Chapter 14

Blinn College Department of Physics

Our StarChapter 141

p. 111Ideas About the Suns Energy ProductionPre-19th centuryFire due to wood or coalRuled out after we knew the actual size and distance to the Sun19th CenturyGravitational ContractionCould maintain energy output for 25 million yearsRuled out when we discovered that the earth was far older than 25 million years

3So Why Does the Sun shine?E=mc2Conversion of matter into energy through nuclear fusionFor fusion to occur, the Sun must generate extreme temperature internally without flying apart

4Fission

Big nucleus splits into smaller pieces.

(Example: nuclear power plants)Fusion

Small nuclei stick together to make a bigger one.

(Example: the Sun, stars)

Nuclear Fission vs. Nuclear Fusion5

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The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.7

The energy comes from the fact that the Helium nucleus has slightly less mass (0.7%) than the sum of two protons and two neutrons.

It appears as kinetic energy and the energy of the gamma-ray photons8Proton-Proton Cycle

Basic reaction:4 1H 4He + energy4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He. Energy gain = m*c2 = 0.43*10-11 Jper reaction.Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons).Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.T 107 K = 10 million KThought QuestionWhat would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?

A. The core would expand and heat up slightly.B. The core would expand and cool.C. The Sun would blow up like a hydrogen bomb.The solar thermostat keeps burning rate steady.10Contraction of the sun due to its own gravity provided the energy that heated the core as Sun was forming.

Once nuclear fusion began, it generated pressure that halted the contraction.

The Sun exists is a state of gravitational equilibrium (also called hydrostatic equilibrium

Gravitational Equilibrium11Solar ThermostatDecline in core temperature causes fusion rate to drop, so core contracts and heats up.Rise in core temperature causes fusion rate to rise, so core expands and cools down.

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Calculations show that the Sun has enough hydrogen in its core to maintain gravitational equilibrium for 10 billion years.We will investigate what happens after that in a later chapter13Good Sun movies to download:

http://es.rice.edu/ES/humsoc/Galileo/Things/g_sunspots.htmlImages compiled from Owen Gingerichs copy of the first edition of Istoria e Dimostrazioni

Wave_fade.mpg photosphere:chromosphere:corona, optical:UV:X-rayhttp://www.lmsal.com/YPOP/FilmFestival/index.html

C2_1mth.mpghttp://lasco-www.nrl.navy.mil/Suns Basic StructureInsert TCP 6e Figure 14.3

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Core:Energy generated by nuclear fusion~ 15 million K15

Radiation Zone:Energy transported upward by photons until they reach a region where the temperature has dropped to about 2 million K. Photons get absorbed there.16

Convection Zone:Absorbed photons heat the gas at the bottom of the convection zone. Energy is transported upward by rising hot gas17

Photosphere:Visible surface of Sun~ 5800 K18

Chromosphere:Middle layer of solar atmosphere~ 104105 K19

Corona:Outermost layer of solar atmosphere ~1 million K20

Solar wind:A flow of charged particles from the surface of the Sun21Radiative DiffusionMost of the energy from the Sun works its way out from the core as photons. They execute a random walk to make their way out.

It takes thousand of years for energy liberated in the core to get to the top of the radiation zone.

22Energy Transport in the SunFollow a single gamma ray as it heads toward the surface.The ray is scattered by electrons in the core until it reaches cooler gasThe cooler gas absorbs the gamma photon and emits two x-ray photons, each of less energy than the gamma photonThe x-rays migrate toward cooler regions where they are absorbed and emitted as still longer-wavelength photons.This process repeats until the single gamma in the core has resulted in about 1800 photons of lower energy.23

Photons absorbed at the plasma near the surface heats the plasma and results in a convection zone (rising hot gas) taking energy to surface.Convection Zone24

The Photosphere: Granules

Bright central regions are hotter than the edges so they appear brighter.Doppler shifts show that hot gas is rising in the center and sinking at the edgesEach granule is about the size of TexasThey last about 10-20 minutes

25Figure 7.2: (b) This model explains granulation as the tops of rising convection currents just below the photosphere. Heat flows upward as rising currents of hot gas and sinking currents of cool gas. The rising currents heat the solar surface in small regions that we see as granules.

