘surface’ the sun’s force of gravitational attraction is 28 times greater than the earth’s.

First requirement for nuclear fusion- extreme pressure

  • So what is it about the sun vast mass that allows ‘nuclear fusion’ to take place in its core?

The sun’s force of gravitational attraction pulls all the overlying layers of gas down towards its center, creating massive weight and huge pressure in the core.

The distance from the core to the ‘surface’  is 696,000 kms.( 432,450 miles)  That’s is a vast amount of gas!

suns interior showing the core Image credit: engineering scale

The pressure in the core reaches an incredible 250 billion atmospheres. By way of comparison the pressure in earth’s core is 3,500,00 atmospheres and the pressure 700 meters underwater is 70.4 atmospheres.

woman wearing an exo suit walking on the sea bed

Second requirement for nuclear fusion- extreme heat

The sun’s core needs to reach a temperature of 9,999,726 obefore nuclear fusion can take place. After nuclear fusion has taken place the temperature in the core rises even higher to 15,000,000 oC!

  • How did the core reach 9,999,726 oC?

To answer this question we need to take a brief look at the formation of the sun which took place 4.6 billion years ago.

The sun was formed out of a swirling cloud of dust and gas known as a ‘nebula’.

part of the carina nebula taken from the Hubble Space telescope

Under the influence of gravity, dust and gas started sticking together (‘accreting’) to form a dense cloud. This dense, swirling cloud accreted more dust and gas until it eventually collapsed under the pressure of its own weight.

After the nebula collapsed the protosun was born. Gas and dust from surrounding circular discs of matter continued to accrete to the proto sun, increasing its mass and raising the pressure at its core.

proto sun surrounded by solar nebula

The circular discs of matter that did not accrete to the sun would eventually form the planets.

As the pressure in the core increased so did the temperature of hydrogen gas in the core.

  • So how did increased pressure create hotter temperatures in the core?

It is a well known phenomenon that increasing the pressure of gas raises the temperature of that gas. For example when you pump up the tyres of your bicycle the air entering the tyre is compressed as it passes through the pump, making the pump feel hot.

air is compressed as it passes through a pump

Third requirement for nuclear fusion- the presence of hydrogen

Only if hydrogen is present in the core could nuclear fusion take place. The core of the sun has hydrogen in abundance.

one proton and one electron in each hydrogen atom

A single hydrogen atom has one positively charged proton in its nucleus and one negatively charged electron ‘orbiting’ each proton.The positively charged protons naturally repel each other.

Protons resist fusing together

At the incredibly high temperatures in the core, the electrons become detached from their ‘orbit’ around the protons; they are no longer bound to a proton and become ‘free’.

A different state of matter called ‘plasma’ is now formed. (other states of matter include solids, liquids and gases)

Hydrogen plasma equal numbers of protons and electrons

Under these conditions of extreme pressure, protons are squeezed tightly together. Not only are they squeezed very tightly, but the high temperatures that exist in the state of ‘plasma’ mean that the protons become very ‘excited’ and move around at great speed.

demonstration of how protons collide together at great

The chances of protons joining together are now greatly increased; magnetic resistance to two positively charged atomic particles joining together is overcome.

enormous pressure fuses protons together

  • What happens next?

Protons collide and NUCLEAR FUSION takes place!! Two protons fuse (join) together and produce one helium atom.

nuclear fusion showing two protons fusing together

Two additional protons are converted into neutrons. The two neutrally charged neutrons (converted from protons) attach themselves to the positively charged protons. This ensures that the positively charged protons remain ‘fused’ and unable to repel each other.

nuclear fusion showing two protons and two neutrons

The newly formed atomic nucleus also attracts two negatively charged electrons. A helium atom has now been formed made up of four protons and two ‘orbiting’ electrons.

