The Sun’s Energy Nuclear Fusion

In the Beginning The Sun and solar system were born, as all stars are, out of a cloud of gas and dust – a nebula.

Gravity causes the nebula to contract and heat up. If it gets hot enough to ignite nuclear fusion, a star is born. Click for computer model animation

Early Theories The first scientific theories involved chemical reactions or gravitational collapse

Early Theories Disproved chemical burning ruled out…it can not account for the Sun’s brightness conversion of gravitational potential energy into heat as the Sun contracts would only keep the Sun shining for 25 million years late 19 th -century geological research indicated the Earth was older than that

The development of Nuclear Physics led to the correct answer. Namely, Albert Einstein and his famous equation E = mc 2 Nuclear Fusion in the core provides the Sun’s power. (Not to be confused with nuclear fission).

Fission vs. Fusion Nuclear fission is the opposite of fusion. –Nuclear fission is a reaction where lighter nuclei are created by splitting heavier nuclei. –Nuclear fusion is the combining of two smaller nuclei to form a bigger nuclei.

FUSION: Lighter elements FUSED into heavier elements FISSION: Heavier elements split into lighter elements BOTH process release ENERGY! Fusion releases more!

Both fission and fusion release energy. Fusion releases much more energy than fission. We have learned to control fission but not fusion. It is easier to destroy than to create.

What’s Nuclear Fusion? First we must understand the three fundamental forces in nature: –Gravity –Electromagnetism –Strong Nuclear Force

Gravity: –Objects with mass attract each other. –Weakest force in nature –But… works over very large distances.

Electromagnetism: –Positive & negative. Opposite charges attract and like charges repel. –Medium strength force –Works over a mid range of distances. From atomic up to planetary in scale.

Strong Nuclear Force: –Binds protons (+) together in the nucleus of atoms –Strongest force by far. –But… works only over very short distances (less than the diameter of a proton).

Normally the electromagnetic force causes protons (+) to repel. At high temperatures and pressures, the nuclei move fast enough to touch. When nuclei touch, the nuclear force binds them together and energy is released!

Nuclear Fusion Needs HIGH temps (around 1- 8 million K) –Because high temp means high Kinetic Energy of atoms (moving fast) –This high kinetic energy enables protons to overcome electromagnetic repulsion and collide. Need high densities so collisions happen frequently

Nuclear Fusion at Sun’s Core Inner 25% of solar radius (the core) can support FUSION Core temp ~ 15 million K Density of hydrogen is extremely high in core Sun converts 600 million tons of H into He every second!

1.Two protons are forced together Proton This takes temps of 8 million+ K. Sun’s core = 15 million K! Proton-Proton Fusion Reaction

2.Strong nuclear force holds protons together. Deutron Neutrino + Positron 3.One proton decays to a neutron. This results in: - formation of a deutron (a proton and neutron bonded). - release of a positron (electron w/ a + charge!). - and the release of a neutrino (no mass, neutral charge, “little neutral one”). Proton

4.The positron (antimatter) collides with an electron. Proton Deutron Proton Annihilation Electron - Neutrino + Positron Proton Both are annihilated, turned into pure energy, in a matter-antimatter explosion. This energy is released as two gamma rays (high-energy photons).

5.The newly-formed deuteron may collide into another hydrogen nucleus, Proton Deutron Proton Annihilation Electron - Neutrino Helium-3 Proton + Positron creating the isotope helium-3, and causing the release of another gamma ray.

6.When two helium-3 nuclei collide (and combine), Proton Deutron Proton Annihilation Electron - Neutrino Helium-3 Protons + Positron Helium-4 two protons are released leaving a Helium-4 nucleus. The sun’s core is not hot enough to fuse helium-4 into a bigger element.

The Complete Proton-Proton Fusion Reaction Proton Deutron Proton Annihilation Electron - Neutrino Helium-3 Helium-4 Proton Protons Neutrino + Positron Electron + Positron Annihilation -

Proton-Proton Fusion Reaction SIMPLIFIED (draw this one) Proton Helium-4 Proton Helium-3 - Multiple Steps + Energy!

Read on to see what Einstein’s Famous equation has to say about how much energy the Sun produces.

