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by: Keegan Goyette


Marketplace > University of Texas at Austin > Astronomy > AST 309N > LIVES AND DEATHS OF STARS
Keegan Goyette
GPA 3.68


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This 35 page Class Notes was uploaded by Keegan Goyette on Sunday September 6, 2015. The Class Notes belongs to AST 309N at University of Texas at Austin taught by Staff in Fall. Since its upload, it has received 56 views. For similar materials see /class/181760/ast-309n-university-of-texas-at-austin in Astronomy at University of Texas at Austin.




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Date Created: 09/06/15
if 1 The Sun A Main Sequence Star Q What Powers the Sun Readings Wheeler pp1626 O ProtoStelar Collapse I t t 39 N t quot5 rgzc t iro quotf es 0 Energy Flow In the Sun 0 Nuclear Fusion in the Sun 4gt and solar neutrinos i a Theories about solar Energy The first scientific theories involved chemical reactions or gravitational collapse 0 chemical burning as in fossil fuel ruled out can t account for the Sun s luminosity lasting long enough 0 conversion of gravitational potential energy into heat as the Sun contracts would only keep the Sun shining for 25 million years would cause a noticeable shrinking over centuries 0 late 19thcentury geological research indicated the lrth was older than this Dinerstein Ast 309N 2 I How the Sun Makes its Energy Nuclear physics provided the answer 0 the Sun generates energy via nuclear fusion reactions 0 Hydrogen is converted into Helium in the Sun s core 0 the mass lost in this conversion is transformed into energy 0 the amount of energy that is created is given by Einstein s equation E mc2 m is the converted mass 0 given the Sun s mass this will provide enough energy for the Sun to shine for 10 billion years ii 3 Dinerstein Ast 309N I at Stellar Collapse Stage 1 ll Stage 2 ll Stage 3 u 4 Stage 4 A Vk 1 Striking a Balance The Sun began as a cloud undergoing gravitational contraction Gravitational energy is converted to thermal energy and eventually the central regions become hot enough for nuclear fusion reactions Energy generated by fusion reactions provides thermal pressure measure The outward thermal pressure balances quotWquot 39 the inward force of gravity this is called gravitational or hydrostatic equilibrium the Sun s radius remains constant the pressure is strongest where gravity is strongest deep inside the Sun the pressure is weakest where grew i weakest near the surface k Dinerstein Ast 309N 5 23 Pressure lt3 Gravity W32 Am wvm moss sch or sun Basic Properties of the Sun Luminosity 38 X 1026 watts Mass 20 X 1030 kg Radius 7 X 105 km Density 141 gcm3 Distance 15 X 108 km 1 AU Dinerstein Ast 309N 7 I lnClass Notes Dinerstein Ast 309N an 13quot x 2 Prussure vs grzvl a d mane balance Temperature 3 dens y Frames 1 Methods of Energy Transport wC ass Nmes O Radiative di usinn O Cnnve inn O Cnnduclinn O Neutrinns a E 1i an M u A n Energy Transport by Radiation energy travels as photons which interact with particles with each such interaction they can change direction random walk and energy hence wavelength 0 This is a slow process It takes about 1 million years for energy to travel from the core to the surface at x it 91505 Dinerstein Ast 309N Energy Transport by Convection Photons arriving from below are absorbed by an opaque layer This region is strongly heated causing hot gas to rise up and cooler gas to sink just like a boiling pot of water The net effect is that energy is carried upward along with the moving material Layers of the Sun Core Central temperature 15 million K Region of nuclear reactions inner 25 by radius Interior T lt 8 x106 K depth 025 086 Rsun Energy is transported through the interior The interior is divided into two zones Radiation Zone inner 70 by radius Convection Zone outer 30 by radius Temperature at the boundary is 2 million K 91505 Dinerstein Ast 309N 15 gtgto How does nuclear fusion work Nuclei are positively charged so the repel each other electric force The nuclei must be moving with high average kinetic energies in order to push past this repulsion If they get close enough nuclear forces overcome electric force and create a new heavier nucleus High energies correspond to high temperatures in m no mm m Hut Dinerstein Ast 309N H Fusion via the ProtonProtonChain 00 A I u N i Mi sun Slap 39 n m Dinerstein Ast 309N Net result 4 H nuclei are converted into a He nucleus energy is released TexI mam a O rcumv 3 Wm m 39 18 H Fusion via the CNO Cycle r O 27 Q 1 4 N g NH He 35quot lnClass Notes an Dinerstein Ast 