Helium atom has two electrons, neutrons and protons

So hydrogen, the lightest element in the universe, is converted into a heavier element, helium. The process of converting hydrogen into helium (repeated on a massive scale) produces light, heat and energy.

shadow of a woman apparently holding the evening sun in her hands©

  •  How is light, heat and energy produced when hydrogen nuclei fuse together to form helium atoms?

One helium atom has a smaller mass that the combined mass of four hydrogen atoms.

Helium atom lighter than four hydrogen atoms

In the same way that cars colliding head on lose bits of their bodywork…..

head to head collision between two cars used as an analogy to demonstrate how protons lose some of their mass when they collide in the core of the sun to produce nuclear fusion

….. so protons smashing into each other at high speeds lose some of their mass.

The mass which is lost is turned into the ENERGY which powers our sun!

birds migrating against a backdrop of the sun

This conversion from mass to energy was famously described by Albert Einstein’s in his equation E = mc2.

albert einstein and e =mc2 written on blackboard

The equation states that the energy generated is equal to the lost mass multiplied by the speed of light and then multiplied once again by the speed of light. So  E = mc2 describes how a small amount of lost mass produces a large amount of energy.

  • So how is this massive ball of superheated gases able to maintain its integrity? (stay intact) What keeps it from falling apart?

The sun showing surface

The answer lies in the sun’s state of hydrostatic equilibrium.

What is hydrostatic equilibrium?

Gas has a tendency to flow from high pressure to low pressure regions. For example when you get a puncture the air flows outwards from the high pressure region inside the tyre to the lower pressure of earth’s atmosphere.

 flat car tyre demonstrating how gas flows from high to low pressure

Given this fact you might expect the sun to explode as all that energy (heat and light) escapes from the higher pressure core to the lower pressure ‘surface’.

  • So why does the sun not explode as a result of outwards pressure created by all the energy escaping from the core?

sun in equilibrium

The answer lies in the balancing force of the inwards pressure created by the force of gravitational attraction. The inwards pressure of gravity exactly matches the outwards pressure.

The two forces balance each other; the sun neither explodes nor implodes because it has reached a state of ‘hydrostatic equilibrium.’

Our sun is very stable; it just keeps on producing the same amount of heat providing the fuel for life on our planet. For the next 4.5 billion years or so, the sun will maintain this state of hydrostatic equilibrium.

  • How is hydrostatic equilibrium affected if, for instance, extra mass is added to the sun?

It has been calculated that there is a 1% chance that Mercury will crash into sun within the next 4.5 billion years.

mercury close to sun

If this happens then the extra matter deposited inside the sun will increase the sun’s mass. This additional mass will result in an increase in the inwards force of gravity.

With the increase in the force of gravity comes additional inwards pressure acting on the sun’s core. Additional inwards pressure would lead to increased nuclear fusion and hotter temperatures inside the core.

mercury crashes into the sun and hydrostatic equilibrium restored

The hotter temperatures in the core will result in additional outwards thermal  pressure. Hydrostatic equilibrium will be restored as increased inwards and outwards pressures are in balance.

  • How long will the sun be in this state of hydrostatic equilibrium?

Only for the next 4.5 billion years or so! After that the supply of hydrogen in the sun’s core will be exhausted and nuclear fusion in the core will cease.

The sun will balloon into red giant….

sun as a red giant comparing size of sun now with future size

…and wipe out all life on earth.

sun as a red giant showing uninhabitable earth

Nuclear fusion on earth- is it possible?

  • If nuclear fusion can take place inside the sun, why not inside the earth?

The short answer is that our planet has neither the extreme temperatures or the mass required for nuclear fusion to take place. The temperature in earth’s core ‘only’ reaches 5,430°C. (9,800 °F)  with a pressure of  ‘only’ 3,600,000 atmospheres.

In any case, the earth’s core is made of iron and nickel and contains no hydrogen. No hydrogen, no nuclear fusion!

Interesting science facts about the sun

Science fair projects for high school

  • Describe the process of nuclear fusion and hydrostatic equilibrium.
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