Supergranules Supergranules which are a little over twice Earths diameter, include about 300 granules each.These supergranules are regions of very slowly rising currents that last a day or two.They appear to be produced by larger currents of rising gas deeper under the photosphere.26Photosphere SpectrumBelow the photosphere, the gas is dense and hot and therefore radiates a continuous spectrum of light.Atoms in the photosphere absorb photons of specific wavelengthsproducing the absorption lines you see.

27ChromospherreThe chromosphere lies above the photosphere. Visible to unaided eye only during a total eclipsePink color due to red, blue, and violet Balmer emission lines of hydrogen.

28Chromosphere SpectrumThe chromosphere produces an emission spectrum.Atoms in the lower chromosphere are ionized, and atoms in the higher layers of the chromosphere are even more highly ionized.Can be used to determine the temperature of various parts of the chromosphere.

29Temperature ProfileJust above the photosphere, the temperature falls to a minimum of about 4,500 K and then rises rapidly to the extremely high temperatures of the corona.

30The Solar CoronaThe outermost part of the suns atmosphere is called the corona, after the Greek word for crown.You can see the inner parts of the corona during a solar eclipse.

31ChronographObservations made with specialized telescopes called coronagraphs on Earth or in space can block the light of the photosphere and record the corona out beyond 20 solar radii.

Such images reveal that magnetic fields link the sunspots with features in the chromosphere and corona.

32Coronal SpectrumComplex coronal spectrumSunlight reflected from dust particles produces a spectrum with absorption lines just like the sunWhen sunlight from the photosphere is scattered off free electrons in the ionized coronal gas, it produces a continuous spectrum without absorption lines.Fast moving electrons produce photons with a large Doppler shift smearing out the absorption linesEmission lines of highly ionized gas are superimposed on this continuous spectrum.Higher in the corona means more ionization implies the temperature rises33Coronal Temperature ProfileJust above the chromosphere, the temperature is about 500,000 K.In the outer corona, it can be as high as 2 million K or more.

34Heating the CoronaMagnetic fields extend from the photosphere to the corona. Turbulence in the photosphere whips the field aroundAs the gas of the chromosphere and corona has a very low density, it cant resist movement in the magnetic fields.This motion heats the gas35

How we know what is happening inside the Sun?36We learn about the inside of the Sun by making mathematical modelsobserving solar vibrationsobserving solar neutrinos37Mathematical ModelsUse basic physics to develop equation to describe the measured properties of the SunUsing a computer we can calculate the temperature, density, and pressure at any depth within the SunFrom these we predict the rate of fusion can predict many measurable properties of the Sun.If these models correctly predict the known properties, then we gain confidence that we really do understand what is going on in the solar interior.HelioseismologyRandom motions in the sun constantly produce vibrations like resonance in organ pipes.These are sound waves of very long wave period. These vibrations are observed by Doppler shifts.Interpretation requires a lot of data to reconstruct the subsurface features.GONG, Global Oscillation Network Group, uses telescopes spread around the world to observe the sun continuously.

Data on solar vibrations agree very well with mathematical models of solar interior.40Download a movie to illustrate solar oscillations from:http://science.nasa.gov/ssl/pad/solar/p_modes.htm

Neutrinos created during fusion fly directly through the Sun.

Observations of these solar neutrinos can tell us whats happening in core.Solar NeutrinosNeutrinos interact very rarely with matter. Trillions pass through you every second. So they are very difficult to detect.41John Updike poem - permissions needed ?Searching for NeutrinosRaymond Davis filled a 100,000-gallon tank with the cleaning fluid perchloroethylene (C2Cl4).Theory predicted that, about once per day, a solar neutrino would convert a chlorine atom in the tank into radioactive argon.This could be detected later by its radioactive decay.Results showed only one event every 3 daysThere were two possible explanationsWe didnt correctly understand how the sun and stars make their energy.There was something about neutrinos that we did not understand.

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Early searches for solar neutrinos failed to find the predicted number, but they were looking only for electron neutrons, the kind produced by fusion in the Sun

We discovered that some neutrinos change into other types (muon and tau neutrinos) in a process called oscillation.

