Astronomy 2010 1 February 7, 2006 Chapter 15: The Sun: A Nuclear Powerhous e.

Astronomy 2010Astronomy 2010 11February 7, 2006February 7, 2006

Chapter Chapter 15: The 15: The Sun: A Sun: A Nuclear Nuclear

PowerhousPowerhousee

February 7, 2006February 7, 2006 Astronomy 2010Astronomy 2010 22

Happy SunHappy Sun


Why Does the Sun Shine?Why Does the Sun Shine?The Sun gives off energy (duh)!The Sun gives off energy (duh)!The energy must come from The energy must come from somewhere - there’s no free lunch.somewhere - there’s no free lunch.Conservation of energy is a Conservation of energy is a fundamental tenet of physics.fundamental tenet of physics.

Where does the energy come from?Where does the energy come from?Until the 20Until the 20thth century only 2 century only 2 possibilities were known:possibilities were known:Chemical reactionsChemical reactionsGravityGravity


The Sun’s Energy OutputThe Sun’s Energy OutputHow bright is the Sun? How bright is the Sun?

The Sun produces The Sun produces 4x104x102626 watts watts• Watt is the unit for the rate Watt is the unit for the rate of energy use, commonly seen on of energy use, commonly seen on light bulbs and appliances. light bulbs and appliances.

Our largest power plants Our largest power plants produce around 5 x 10produce around 5 x 1099 watts watts of power (5,000 megawatts or of power (5,000 megawatts or 5 gigawatts)5 gigawatts)

Sun = 8 x 10Sun = 8 x 101616 of these power of these power plants (80,000 trillion) plants (80,000 trillion)

Anyway you look at it, the Anyway you look at it, the Sun gives off a lot of Sun gives off a lot of energy.energy.


Is the Sun Powered by Is the Sun Powered by Chemical Reactions?Chemical Reactions?

What are chemical reactions?What are chemical reactions?Rearrange the atoms in molecules, as in Rearrange the atoms in molecules, as in 2H2H22 + O + O22 2H 2H22O + energy. This reaction O + energy. This reaction combines hydrogen and oxygen gas to combines hydrogen and oxygen gas to produce water plus produce water plus energyenergy..

Reverse the process: 2HReverse the process: 2H22O + energy O + energy 2H 2H2 2 + + OO22. By adding energy we can dissociate . By adding energy we can dissociate water into hydrogen and oxygen.water into hydrogen and oxygen.

The energy factor is often left out of The energy factor is often left out of chemical reaction formulas, for chemical reaction formulas, for convenience.convenience.


Is the Sun Powered by Is the Sun Powered by Chemical Reactions?Chemical Reactions?

If the Sun is powered by burning If the Sun is powered by burning coal or oil, how long could its coal or oil, how long could its fuel last?fuel last?

Only a few thousand years!Only a few thousand years!

A process that uses fuel more A process that uses fuel more efficiently is needed - something efficiently is needed - something that gets more energy out of every that gets more energy out of every kilogram of material.kilogram of material.


Gravity Squeeze?Gravity Squeeze?Gravitational contraction: falling layers Gravitational contraction: falling layers of the Sun's material compress the Sun of the Sun's material compress the Sun heat energyheat energy

drop a book drop a book noise! Gravitational potential noise! Gravitational potential energy.energy.

A contraction of 40m per day would account for A contraction of 40m per day would account for the Sun’s energy output.the Sun’s energy output.

Efficiency ~ 1/10000 %Efficiency ~ 1/10000 %

Gravity could power the Sun for about 100 Gravity could power the Sun for about 100 million years million years but the Sun is at least 4 but the Sun is at least 4 billion years old!billion years old!

Gravity can't be the Sun's main energy Gravity can't be the Sun's main energy sourcesource

But it did help But it did help igniteignite the Sun when it formed the Sun when it formed


15.2 Mass, Energy, and the 15.2 Mass, Energy, and the Special Theory of Special Theory of

RelativityRelativityTo understand the way the Sun To understand the way the Sun produces energy, we need to learn a produces energy, we need to learn a little about little about nuclear physicsnuclear physics and and the the special theory of relativityspecial theory of relativity..Nuclear physics deals with the Nuclear physics deals with the structure of the nuclei of atoms.structure of the nuclei of atoms.The special theory of relativity The special theory of relativity deals with the behavior of things deals with the behavior of things moving at close to the speed of moving at close to the speed of light.light.