309N xi 4 1 Wavelength amp Frequency 33 Dinerstein Ast 309N 1 EM SpectrumrAll Waves i i i fi ii U i ii if 39i Kirchhoff s Laws 1 A hot dense glowing object emits a continuous spectrum N A hot lowdensity object produces an emissionline spectrum W When light with a continuous spectrum passes through a cool gas an absorption line spectrum is produced it 2 tr 3 an Dinerstein Ast 309N Kirchhoff s Laws continuous s not light source may gas My 6 emission Iin la n mamamm i I u iw umusmnmuum thin cloud of k 7 k k 7 r HowAtoms Absorb and Emit Photons Light Light is both wave amp particle 0 Light as travelling waves 0 Light as photons energy packets 0 Both aspects needed to describe the actual behavior of light 4 InClass Notes Kinds of Spectra 0 Continuous no gaps in wavelength special case 0 Brightline or Emission lines arises from gas 0 Dark or Absorption line shadows ht H a arises from gasxk Dinerstein Ast 309N I Kig z quot Kg k Blackbody Spectra Blackbody Spectra 0 Characteristic shape Q Hotter objects peak a v He n while cooler ones peak at plot of intensity vs wavelength lt51 72 InClass Notes Blackbody Spectra Q objects emit energy at all wavelengths in proportion to temperature4 a D 91305 Dinerstein Ast 309N w 12 if Uses of Spectral Lines 0 Composition match with speci c elements 0 Relative speed shifts due to Doppler erect M IITCIaSS NoleS a D was Dmn um i Approaching object wave fronts pushed togetheraiii frequency Receding object waves fronts stretched apart ency GD Doppler Effect r Examples o pectra Structure of Matter Phases Blackbody emitters Planets rocks stars Emissionline spectra Nebulae illuminated Q 3039 39 by hot stars H II regions planetary 0 Liquid nebulae Gasfreel movin articles Absorptionline O y g p spectra Star atmospheres interstellar clouds neUtral 3 plasma 91305 DinersteinAst309N 17 5 25 InClass Notes Types of Pressure Q Thermal ordinary or ideal gas Q Degenerate very dense gas 0 Radiation momentum of photons if at it 1 a 91305 Dinerstein Ast 309N 20 it 79339 3 The Sun A Main Sequence Star Q What Powers the Sun Readings Wheeler pp1626 O ProtoStelar Collapse Instructor s Notes Section 0 Energy Flow in the Sun 0 Nuclear Fusion in the Sun 4gt and solar neutrinos 1 k xv Sun s Source of Energy C Chemical Reactions 0 Contraction uses grav potential energy Nuclear Reactions G if 2 A 7K 23 Stellar Collapse Stage 1 ll Stage 2 ll Stage 3 u 13 Stage 4 it InClass Notes yA a vD 91305 Dinerstein Ast 309N 24 379 Pressu re 9 G ravity 1 fr Hydrostatic Equilibrium Pressure vs gravity a delicate balance i1 Temperature amp density pro lesquot x ianiaSS Notes 3 ezi a D mm 1 mm a 1 75139 Solar Interior Lnodel Conditions for fusion As Temperature distribution 0 High densities r gt 0 MW tempem iweg gt Temperature 106 K I 0 Quantum Eun nne ng ee HIM 1 H W H vuv 1 IN D 39t 139 t 39b t39 enSI y K 15 n u 10 5 Energy production TH E 100 V E an K 807 5 a i E E g 80 as U u 4 g C E 40 k m quotd 2 20 k k 0 on 0 015 02 025 0 3 035 n DIu5 I DL 1 403 0 2 l ml w y 8 5 c I 330 I 0 RRo E 1 Forms of Energy Potential energy Kinetic Thermal Chemical Electromagnetic 0000 2 Conservation of Energy 0 Energy can be converted from one form to another O The total amount of energy sum of energy of all kinds remains constant 1 Momentum the third property Q Forms Linear straight line 1 ngular of rotation Angular momentum the momentum involved in l 2 Conservation of Momentum No change in velocity speed direction or both unless which causes an acceleration Conservation of angular momentum makes collapsing objects 1 a ma 1 m x v ulnaa arm Iman mm mm m uvuunr mun cl wlluun 7f 3 Structure of Matter Subatomic Particles 0 Massive particles baryons eg protons 7 neutrons 7 O Lighter particles leptons eg electrons 7 neutrinos 7 M Matter is made of Atoms Nucleus electrons protons neutrons Ms awn 90805 DinersteinAst309N 6 Hydrogen Helium atomic number 1 atomic number 2 atomic mass number 1 if atomic mass number 4 if aD 90805 DinersteinAst309N 7 GED 90805 Dinmdn39m wm 8 Deuterium another isotope of hydrogen 6 atomic number 1 atomic mass number 2 gt ilk XL awn 90805 DinersteinAst309N 9 What if an electron is removed ion has an atomic electric charge number 2 He1 atomic mass number 4 r 3 Gib 90805 Dinerstein Ast 309N 10 The particles in the nucleus determine the element amp the isotope an11cm AW 1 9mm nmlammmr a mutual pm mm Hydrogen H Hallum Hoi carbon quotCi 8 re 4 quot1 19 w new nlduclmhamnllaImI ummmm numb In 1411 r 9 a 1 r c 11 1 5 1 1 Wm m M a t k munwwwqu Wiwnu Iwm nLlthwm pmlmi but dimler mmnors urnmm aiD 90805 DmerstemAst309N 11 InClass Notes 8 4 if Gib 90805 Dinerstein Ast 