More recent observations find the right number of neutrinosSolar Neutrino ProblemIllustrates the great synergy between astrophysics and subatomic physics43This solution to the solar neutrino problem is exciting because neutrinos cant oscillate unless they have mass.Neutrinos were long thought to be massless.However, if they have even a small mass, they are so common their gravity could affect the evolution of the universe as a whole.44

Solar Activity45Solar activity is like weather. SpiculesSunspotsSolar flaresSolar prominences

All these phenomena are related to magnetic fields.They can at times affect our daily lives.46SpiculesSpicules are small plasma burstfrom the photosphere into the chromosphere.

They are created by periodic sound waves leaving the sun.The plasma is concentrated to a small thread like jet due to strong magnetic fields.that last 5 to 15 minutes

47Cooler than other parts of the Suns surface (4000 K)

Regions with strong magnetic fields

They appear dark by comparisonSunspots

48Observing the SunIn the early 17th century, Galileo observed the sun and saw spots on its surface.Day by day, he saw the spots moving across the suns disk.These are sunspots.He rightly concluded that the sun was rotating.

49Sunspot Rotation

http://www.youtube.com/watch?v=U0Lt3SgiEQ8

The TRACE satellite can detect the hot gas trapped in the magnetic fields arching above the sunspot group. (NASA/TRACESunspots tend to occur in pairs connected by magnetic fields, represented as lines in this drawing.51Magnetic Loops

Magnetic field lines0Sunspots tend to occur in pairs resembling a bar magnetPolarity different on opposite sides of the solar equatorAt the end of an 11-year cycle, the new spots appear with reversed magnetic polarity.

53We can measure magnetic fields in sunspots by observing the splitting of spectral lines.

Zeeman Effect54

Gas in the chromosphere and corona can become trapped in the magnetic fields of sunspots resulting in large prominences that can rise far above the surface.Solar Prominences55Prominences

Looped prominences: gas ejected from the suns photosphere, flowing along magnetic loopsRelatively cool gas (60,000 80,000 oK)May be seen as dark filaments against the bright background of the photosphere0

Prominence in UV Hot plasma trapped in the magnetic field loop above the chromosphere into the lower corona.In the visible region they look pink due to the three visible Balmer lines. From above these look dark against the surface and are called filamentsFilaments

Magnetic fields can become twisted. At some point the field will suddenly reorganize itself and in the process release excess energy heating the plasma to 100 million K releasing a burst of X-rays and accelerating charged particles to nearly the speed of lightSolar Flares59Some magnetic field lines extend far out into spaceGas from the solar atmosphere follows along the magnetic fields that point outward and flows away from the sun in a breeze called the solar wind.The Solar Wind

This X-ray photograph shows dark regions called coronal holes from which much of the solar wind escapes.60

AurorasSolar winds, guided by the Earth magnetic field, and low density gases at ~130 km above the surface create conditions like the gas discharge tubes.

Coronal Mass EjectionsFlares and other solar storms sometimes eject large numbers of charged particles from the solar corona.Called a coronal mass ejectionIf aimed toward Earth, they can create a geomagnetic storm,Particularly strong auroraDisrupt power deliveryInterfere with communicationsDamage satellites

Variation in Sunspot Activity with Time: The Sunspot CycleAverages 11 years, min to min, but varies from 7 to 15 yearsSunspot minimumFew, if any, sunspots visibleWe just emerged from one a year or so agoSunspot maximumMay see dozens of sunspots simultaneouslyThe frequency and intensity of solar flares, prominences, and CMEs, follow the sunspot cycle

65Maunder Butterfly DiagramSunspots appear at higher latitudes (farther from the equator) early in the cycle, and at lower latitudes later in the cycle.

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Maunder MinimumThe Suns Magnetic CycleThe Babcock modeAs the electrons in the ionized gas carry any magnetic field. The field is is frozen into the gas.

68Where these magnetic tubes burst through the suns surface, sunspot pairs occur.After about 11 years the field is so tangled that it begins to rearrange itself into a simpler structure, but with opposite polarity

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