Module :

Activity 1:

Solar is Nuclear

Module 20: Inside the SunSwinburne Online Education Exploring the Solar System

© Swinburne University of Technology

Summary:

In this Activity, we will investigate

(a) the method by which the Sun produces energy...

If it’s not chemical or gravitational, what is it?

How reliable is our understanding of it?

(b) the temperature and pressure inside the Sun, and how these maintain the Sun’s energy production.

Early ideas

It was pretty obvious to humans in prehistoric times that the Sun was highly unusual and obviously not a normal “Earth-like” object.

The Sun seemed to glide slowly and regularly through the sky while other daytime objects either fell to Earth, or changed from minute to minute (like clouds) or from day to day (like the Moon).

Gravitational collapse?

As we saw in the Activity The Sun: Ruler of the Solar System, much later on, scientists wondered whether gravitational collapse has caused the material in the Sun to heat up.

If that was the case, the Sun would not burn but instead glow.

Sun shrinks,

and in doing so

radiates a lot

of heat and light

Time problems

However if the Sun glowed due to gravitational contraction alone, then it would never have kept shining for the last 5 billion years, nor could it be expected to last much longer.

DELETE or overwrite

visible lightother

electromagnetic radiation

neutrinos

electrons

protons, ions

“solar wind”

Another possibility

We now know that the Sun not only emits light but also emits other forms of electromagnetic radiation and streams of particles.

This happens in “nuclear reactions” too.

But ...

Nuclear interactions and its associated energy first came to people’s attention about a century ago. It became obvious that the Sun must be energised by these interactions. From then on the Sun was often referred to as a “nuclear furnace”.

Proceed with Caution!

There are several parts of the term “nuclear furnace” that are misleading.

We have to treat the term with extreme caution … or better still, avoid using it altogether.

Proceed with Caution!Proceed with Caution!

“Furnace”

First, the word “furnace” (which comes from an old Latin term, fornax, meaning “warm”) is usually associated with burning, which is a chemical reaction.

“Burning” usually means the combination of a substance (such as carbon, paper, wood) with oxygen.

You start off with carbon (for instance, in the form of coal) and oxygen (from the air) and end up with carbon dioxide (and ash) … and a good deal of heat, as well.

Chemical reactions occur in and between molecules (which are collections of atoms). Chemistry is the study of molecules and chemical reactions.

“What does CH

4 mean?”

CH4 + 2O2 react to form 2 H2O + CO2

(methane) (oxygen) (water) (carbon dioxide)

In a chemical reaction, atoms join, leave or move between molecules to make new molecules. For instance …

Chemical reactions rely on strong bonds existing between the atoms inside molecules: these bonds occur when those atoms share electrons.

In the methane molecule, a carbon atom shares electrons with four hydrogen atoms.

After the chemical reaction, it shares electrons with two oxygen atoms instead.

Strong bonds

No molecules, thanks

However it is far too hot for molecules to exist in the Sun. The molecules simply fall apart in the incredibly high temperature, so in the Sun there can be no chemistry.

too hot for molecules

molecule made of atoms

molecule is dissociated

So no chemistry, thanks

So we must avoid using the term “furnace” with respect to the Sun.

too hot even for atoms

plasma

In fact, it is even too hot for atoms as we know them to exist. The heat causes atoms to fall apart too, and what you have instead is a kind of “melt” called plasma.

molecule is dissociated

No fission either, thanks!

Second, most people think that “nuclear” reactions in the Sun are fission reactions.

However fission mostly occurs when elements are pretty heavy (for example, uranium and plutonium)...

hydrogen nucleus

helium nucleus heavier

nucleus

Fission

electromagnetic radiation

electromagnetic radiation

particles

particles

Fis sionFis sionFission is the process whereby the nucleus of an atom splits into several different pieces. These pieces usually include several particles and some sort of electromagnetic radiation.

Such radiation may be used for good purposes (e.g. radiotherapy), although the results can also be unwelcome (e.g. mutation, radiation poisoning, cancer).

Although fission can and does occur in some much lighter elements, the lighter the element the less likely it will be that fission can occur.

other2%

He26%

H72%

Since the Sun is 98% hydrogen and helium (the lightest elements), there can be hardly any fission in the Sun.