How Much Energy does the sun produce? As Einstein Said… E = mc 2 or Energy = mass x (speed of light) 2

Our Proton-Proton reaction can be summed up like this: 1 helium nuclei (2 p & 2 n) 4 protons

The four hydrogen nuclei on the left side, weigh more than the products on the right side amu4.031 amu 1 helium nuclei (2 p & 2 n) 4 protons

Subtracting we find… u4.031 u= u  This mass is gone! Turned into pure energy! E = m x c 2 1 helium nuclei (2 p & 2 n) 4 protons

How much energy you ask… E = mc 2 In each fusion reaction, 0.07% of the mass is converted directly into energy. So m is a big number and c is a big number, therefore E is a really darn big number. Roughly equal to 6,670,000,000,000, that’s 6.67 trillion A-bombs per second. damn

For you physics jocks… = 3.86 x Joules/second (or Watts) Luminosity = Energy released second = 7,720,000,000,000,000 typical (500 mW) power plants

Gravitational Equilibrium Striking a Balance The Sun began as a cloud of gas undergoing gravitational collapse. the same heating process, once proposed to power the Sun, did cause the core of the Sun to get hot & dense enough to start nuclear fusion reactions

Once begun, the fusion reactions generated energy which provided an outward pressure. This pressure perfectly balances the inward force of gravity. This balance is called gravitational equilibrium. it causes the Sun’s size to remain stable

The steady Sun Constant properties of the Sun –Luminosity (energy output) –Radius (size of the Sun) How can the Sun stay SO stable over time?

Gravitational Equilibrium Tug of War between… PRESSURE Pushing outwards GRAVITY Pulling inwards

Acrobats are in gravitational equilibrium Bottom person feels most weight from above and must exert the largest “pressure” upwards to maintain balance.

Near the Core: Pressure outwards high Weight of overlying material is BIG Near the Surface: Pressure outwards lower Weight of overlying material is smaller

Gravitational Equilibrium Pressure FORCE Originates from energy source BELOW that layer Gravitational FORCE From weight of matter ABOVE that layer As long at those are balanced at EVERY LAYER in the Sun, the Sun should not expand nor contract very much. Warning: expansion/contraction will occur when this equilibrium is violated.

THE END!

ADDITIONAL TOPICS Feel Free to Browse these

Chain of Reactions within the Sun’s core to produce Energy Proton-Proton (pp) Chain Note: Sun’s core is so hot, gas is a plasma (completely ionized)

Proton-Proton Chain Fusion Chain of reactions fuse H  He Bottom line: –4 protons  1 Helium Atom (2 protons & 2 neutrons) –Mass of 4 protons > mass of Helium –Excess mass was converted to Energy!

The Solar Thermostat The rate of fusion reactions depends on temperature. the higher the T, the faster the rate, the more energy is produced This fact, coupled with gravitational equilibrium, acts as a mechanism which regulates the Sun’s energy output. its energy output (luminosity) remains stable

The Solar Luminosity The Sun’s luminosity is stable over the short-term. However, as more Hydrogen fuses into Helium: four H nuclei convert into one He nucleus the number of particles in Sun’s core decreases with time the Sun’s core will contract, causing it to heat up the fusion rate will increase to balance higher gravity a new equilibrium is reached for stability at a higher energy output the Sun’s luminosity increases with time over the long-term Models indicate the Sun’s luminosity has increased 30% since it formed 4.6 billion years ago. it has gone from 2.9 x watts to today’s 3.8 x watts

“Observing” the Solar Interior The Sun’s interior is opaque… we can not see directly into it with light We can construct mathematical computer models of it. the models are a grid of temperature, pressure, & density vs. depth these values are calculated using known laws of physics they are tested against the Sun’s observable quantities We can directly measure sound waves moving through the interior we observe “sunquakes” in the photosphere by using Doppler shifts motion of sound waves can be checked against interior conditions predicted by models There is another way to see directly into the core…neutrinos!

The Neutrino Problem Neutrinos come to us directly from the core of the Sun, a product of the proton-proton chain. We have detected them, proving that the theory of nuclear fusion reactions is correct! But we only detect about 30% - 50% of the neutrinos which are predicted by theoretic models. either our understanding of nuclear fusion reactions or our understanding of neutrinos is wrong! We have since discovered three types of neutrinos: electron ( e ), muon (  ), and tau (  ) our neutrino detectors can register only electron neutrinos If neutrinos can change type after being created, this could solve the “neutrino problem.”

How can we see inside the Sun?

“Observing” the Solar Interior The Sun’s interior is opaque… we can not see directly into it with light We can construct mathematical computer models of it. the models are a grid of temperature, pressure, & density vs. depth these values are calculated using known laws of physics they are tested against the Sun’s observable quantities

“Observing” the Solar Interior We can directly measure sound waves moving through the interior we observe “sunquakes” in the photosphere by using Doppler shifts motion of sound waves can be checked against interior conditions predicted by models

Solar Quake

“Observing” the Solar Interior There is another way to see directly into the core…neutrinos!