309N 12 21gt Structure of Matter Atoms 5 Nucleus protons neutrons I Q1 Q Electrons in xed orbitsorbital clouds cmj wrmlgu k as if Structure of Atoms 21gt A Short History of antimatter Q Opposite charge but same mass etc Antielectron positron predicted by Dirac 1933 Nobel Prize measured by Anderson 1936 prize CD Antiproton detected in 1950 s Bevatron particle accelerator LBL 71 Creation of antihydrogen C Antihydrogen atoms created in 1995 In 2002 2 experiments using CERN s Antiproton Decelerator made lots of antiH Importance k z 4 Wavelength amp Frequency L 7 K Dinerstein Ast 309N A t a y I hm fag3 HHWL wakL x1M w 1 1 w 777 Shouwaves Langwaves hlghlrequancv lawhcquennv L 39 quot I F 17 Wavelength amp Frequency The Electromagnetic Spectrum L m EIeclIamagnetc Spectrum u llmbl I 39 a mvlo N I n ram Wm 1039 m m r 10 Wavulcngm m mew Speed 3 x10I mls InClass Notes 2 6 if Dinerstein Ast 309N CID Kirchhoff s Laws 1 A hot dense glowing object emits a continuous spectrum A hot lowdensity object produces an emissionline spectrum N A When light with a continuous spectrum passes through a cool gas an absorpti n line spectrum is produced gt I Dinerstein Ast 309N 21 Kirchhoff s Laws continuous sectum h m M a 3 hillnclouddj39 o g 5 me wimzl a ga emission line spectrum anngr mi Adm Waslsj wmw umusmn Astguyn 22 k How Atoms Absorb and Emit Photons Light xv Light is both wave amp particle 0 Light as travelling waves 0 Light as photons energy packets 0 Both aspects needed to describe the actual behavior of light Rye 2 k f 7 Kinds of Spectra Blackbody Spectra 0 Continuous no gaps in wavelength 3 Characteristic shape special case 5 Brightline or Emission lines PIOt 0f intenSity vs wavelength arIses from gas 3 Dark or Absorption line shadows zi f 23gt arises from gas k Xlt z agarquot Blackbo Spectra Blackbo Spectra O objects emit energy at all 0 H tt b39 t k ato er 0 Jec s pea wavelengths in proportion to temperature4 HE peak at Examples ofSpectra quot Blackbody emitters Planets rocks stars quot Emissionline spectra Nebulae illuminated by hot stars H II regions planetary nebulae quot Absorptionline spectra quot Star atmospheres interstellar clouds a 90805 Dinerstein Ast309N 29 24gt Uses of Spectral Lines 0 Composition match wavelengths with speci c elements ngerprints C Relative speed shifts due to Doppler effect change in A proportional to r4 Structure of Matter Phases 0 Solid 0 Liquid Q Gas freely moving particles neutral plasma it Types of Pressure Q Thermal ordinary or ideal gas 0 Degenerate very dense gas C Radiation momentum of photons ll 11m 0 Ast 309N quotStar Bizarrequot THE SUN A MAIN SE UENCE STAR We begin with the ordinary before moving onto the extraordinary so we begin with the suna typical MS star But even ordinary stars are quite remarkable A What is the Source of the Sun s EnergV 1 Chemical Reactions In the 19th Century people proposed combustion reactions as the Sun s method for generating energy 2H2 02 ZHZO 13 x 1011 ergsgm To provide the solar luminosity would take using up mass at a rate of L 33 m 2 o 4gtlt10 ergsec 22 E 13 X lonerggm 3 x 10 gmsec This could last a maximum of t Mo 2X1033gm m 3X1022gmsec gt 2150 years 2 Gravitational Potential Energy If the Sun contracts shrinks slowly gravitational energy is released converted into kinetic energy of infall then to random or thermal kinetic energy and then to radiation This does occur in other stages of stellar lives particularly pre Main Sequence statges protostars and in red giants and supernovae m it still would last only 20 million years 3 Nuclear Reactions Provide the only energy reservoir large enough to keep the Sun radiating for billions of years Also provide an energy source for massive Main Sequence stars with higher luminosities though they use it up faster 2 The Cycle of Stellar Collapse What happens when the star contracts Stage 1 Sta e 2 Stage 3 Gravitational BulkKinetic lnternal Kinetic potential energy Energy Thermal Energy free fall randomized motion By Stage 3 thermal pressure resists collapse The Virial Theorem tells us that the amount of thermal energy generated is half of the original gravitational energy The rest is radiated away However hot bodies continue to radiate energy By stage 3 thermal pressure resists collapse Problem As the star keeps radiating loses energy the gas pressure drops P cc T so gravity wins again and the star contracts again What s to stop this process from continuing over and over again with the star collapsing to an arbitrarily small size and high density a black hole Where does it end 1 When nuclear reactions begin providing a different source