The large, complex collections of particles in a big nucleus are far more likely to break up.

While fission occurs when nuclei split up into smaller particles, there is a type of nuclear interaction where the reverse happens.

Another type of “nuclear”

There are actually a number of different kinds of “nuclear” reaction, involving different forces, particles and energies.

This type of nuclear interaction is called

fusion.

Fusion

It is very difficult under Earth conditions to make fusion occur: the particles being fused often have the same electrostatic charge (positive, in the case of nuclei) and therefore repel each other very strongly.

So a cloud of gas has to be very compressed (or collapse a great deal under its own weight) before the high pressure and temperature can overcome this repulsion, and fusion can begin.

Electrostatic repulsion stops impact

… but high pressure and temperature

encourage impact

When fusion does occur, it not only involves the formation of a new atom from several old ones, but there is also the release of some energy in the form of electromagnetic radiation (heat, light, x-rays and so on) and perhaps particles such as neutrinos, electrons etc.

electromagnetic radiation

electromagnetic radiation

particle

new nucleus

particle

To see an animation click here

Change figure to match movie

Ninety percent of the time, fusion in the Sun involves hydrogen nuclei being fused to make helium:

Start with 4 protons under enormous

pressure and temperature

End up with a “normal” helium nucleus,

two gamma rays, two positrons and

two neutrinos

Several Reactions

Here is that process broken into its three steps:

1. Two protons fuse to make deuterium, releasing a positron

and a neutrino

2. The deuterium fuses with another proton to make

a light helium nucleusand a gamma ray

3. Two light helium nuclei fuse to make “normal”

helium, plus two protons

protonhydrogen nucleus

one positive charge

neutronlike a proton

but with no charge

positron “positive electron”one positive charge

neutrinono charge

and no mass

gamma raya very energetic

photon

Here are the symbols and equations used by physicists to show how the various particles and so on “add up” for this reaction:

Two hydrogen nuclei combine to make one

“heavy” hydrogen nucleus (also called deuterium).

A positron and a neutrino are emitted.

A hydrogen nucleus combines with a “heavy”

hydrogen nucleus to produce helium-3.

A gamma ray is emitted.

Two helium-3 nuclei

combine to make a

helium-4 nucleus.

Two hydrogen nuclei

are emitted.

This reaction starts with protons (bare hydrogen nuclei) and so is called the proton-proton chain.

61H+ 4He++ + 21H+ + 2e+ + 2 + 2

If you combine all of the equations for the entire chain, you find that six protons end up producing a helium nucleus, two positrons, two neutrinos and two gamma rays, with two left-over protons released as well.

[By the way, the positrons don’t just sit there.

They fly off and combine with electrons, but that’s another story.]

Here it is in one diagram:

Click here to see animation

Energy production Now, just for a moment remember what started all this talk about fusion and fission and nuclear reactions:

it was to work out how the Sun produces so much energy.

It turns out that if you compare the mass that you start with and the mass you end up with there is a difference …

Although there is an exchange of energy in most of the steps, it is the step where a gamma ray is emitted that is of most interest.

… and that difference is exactly accounted for by one of the most widely-known and least-understood equations in physics:

E = mc2

According to this equation, energy (E) and mass (m) may be interchangeable: for example, in fission reactions and in fusion reactions like the proton-proton chain.

c is the speed of light in a vacuum: 3 x 108 ms-1.

Here is that equation at work with respect to the proton-proton chain:

BEFORE: four protons

AFTER:helium nucleus

plus two positrons plus two neutrinos

… and two gamma rays

Initial total mass = 6.693 x 10-27 kg

Final total mass = 6.645 x 10-27 kg

Difference = 0.048 x 10-27 kg

… and according to E = mc2 this is equivalent to ...

Energy = 0.43 x 10-11 joules

… which is just the energy observed in the two gamma rays

Energy = 0.43 x 10-11

joules

E = mc2

Big deal! 0.43 x 10-11 joules sounds tinyeven with the help of c2 (I hear you say). However to produce the Suns luminositya huge 6 x 1011 kg of hydrogen must be converted into helium each second.