15.2.1 Converting Mass to 15.2.1 Converting Mass to EnergyEnergy

Out of the special theory of relativity Out of the special theory of relativity comes the most famous equation in comes the most famous equation in science:science:

This equation tells us that mass (m) is This equation tells us that mass (m) is just another form of energy (E)!just another form of energy (E)!

The cThe c22 is the square of the speed of is the square of the speed of light.light.

1 gram of matter is equivalent to the 1 gram of matter is equivalent to the energy obtained by burning 15,000 energy obtained by burning 15,000 barrels of oil.barrels of oil.


……but there are rulesbut there are rulesWe can’t simply convert atoms into energy.We can’t simply convert atoms into energy.

We rearrange the protons and neutrons in We rearrange the protons and neutrons in nuclei to get a lower-mass configuration.nuclei to get a lower-mass configuration.

The difference between initial mass and The difference between initial mass and final mass is converted to energy.final mass is converted to energy.Chemical energy comes from rearranging atoms to Chemical energy comes from rearranging atoms to configurations of lower energy (mass).configurations of lower energy (mass).

Nuclear energy comes from rearranging nuclei to Nuclear energy comes from rearranging nuclei to configurations of lower mass (energy).configurations of lower mass (energy).

In each case, we get out the energy difference.In each case, we get out the energy difference.


Elementary Particles Elementary Particles (condensed)(condensed)

5 particles play 5 particles play a fundamental role a fundamental role in the Sun.in the Sun.

Protons and Protons and neutrons make neutrons make atomic nucleiatomic nuclei

Electrons orbit Electrons orbit nuclei of atomsnuclei of atoms

Photons are Photons are emitted by the Sunemitted by the Sun

Neutrinos are Neutrinos are also emittedalso emitted

Particle name

Mass (MeV/c2

)

Charge (e)

Proton 938.272 +1

Neutron

939.565 0

Electron

0.511 -1

Neutrino

<10-6 0

Photon 0 0


15.2.3 The Atomic Nucleus15.2.3 The Atomic NucleusTwo ways to rearrange nuclei and get Two ways to rearrange nuclei and get energy:energy:FissionFission •produces energy by produces energy by breaking upbreaking up massive nuclei massive nuclei like Uranium into less massive nuclei like like Uranium into less massive nuclei like Barium and KryptonBarium and Krypton

•A-bombs, nuclear reactorsA-bombs, nuclear reactors•needs Uranium 235, Plutonium 238needs Uranium 235, Plutonium 238•Problem:Problem: no Uranium or Plutonium on the Sun no Uranium or Plutonium on the Sun

FusionFusion•produces energy by produces energy by combining combining light nuclei light nuclei like Hydrogen to make more massive nuclei like Hydrogen to make more massive nuclei like Helium. like Helium.

•H-bomb, tokamak, internal confinement fusionH-bomb, tokamak, internal confinement fusion•Sun has lots of Hydrogen!!Sun has lots of Hydrogen!!


How Does Fusion Work?How Does Fusion Work?Nuclear fusion: Nuclear fusion: a process by which two light nuclei combine a process by which two light nuclei combine to form a single larger nucleus.to form a single larger nucleus.

However: nuclei are positively charged However: nuclei are positively charged Like charges repelLike charges repel

Two nuclei naturally repel each other and Two nuclei naturally repel each other and thus cannot fuse spontaneouslythus cannot fuse spontaneously

For fusion, electrical repulsion must be For fusion, electrical repulsion must be “overcome”“overcome”

When two nuclei are very close the When two nuclei are very close the strong nuclear forcestrong nuclear force takes over and takes over and holds them together.holds them together.

How do two nuclei get close enough?How do two nuclei get close enough?


Fusion needs Fusion needs fastfast moving nuclei moving nucleiFast moving nuclei Fast moving nuclei can overcome the can overcome the repulsion - they repulsion - they get a running get a running start.start.

Lots of fast Lots of fast moving nuclei means moving nuclei means high temperature.high temperature.

The core of the The core of the Sun has a Sun has a temperature of 15 temperature of 15 million degrees million degrees Kelvin. (ouch!)Kelvin. (ouch!)