of energy to replenish the energy lost through radiation E 2 The gas pressure becomes degenerate so that the pressure balances gravity but doesn t change when it cools 3 Hydrostatic Eguilibrium In the above we have implicitly applied the principle of hydrostatic equilibrium or balance between gravitational force inward and 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution pressure 2 force per unit area outward This condition must be satis ed throughout the star why Gravity depends on mass M radius R I Thermal pres sure depends on temperature T and density p which is related to M R3 gravity inward due to enclosed mas s This sets up a gradient or smooth variation of conditions in the solar interior A calculation which predicts the values of the physical quantities such as p T P at each level is called a stellar model or stellar interiors model For the Sun eg 100 15 A 9 mg 10 10 E E Q 1 F 5 O radius a Re radius a Re Aside These particular temperature and density pro les assume that support is provided by thermal gas pressure the kind given by P cc pT This is not the only possibility A star can also be supported by electron degeneracy pressure balancing gravity Such a star has a particular density profile variation of p with radius but its temperature will depend on its previous history For example consider a low mass cloud that is collapsingcontracting and trying to become a star If it becomes so dense that electron degeneracy pressure stops the collapse before the central temperature gets high enough to start nuclear reactions you have a brown dwarf 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution This will happen if the star begins with less than 08 Mo Such a star will be cool at the surface T lt 2000 K On the other hand late in the life cycles of stars up to a few solar masses the core regions become degenerate while the outer layers are expelled into space This leaves a degenerate star of about 06 1 Mo called a white dwarf It starts out hot because of heating during earlier phases of nuclear burning Both types of degenerate stars remain stuck at a fixed radius They continue to radiate they are blackbodies but have no means to generate more energy so they get cooler and fainter following a cooling track on the H R diagram C NUCLEAR FUSION IN THE SUN 1 Energy Production The overall process of H burning converts four protons 1H into a helium nucleus He also known as an alpha particle or releasing a total of energy of 267 MeV 267 x 107 eV 2 4 x 1039 ergs This energy comes from the amount of mass lost in converting H a He the mass defect Fuel 4 protons 4 x 1008 atomic mass units amu 4032 amu Product He 2 4004 amu Mass converted 028 amu m conv Energy yield 2 mcmc2 028 amu x 16 x 10 E x 3 x 1010cmsec2 amu 4 x 10395 erg Much of the energy comes out as photons Compare this energy with the energy of a typical photon emitted at the sun s surface with 9 2 5000A E 5000A hf h gtlt 66 X10727ergsec X 3 X 1010 cmsec 5 gtlt103 gtlt10398cm 20gtlt10 17 5gtlt10 5 erg 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution 4 x 10 12 erg per photon E photon solar core 4 gtlt10 5 Ratio 712 E photon solar surface 4 X 10 107 The newly created photon has 10 million times the energy of the surface photon it s a gamma ray 2 Energy Transport We do not of course directly amp those newly created photons Although emitted at the speed of light of coursel they are soon absorbed by nearby nearby nuclei typically after travelling a mere 1 cm path How does the energy get to the surface a Radioactive transport or diffusion Photons are absorbed by nearby matter but that matter then in turn emits photons The latter come out in random directions but averaged over a long time many absorptions and emissions the photon will follow this random walk all the way to the surface This takes about 104 years and nearly all of the photon s energy is lost deposited as thermal energy in the intervening layers see last page center quotrandom walkquot b Convective Motions The energy is absorbed and converted to thermal energy If the energy changes quickly with depth parcels of hotter gas 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution from below will rise up bubble displacing cooler material that then will sink before the radiation has time to carry the energy by diffusion cool gas sinks g hot gas rises cooler layer hotter layer c Conduction In a solid or a degenerate gas particles mainly Vibrate around a relatively fixed position but can transmit or pass along their thermal energy to adjacent particles through electric forces This is how heat is transmitted