This turns out not to be a problem. The Suns mass is 2 x 1030 kg. Based on this, it has adequate fuel to have been undergoing nuclear fusion for the last 4.6 billion years (the age of the Solar System), and to continue for another 5 billion years!

Other important fusion reactions Although in our Sun it is the proton-proton chain which dominates (91%), in other stars other reactions are very important. Here are the main ones:

CNO cycle

Helium “burning”

A complex series of reactions in which the transformation carbon - nitrogen - carbon - nitrogen - oxygen - nitrogen - carbon facilitates the conversion of four protons to one helium nucleus

(plus energy)

Three helium nuclei fuse to create one carbon nucleus (plus energy).

This is also called the “triple-alpha reaction”.

Carbon “burning”Carbon is fused to form heavy elements (plus energy):

in particular, iron is the final product of much carbon burning.

Why all the p-p in our Sun?

Why in our Sun is there mostly proton-proton reaction and hardly any of the other types of reaction?

It is because of the difficulty in making these reactions occur, and that depends on three things ...

Q:

A:

Pressure, temperature and chemicalcomposition

All nuclei contain protons and so are positively-charged, and things with the same charge will repel each other.

So unless the nuclei are forced to come so close to each other that this electrostatic repulsion is overcome, fusion will not occur.

Higher pressure within a gas makes the particles in the gas be, on average, closer to each other.

Higher temperature within a gas makes the particles in the gas move a great deal faster and so they may randomly come closer to each other.

Finally, if the elements aren’t there, they can’t react!

A closer look

Let’s watch two positively-charged nuclei approaching each other and see what happens.

nono

noNo!No!

No!No!Oh,

all rightOh,

all right Okay!Let’s fuse!

… but suddenly, when the nuclei are very close to each other the nuclear force takes over and fusion can occur. Censors Note: Nuclei used in this slide were older than the age of thermonuclear consent

As the nuclei get closer, they repel each other more and more strongly because of the electrostatic repulsion …

More P and T required

The more positively-charged a nucleus is, the harder it will be to get it close enough to another nucleus that fusion can occur. More pressure (P) and temperature (T) will be needed.

Reaction... becomes significant at

Proton-proton reaction 8 million degrees Kelvin (K)

CNO cycle 20 million K

triple-alpha reaction 100 million K

carbon “burning” 600 million K

A proton has only one positive charge; carbon has 6, nitrogen 7 and oxygen 8; and an alpha particle has two.

The heart of the Sun

Mathematical models can be applied to estimate what the temperature and pressure will be in various zones inside the Sun (and other stars).

Fusion can only occur in the core of our Sun. The heat produced in the core is transported to the surface through the other layers, and we’ll have a look at that in the next Activity.

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Radius (fraction of whole)

Den

sity

(g

/m3)

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mill

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K)

density

temperature

Temperature and density in the Sun

Fusion in the Sun occurs where the temperature exceeds 8 million K

… that is, only in the innermost 25% of the Sun One quarter of

the radius is justone 64th of the

volume!

The journey ahead

In this Activity, we looked at some ways that energy is produced in the natural world, and explored why fusion is the only option to explain the energy production of the Sun.

Because of the extreme temperature and density required, this can only occur in the interior region which we call the core of the Sun.

In the next Activity we will look at how energy is transported to the surface of the Sun once it is produced.

Image Credits

Temperature variations in the corona August 1998 (colour enhanced)

http://antwrp.gsfc.nasa.gov/apod/image/9808/activesun_trace.jpg

Now return to the Module home page, and read more about the Sun in the Textbook Readings.

Hit the Esc key (escape) to return to the Module 20 Home Page

Chemical symbols

A molecule is a combination of atoms. The “formula” for the molecule is written using a subscript if there is more than one atom of an element.

When astronomers, chemists, biologists and other people working in the sciences want to write about particular elements such as hydrogen, helium, oxygen and so on, they use a letter or letters which relate to the Latin name for the substance.

O = oxygen

C = carbon

COcarbon monoxide

CO2

carbon dioxide

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