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Agenda for Ast 309N, Sep. 6

• Feedback on card of 9/04• Internal structure of the Sun• Nuclear fusion in the Sun (details)• The solar neutrino “problem” and its solution• Index card: photons vs. neutrinos• Next Tuesday: review basic properties of light

The Sun’s Core: Site of Nuclear Fusion

• The Sun generates energy via fusion reactions• Hydrogen is converted into Helium in the Sun’s core

(innermost regions, out to about 0.25 R!)• The helium has less mass than the initial 4 H nuclei• The difference in mass (sometimes called the “mass

defect”) is converted into energy, according to E = m c2

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Transporting Energy by Radiation

• Deep in the Sun, energy is transported by photons whichare absorbed, then re-emitted by the atoms

• With each such interaction, they can change direction(random walk), and energy (hence wavelength)

• This “random walk” isa slow process! Ittakes about ahundred thousandyears for the energyto reach the surface.

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Transporting Energy by Convection

• Photons from below are absorbed by an opaque layer thateffectively “blocks” radiative transport

• This layer is strongly heated, causing hot gas to rise upand cooler gas to sink …a convection current

• The net effect is that energy is carried upward along withthe moving material

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Convection Zone:where the energy istransported by risingcells of hot gas

Radiative Zone:region where energyis transported mainlyby photons

Core: where theenergy is createdby nuclear fusion

Summary: Inner Layers of the Sun

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Inside the Sun: Striking a Balance

There must be astable balancebetween inward(gravity) and outwardforces (resistance tocompression due tothermal pressure), forthe Sun to stay inequilibrium, neithercollapsing nor blowingitself apart!

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How did the Sun reach its present state?

• The Sun began as a cloud of gas, contracting undergravity. It had the same composition throughout, amixture of approximately 75% H, 25% He by mass.

• The contraction released energy that was ultimatelyconverted to thermal energy, heating the central regions.

• When the center became hot and dense enough, Hbegan fusing into He, supplying a new energy source.This fuel will last until all the H is converted into He, inthe core, the region that’s hot enough for fusion.

• This energy kept up the outward pressure to balancegravity’s inward force, so the Sun achieved a long-termbalance, with its present radius and luminosity.

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Changing conditionswith greater depth:

The deeper levels mustbe hotter and denser, tosupport the layers above.

– pressure is strongestwhere gravity isstrongest (in the centralregions)– the pressure is weakestwhere gravity is weakest(near the surface)

Models of the Solar Interior

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Current models of theSun explain observedsolar vibrations

They also predictthe nuclear fusionreaction rate, hencethe number ofneutrinos emitted

Current Solar Model

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Helioseismology: A Tool forStudying the Solar Interior

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http://solarscience.msfc.nasa.gov/helioseismology.shtmlhttp://solarscience.msfc.nasa.gov/interior.shtml

• Both nuclei have positivecharge, so they repel eachother.

• For fusion to occur, the nucleimust have enough kineticenergy to overcome thisrepulsion

• High kinetic energy means hightemperature (and pressure)

• When the nuclei get closeenough, the strong nuclearforce overcomes the repulsionand fuses the nuclei together

Why Fusion Happens Only in the Core

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Solar neutrinos starin a Hollywooddisaster movie:junk science !

The Real Story of Solar Neutrinos

• Starting in the 1950’s, astronomers proposed that theSun and stars produce energy by nuclear fusion

• But we do not directly see this happening – why not?• Is there a way we can be sure about it?

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Hydrogen Fusion into Helium

IN4 protons

OUT4He nucleus

2 gamma rays2 positrons2 neutrinos

Converted mass(called the “massdefect”) is 0.7%

of the initial mass

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Neutrons and Protons

Neutrons can change into protons, and vice versa.

n p+ + e- + !e ("-decay)

p+ n + e+ + !e (inverse "-decay)

!e is a neutrino ---- a weakly interacting particle whichhas low mass and travels at nearly the speed of light.