in a metal for example It plays very little role in the Sun but is important for white dwarfs etc cool hot d Neutrinos A small fraction of the energy from nuclear reactions comes out in the form of neutrinos another kind of lepton like electrons Neutrinos interact only weakly through the weak nuclear force with matter they have zero or very small rest massesand travel at nearly the speed of light Overall the p p chainwhich is the specific H burning reaction in the Sun produces 4pa4He2e2vey 2 positrons 20 MeV 06 Me the rest of the energy 1 MeV 106eV 16 x lO396ergs The neutrinos basically pass out of the Sun as if it were transparent 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution R 10 This takes t o 7X10 em 23 sec C 3 gtlt101 cmsec If we could detect them we could peer directly at the region undergoing fusion in the solar core 3 Conditions for Nuclear Fusion Nuclear fusion can occur at a rate that produces a significant amount of energy only under conditions of high temperature and density This is usually explained by saying that the kinetic energy of the nuclei must be great enough to bring the nuclei close together overcoming the electric repulsion until the strong nuclear force which operates over only about 10 13cm can take effect The real story is more complex Actually even at 107 K the typical thermal energy per nucleus is a factor of 100 or more less than the energy of electric repulsion the Coulomb barrier However due to quantum mechanical effects the tendency for matter to be somewhere else a small fraction of the time the nuclei can tunnel through this barrier and get close enough for the nuclear force to take over p Energy repulsron by electr1c force repels tunnels through the barrier initial energy attracts separatron of p the nuclei 3 gt lt 3 Even in the Sun this is a rare event but there are enough chances for it to happen that energy is produced at a steady rate Higher temperature hence initial energy helps by making tunneling more likely 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution 4 Reactions in H burning with an eye on the neutrinos a Digression Notation for nuclear reactions 2 1H proton n neutron baryons 3El 2 nucleus with proton given by the element name see table a p s n s 4He 2 or alpha particle e39 B electron e positron or anti electron annhilates with matter 7 photon v8 neutrino produced along with an electron similarly v with a muon v with a tau particle b The main route for the p p chain is the following 1 p p a 2H e ve low energy neutrino E S 04 MeV 22Hpa3Hey 3 3He 3He 4He p p y from 2 sets of reactions 1 and 2 This creates one helium nucleus plus two low energy neutrinos Reaction 1 is called the pp reaction so its neutrino is called a pp neutrino Reaction 2 is always part of the chain reaction 3 follows in 85 of the cases The rate for 3 was poorly known a few years ago but is now well known Less often once per 250 p p reactions 2H deuterium is made by the pep reaction l p e39 p a 2H ve pep neutrino E 14 MeV These neutrinos are much rarer but each has a larger energy They are easier to detect Once 3He is formed there are actually 3 different ways to get 4He which is the stable isotope Step 3 above is the route taken 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution 85 of the time it produces no extra neutrinos c side chains alternate ways to get 3He 4He 3 3He 4He 7Be 7 about 15 of the time Then most of the time 7Be e a 7Li ve E2086 MeV 7Li p a 4He 4He aTwo helium nuclei job is done 3 7Be p 8Be 7 One time in 5000 8B 8Be e ve Boron decays fission 8Be 4Her 4He very high energy neutrino S 15 MeV Two helium nuclei again Finally an extremely rare branch is 3 3He p irHe e ve Highest energy V one helium nucleus This is called the hep reaction All of these reactions go on simultaneously some much more frequently than others The result is a range of many v s many with low energies some with high energies a few with specific energies neutrino lines just like emission lines for photons The next page shows the predicted neutrino spectrum for the standard solar model d The CNO Cycle H burning Via the CNO cycle involves essentially building a 4He nucleus on a 12C platform then splitting it off 4p 12C a 4He 12C many steps several V s The neutrinos that come out have relatively high energies up to 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution 12 and 17 MeV The CNO cycle however is now believed to constribute only 16 of the Sun39s en rgy th rest coming from the pp 39 e un one important process c ain esc ove is incomplete CNO cycle processing which converts C into N y I E 1 