Low speed

High speed


Fusion Power on EarthFusion Power on EarthFusion is the Fusion is the source of energy for source of energy for hydrogen bombs.hydrogen bombs.We are trying to We are trying to harness fusion to harness fusion to generate generate electricity:electricity:

tokamak - magnetic tokamak - magnetic confinement machine confinement machine as envisioned for as envisioned for ITER shown to the ITER shown to the rightrightinertial confinement inertial confinement fusion - Lawrence fusion - Lawrence Livermore National Livermore National LabLab

ITER reactor


Fusion Powers the SunFusion Powers the SunTemperatures in the cores of stars Temperatures in the cores of stars are above the approximately 8 are above the approximately 8 million K needed to fuse hydrogen million K needed to fuse hydrogen nuclei together.nuclei together.

Calculations: observed power output Calculations: observed power output of the Sun consistent with fusion of of the Sun consistent with fusion of hydrogen nuclei. hydrogen nuclei.

Observation: neutrinos from Sun Observation: neutrinos from Sun produced by fusion reactions.produced by fusion reactions.

Hypothesis: all stars produce Hypothesis: all stars produce energy by nuclear fusion.energy by nuclear fusion.


proton-proton proton-proton chain chain

ν++→+ +eHHH 2

γ+→+ HeHH 32

HHHeHeHe ++→+ 433

1. fuse two hydrogen, H (1 proton) to make deuterium, 2H (1 proton, 1 neutron), neutrino and positron

2. fuse one deuterium and one hydrogen to make helium-3 3He (1 proton, 2 neutrons), gamma ray (energetic photon)

3. fuse two helium-3 to make helium 4He plus two hydrogen


Why a complicated chain?Why a complicated chain?Fusion would be simpler if four protons Fusion would be simpler if four protons would collide simultaneously to make one would collide simultaneously to make one helium nucleus helium nucleus

Simpler, but less Simpler, but less likelylikelyrare for four objects to collide rare for four objects to collide simultaneously with high enough energy simultaneously with high enough energy

chance of this happening are very, very smallchance of this happening are very, very small

rate too slow to power the Sun rate too slow to power the Sun

proton-proton chain: each step involves proton-proton chain: each step involves collision of two particlescollision of two particles

chance of two particles colliding and fusing chance of two particles colliding and fusing is much higher, so nature slowly builds up the is much higher, so nature slowly builds up the helium nucleus. helium nucleus.


fusion fusion only in only in Sun's coreSun's coreonly only place its place its hot enoughhot enoughheat from heat from fusion fusion determines determines Sun's Sun's structure structure

Fusion and Solar Fusion and Solar StructureStructure


Heat from CoreHeat from Core determines Sun's Sizedetermines Sun's SizeForce equilibriumForce equilibriumHydrostatic equilibrium: balance Hydrostatic equilibrium: balance between between thermal pressure from the hot core thermal pressure from the hot core pushing outwards pushing outwards gravity squeezes the star collapse to gravity squeezes the star collapse to the very centerthe very center

Nuclear fusion rate is very Nuclear fusion rate is very sensitive to temperature. sensitive to temperature. A slight increase/decrease in T A slight increase/decrease in T causes fusion rate to causes fusion rate to increase/decrease by a large amount.increase/decrease by a large amount.


Gravity and PressureGravity and Pressureforce equilibriumforce equilibrium

Newton's second law: Newton's second law: F = maF = ma static equilibrium: no static equilibrium: no acceleration if forces on object acceleration if forces on object balancebalancegravity tries to pull 1/4 pounder gravity tries to pull 1/4 pounder to center of the Earthto center of the Earthpressure from table opposes pressure from table opposes gravity gravity

hydrostatic equilibriumhydrostatic equilibrium on Sun on Sun"cloud of gas" (like 1/4 pounder)"cloud of gas" (like 1/4 pounder)gravity pulls cloud to the centergravity pulls cloud to the centerpressure from gas below opposes pressure from gas below opposes gravitygravityheat from fusion in the hot core heat from fusion in the hot core increases pressure increases pressure energy output controls size of energy output controls size of sun!sun!

pressure from table

weight from gravity

cloud

pressure from hot gas

weight from gravity


Temperature and Pressure Temperature and Pressure temperature: random temperature: random motion of atoms in motion of atoms in a gasa gas

pressure: amount of pressure: amount of force per unit area force per unit area on piston from gason piston from gasgenerally pressure generally pressure increases with increases with increasing increasing temperaturetemperature