!

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Hydrogen Fusion in the Sun

In the first step of the p-p chain, a proton turns into a neutron,yielding deuterium, a positron (anti-electron), and neutrino.The positron is anti-matter; when it meets an electron, they“annhilate,” and all their mass turns into energy (photons).

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The “Main Highway” of H fusion (p-p I)

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“Side Roads” of the p-p reaction

p-p I

p-p II & III

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H Fusion via the CNO Cycle

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Only a smallfraction (1%) ofthe fusion inthe Sun occursvia the CNOcycle, but it isthe main fusionreaction in highmass stars

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Credit: G. Orebi Gann, APS presentation (see slide 34)

An experiment started inthe late 1960’s in theHomestake, S.D. goldmine, detected neutrinosvia: Cl + ! # ArThey found only about1/3rd of the expectednumber of neutrinos.

The Hunt for Solar NeutrinosI. - The Davis Experiment

This discrepancy came tobe known as the solarneutrino problem.

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Using a large tank of H2O to detect!-e- collisions via lightflashes,(Cerenkov radiation),Kamiokande proved that the !’s camefrom the Sun, but also saw “too few”of them.

Kamioka mine in Japan

The Hunt for Solar Neutrinos II. –(the original) Kamiokande

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Solar !‘s: Super Kamiokande

An accident in 2001 blewout most of the sensors;but they were repaired,and operations resumed

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Which Neutrinos Could they See?

Both the Davis and Kamiokande experiments could detect only high-energy neutrinos; they could not detectthe neutrinos produced in the first step of the p-p chain(which makes deuterium), but only some of the rare onesmade by the alternate paths, p-p II and p-p III.

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Neutrinos of Different Energies

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The Solar Neutrino “Spectrum”

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The shadedareas showwhich neutrinoscan be detectedby differentmethods orexperiments:Davis (chlorine),SuperK (water),and gallium.

Chasing the Low-Energy Neutrinos:The Gallium Experiments

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The Solar Neutrino Problem

TOO FEW SOLAR NEUTRINOS DETECTEDCOMPARED TO PREDICTIONS FROM MODELS

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The Solution: Neutrino Oscillations

• Solar fusion reactions create only one type of neutrinos,“electron neutrinos” but two other kinds (muon, tau) exist• Some people theorized that some of the neutrinos werechanging into other kinds en route from the Sun’s core• The Davis experiment could see only electron neutrinos• Kamiokande was used to detect a beam of neutrinoscoming from 250 km away (“Kam-LAND”), proving thatthe neutrinos were indeed changing their types.

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Sudbury Neutrino Observatory (SNO)

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The definitive experiment; in Sudbury, Ontario (Canada)

Used heavy water (D2O) as the detector

Counted !x, !e separately: (!x means any !)

Two experiments in one: could clearly seehow many of the solar ! ‘s “oscillate”

D + !e ! p + p + e-

D + !x ! p + n + !x

Sudbury Neutrino Observatory (SNO)

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SNO proves the predictions were right!

The blue bar(measurementby SNO) is thesame height asthe yellow bar(theory), withinthe uncertainty(shaded region)

Nobel Prize in Physics, 2002

Was shared byRay Davis (left)and M. Koshiba(leader of theKamiokande andKamLAND team)for their majordiscoveries aboutthe Sun and thenature of neutrinos

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The full, highly technical presentation is posted athttp://absuploads.aps.org/presentation.cfm?pid=10380

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Credit: G. Orebi Gann, APS presentation (see slide 34)9/06/12 Ast 309N (47760) 36

Credit: G. Orebi Gann, APS presentation (see slide 34)

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Borexino is within a factor of 2 or 3 of detecting(or not!) the CNO neutrinos from the Sun.

Credit: G. Orebi Gann, APS presentation (see slide 34)

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Credit: G. Orebi Gann, APS presentation (see slide 34)

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Credit: G. Orebi Gann, APS presentation (see slide 34)

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