Solar neutrino i spectrum T 39 m l a 39 I E c l E i k 5 5 a 10 E 5 M 1 P315 10 I E 10 I 11 01 10 Neutrino energy 9 in MeV John N Bahcall Neutrino Astrophysics 1989 p 154 e The Standard Solar Model The neutrino spectrum on the previous page was calculated using the best possible mode of the solar interior The model is a detailed p ription for the conditions throughout the solar interior including density emperature pressure and rate of energy generation at each la er To be v 39 it must predict the correct radius luminosity etc for the presentrday Sun A few major characteristics are Central temperature 156 million K 1998 Harriet L Dinerstein sIIonomy Central density 150 g m cm 984 ofenergy from pp chain 16 from CNO cycle The luminosity in neutrinos is 23 oqumL The Sun is assumed to start with an initial compostion that is the same throug out and the same as the resentday surface composition roughly 27 Helium by mass nearly a t e rest H In the core about half the initial H has been converted to He In the innermost 074 by radius energy is transported mainly by photons in the outer quarter by convection Table 46 Properties of the solar model as a function of time Time Luminosity Te Radius Tc 5B 109 yr L0 K Re 107 K 106 our1 5 000 07095 5625 089 134 014 006 07198 5642 0 89 134 0 l4 0 28 07339 5655 0 B9 135 0 17 0 50 07457 5660 090 136 021 150 07955 5688 092 L40 0 45 2 50 08517 5718 0 94 144 0 98 350 09163 5745 097 149 224 460 10000 5772 100 156 575 IohnN BahcallMwmmAmgphxmi1989gtP 1 C THE SOLAR N39EUTRTNO PROBLEM 1 The Davis Exlgriment In the late 196039s some Ventutesolne folks had the bold ide a of setting up an experiment to measute the neutrinos from the Sun to test our drycleaning uid 2C1 Was set up in the Homestake Mine in Lead South Dakota One ofthe isotopes of Cl Cl reacts as follows v Cl gt e39 Ar 1998 Hanlet L Dlnelsteln if the neutrino energy is 2 0814 MeV The new 37Ar nucleus is radioactive and decays after typically 35 days The experiment is run by letting 37Ar atoms accumulate for a while 1 3 months then removing them by bubbling He gas through the tank which sweeps out the noble gas Ar Results 37Ar is indeed produced by solar neutrinos at the rate of lss than one one new 37Ar one detection every other day or Experimental result 26 i 02 SNU 1 SNU 2 solar neutrino unit 10 36 detections per target atom per second Compare to Prediction standard solar model 95 i 13 SNU This is the solar neutrino problem the fact that the experimental results disagreed with the prediction The initial hope that the early results were a fluke faded with time the the Davis experiment has now been running for three decades and the discrepancy has persisted In fact within the last few years several independent experiments yielded similar results Kamiokande H in the Japanese Alps originally built to search for proton decay was modified in 1986 to detect solar neutrinos This is a large water tank 3000 in which neutrino electron collisions are sensed by Cerenkov radiation visible light pulses from the rebounding electron lt al found too few solar neutrinos about half as many as expected although it is sensitive to only neutrinos of even higher energy than the Davis experiment 3 Possible Solutions In searching for an answer to this discrepancy the rst point to remember is that the Davis experiment is only sensitive to neutrinos with energies greater than about 08 MeV Therefore it simply doesn t g the p p neutrinos most of the neutrinos it detects come from 8B and 7Be Likewise the Kamiokande experiment also only senses the high energy neutrinos from 8B These are only a small fraction of all the neutrinos produced in the Sun Look again at the neutrino spectrum on page 20 Therefore one could account for the results if for some reason the route leading through 7Be and 8B happens a smaller fraction of the time than predicted by the standard solar model Possibility 1 The standard solar model is wrong Blame the astronomy 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution In order to change the prediction one has to assume some rather peculiar things For example one suggestion was that the solar interior was rotating 1000 times faster than the surface This can probably be ruled out today from studies of oscillations seen at the solar surface which result from the pressure waves that bounce back and forth between the surface and the bottom of the convective layer The study of these oscillations is called helioseismology These oscillations are affected by the internal structure and if the Sun s interior were rotating this fast we d know it Another especially