Balancing Fusion, Gravity Balancing Fusion, Gravity and Pressureand Pressure

If the rate of fusion increases, then:If the rate of fusion increases, then:1.1. thermal pressure increases causing the star to thermal pressure increases causing the star to

expand.expand.2.2. star expands to a new point where gravity would star expands to a new point where gravity would

balance the thermal pressure.balance the thermal pressure.3.3. the expansion would reduce compression of the corethe expansion would reduce compression of the core4.4. the temperature in the core would drop the temperature in the core would drop 5.5. the nuclear fusion rate would subsequently slow downthe nuclear fusion rate would subsequently slow down6.6. the thermal pressure would then drop the thermal pressure would then drop 7.7. the star would shrinkthe star would shrink8.8. the temperature would rise again and the nuclear the temperature would rise again and the nuclear

fusion rate would increasefusion rate would increase9.9. Stability would be re-established between the Stability would be re-established between the

nuclear reaction rates and the gravity compressionnuclear reaction rates and the gravity compression


Hydrostatic EquilibriumHydrostatic Equilibrium

balance between pressure, heat from fusion and balance between pressure, heat from fusion and gravity determines Sun's sizegravity determines Sun's sizebig stars have cooler cores, small stars have big stars have cooler cores, small stars have hotter cores – more compressedhotter cores – more compressed


Other ParticlesOther Particles

Helium is not the only product in Helium is not the only product in the fusion of hydrogen.the fusion of hydrogen.

Two other particles producedTwo other particles producedPositronPositron

NeutrinoNeutrino


Gamma Ray Propagation in the Gamma Ray Propagation in the SunSun

Positrons quickly Positrons quickly annihilateannihilate with with electrons.electrons.Photons produced in core of the Sun Photons produced in core of the Sun take about a million years to move to take about a million years to move to the surface. the surface. Slow migration because they scatter Slow migration because they scatter off the dense gas particles off the dense gas particles move about only a centimeter between move about only a centimeter between collisions. collisions. In each collision, they transfer some of In each collision, they transfer some of their energy to the gas particles. their energy to the gas particles.

As they reach the photosphere, gamma As they reach the photosphere, gamma rays have become visible photons.rays have become visible photons.


NeutrinosNeutrinosNearly massless particles with no Nearly massless particles with no charge.charge.

Rarely interact with ordinary matter. Rarely interact with ordinary matter.

Neutrinos travel extremely fast Neutrinos travel extremely fast Almost at the speed of light if small mass.Almost at the speed of light if small mass.

Neutrinos pass from the core of the Neutrinos pass from the core of the Sun to the surface in only Sun to the surface in only two secondstwo seconds. .

They take less than 8.5 minutes to They take less than 8.5 minutes to travel the distance from the Sun to the travel the distance from the Sun to the Earth. Earth.


Neutrino CountingNeutrino CountingIn principle:In principle:Use neutrino count at Earth as Use neutrino count at Earth as indicator of the Sun’s energy outputindicator of the Sun’s energy output

The problem:The problem:Neutrinos have a very low Neutrinos have a very low probability of interacting with probability of interacting with matter. matter.

Could pass through a light year of Could pass through a light year of lead and not be stopped by any of lead and not be stopped by any of the lead atoms!the lead atoms!


Neutrino AbundanceNeutrino AbundanceThe Sun produces A LOT of The Sun produces A LOT of neutrinos.neutrinos.

In one second several mIn one second several million illion billionbillion neutrinos pass through neutrinos pass through your body.your body.Do you feel them?Do you feel them?

Not to worry!Not to worry!•The neutrinos do not damage anything. The neutrinos do not damage anything.

•The great majority of neutrinos pass The great majority of neutrinos pass right through the entire Earth as if it right through the entire Earth as if it wasn’t there.wasn’t there.


Detecting NeutrinosDetecting NeutrinosIncrease the odds of detecting Increase the odds of detecting neutrinos by using a neutrinos by using a LARGE amountLARGE amount of of a material that reacts with a material that reacts with neutrinos in a measurable way. neutrinos in a measurable way. A chlorine isotope changes to a A chlorine isotope changes to a radioactive isotope of argon when hit radioactive isotope of argon when hit by a neutrino.by a neutrino.

A gallium isotope changes to a A gallium isotope changes to a radioactive isotope of germanium. radioactive isotope of germanium.

Neutrinos can interact with protons and Neutrinos can interact with protons and neutrons and produce an electron. The neutrons and produce an electron. The electron can be detected.electron can be detected.


Neutrino DetectorsNeutrino DetectorsNeutrino detectors use Neutrino detectors use hundreds of thousands of hundreds of thousands of liters of these liters of these materials in a container materials in a container buried under many tens buried under many tens of meters of rock to of meters of rock to shield the detectors shield the detectors from other energetic from other energetic particles from space particles from space called called cosmic rays.cosmic rays. Even the largest Even the largest detectors detect only a detectors detect only a few neutrinos per dayfew neutrinos per day.. Kamiokande

Ray Davis

Nobel Prize in 2003


Solar Neutrino ProductionSolar Neutrino ProductionNumber of neutrinos produced in the Number of neutrinos produced in the Sun is directly proportional to the Sun is directly proportional to the number of nuclear reactions taking number of nuclear reactions taking place in the Sun's core. place in the Sun's core.

Same principle with neutrinos Same principle with neutrinos produced via the Carbon-Nitrogen-produced via the Carbon-Nitrogen-Oxygen chain.Oxygen chain.

The more reactions there are, the The more reactions there are, the more neutrinos are produced and the more neutrinos are produced and the more that should be detected here on more that should be detected here on the Earth. the Earth.


Solar Neutrino Production Solar Neutrino Production (cont’d)(cont’d)

Physicists find that the number of Physicists find that the number of neutrinos coming from the Sun is neutrinos coming from the Sun is smallersmaller than expected. than expected.

Early experiments detected only Early experiments detected only 1/3 of the expected number of 1/3 of the expected number of neutrinos. neutrinos.

These experiments used hundreds of These experiments used hundreds of thousands of liters of cleaning thousands of liters of cleaning fluid (composed of chlorine fluid (composed of chlorine compounds) or very pure water. compounds) or very pure water.


Solar Neutrino Production Solar Neutrino Production (cont’d)(cont’d)

Later experiments using many tons Later experiments using many tons of gallium were able to detect the of gallium were able to detect the more abundant low-energy neutrinos. more abundant low-energy neutrinos.

However, those experiments also However, those experiments also found the same problem: too few found the same problem: too few neutrinos (the gallium experiments neutrinos (the gallium experiments found about 2/3 the expected found about 2/3 the expected number). number).

The puzzling lack of neutrinos from The puzzling lack of neutrinos from the Sun is called the the Sun is called the solar neutrino solar neutrino problemproblem. .


Possible Explanations:Possible Explanations:Nuclear fusion is not the Sun's power Nuclear fusion is not the Sun's power source. source.

Not supported by observations, not likely to be Not supported by observations, not likely to be the correct reason.the correct reason.

The experiments were not calibrated The experiments were not calibrated correctly. correctly.

Unlikely that all carefully-tuned experiments Unlikely that all carefully-tuned experiments were tuned in the same wrong way. Experiments were tuned in the same wrong way. Experiments independently verified by many other scientists; independently verified by many other scientists; astronomers think that the results are correct. astronomers think that the results are correct.

The nuclear reaction rate in the Sun is The nuclear reaction rate in the Sun is lower than what our calculations say. lower than what our calculations say.

Possible but many people have checked and re-Possible but many people have checked and re-checked the physics of the reaction rates. checked the physics of the reaction rates. Strong constraints in how much one can lower the Strong constraints in how much one can lower the temperature in the core of the Sun to slow down temperature in the core of the Sun to slow down the reactions.the reactions.


Solar Neutrino SolutionSolar Neutrino SolutionThree types of neutrinos exist.Three types of neutrinos exist.

The Sun produces only one type, called The Sun produces only one type, called electron neutrinoselectron neutrinos

The experiments detect only electron-The experiments detect only electron-type.type.

On their way from the Sun, neutrinos On their way from the Sun, neutrinos can transform from one type to another can transform from one type to another we only detect 1/3 of the mix at we only detect 1/3 of the mix at Earth.Earth.

This also implies that neutrinos have This also implies that neutrinos have mass, very small, but not zero.mass, very small, but not zero.


Neutrino OscillationsNeutrino Oscillations

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

The Sun produces only The Sun produces only

ννee neutrinos (green). neutrinos (green).

This becomes one of This becomes one of the types the types νν11, , νν22, or , or νν33 on its way to Earth.on its way to Earth.

These 3 mix on their These 3 mix on their way to Earth.way to Earth.

When we look at the When we look at the neutrinos on Earth, neutrinos on Earth, some of the original some of the original green is now blue or green is now blue or yellow.yellow.

Astronomy 2010 1 February 7, 2006 Chapter 15: The Sun: A Nuclear Powerhous e.

Documents

Transcript of Astronomy 2010 1 February 7, 2006 Chapter 15: The Sun: A Nuclear Powerhous e.