bizarre idea is that there is a small black hole at the center of the sun and some of the solar luminosity is produced by the release of gravitational energy as matter falls into the black hole This one is worrisome because such a black hole would grow at an accelerating rate and would eat up the entire sun in the next few years Alternatively perhaps the astronomers are correctly predicting the rate of neutrinos produced but the neutrinos turn into something else on the way to Earth Possibility 2 Our understanding of the nature of neutrinos is incomplete we need a new theory of subatomic particles Blame the physics It was suggested that neutrinos decay or metamorphose into something else as they travel from the solar interior to the surface of the Earth To understand this idea consider that there are knownto be three varieties or flavors of neutrinos those associated with the production of electrons V8 muons Vu and tau particles VT The neutrinos produced in H burning in the solar core are electron neutrinos The MSW effect Mikheyev Smimov Wolfstein suggests that as they travel from a region of high electron density the solar core to a region of low electron density empty space some of them change identity and become neutrinos of one of the other two avors This explains the experimental results because the detectors used so far see onlxthe electron neutrinos Somewhat confusingly physicists call this phenomenon neutrino oscillations because the neutrinos shift back and forth from one form to another This effect requires something more than the standard electroweak theory see p 5 Only some of the proposed Grand Unification theories which unify the strong weak and electromagnetic forces predict it Another consequence of this idea is that neutrinos have a non zero mass That has important cosmological consequences and might solve to puzzle of the dark matter that plagues astronomers 3 The Gallium Experiments 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution A major advance in this area has come with the recent completion of construction of a new kind of neutrino detector using gallium instead of chlorine to measure solar neutrinos The reaction is ve 71Ga a e 71Ge for any neutrino with energy 2 023 MeV The resulting germanium is radioactive and can be easily counted Most important of all because of the low minimum threshold energy for the neutrinos this experiment can detect V s produced by the fundamental p p reaction Therefore you can t blame a deficiency of these on something like the branching rate as for the 8B neutrinos The rate of production of p p neutrinos is directly tied to the total energy production rate of the Sun and therefore to its luminosity There is no wiggle room here The first gallium experiment to become operational was SAGE the Soviet American Gallium Project note the obsolete name which obtained its rst results in 1991 These results were striking they confirmed the Homestake experiment by also nding a deficit of energetic solar neutrinos The SAGE experiment used solid gallium as a detector located in a mine in the Caucaus mountains After the gallium is converted to germanium the experimenters have to dissolve the metal in Hcl acid to measure the result A second gallium based experiment called GALLEX was set up in Gran Sasso in Italy also underground This is a collaboration among several European nations the United States and Israel For this experiment the gallium is suspended dissolved in liquid making it easier to extract the product germanium Both gallium experiments which can detect at least some of the key p p neutrinos nd a deficiency of neutrinos They detect some but not as many as predicted After several years of operation for SAGE and GALLEX both are detecting about 70 SNU while the predicted rate is about 140 SNU To summarize the results of these four established experiments the results disagree by factors of 2 4 depending on the energy range of the neutrinos detected This gets pretty tricky The deficit for low energy p p neutrinos measured only by SAGE and GALLEX is a factor of 2 higher energy neutrinos fall short by a factor of 4 while the highest energy neutrinos seen by Kamiokande are deficient by only a factor of 25 It seems that the only way out of this paradox is for the neutrinos to suffer conversion either due to the MSW effect pg 24 or even to vacuum oscillations neutrinos changing types or avors in empty space Furthermore this effect must be different for different 1998 Harriet L Dinerstein Astronomy 309N Stars and Stellar Evolution


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