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PHYS420:Astrophysics &Cosmology Dr RichardH.Cyburt AssistantProfessorofPhysics Myoffice:402cintheScienceBuilding Myphone:(304)384-6006 Myemail:[email protected] Mywebpage:www.concord.edu/rcyburt Inpersonoremailisthebestwaytogetaholdofme. MyOfficeHours TR5:30-17:00am W5:00-5:00pm Meetingsmayalsobearrangedatothertimes,byappointment Midterm1 Thurs.Feb9,4-5:15pm CosmicPerspective&ScientificMethod(s) History&developmentofModernScience&Cosmology InformationfromLight DouglasAdams Hitchhiker’sGuidetotheGalaxy WhatistheUniversemadeof? InformationfromLight Starsarelightbulbs ◦ Ifweunderstandstarsandhowbrighttheyare,wecandeterminetheirdistances Star-lightcarriesinformation ◦ Grossfeaturesofthebrightness(orintensity)spectrumtellsussurfacetemperature ◦ Absorptionoremissionspectratellusaboutthecompositionofthecosmos WhatistheUniversemadeof? 1 Hydrogen Carbon Helium Relative abundance 10–2 Oxygen Neon 10–4 Nitrogen Magnesium Iron Silicon 10–6 Sulfur 10–8 Boron 10–10 Lithium Beryllium 10–12 1 10 20 30 40 Atomic number 50 Discoveries 1800:WilliamHerscheldiscovers“heatrays” 1801:JohannRitterdiscovers“chemicalrays” ◦ Laterbothrenamedinfared andultraviolet 1895:UVradiationbelow200nm 1895:WilhelmRö ntgen discoversX-rays 1897:J.J.Thomsondiscoverselectron 1899:E.Rutherforddiscoversalphaparticle 1900:P.Villarddiscoversgammarays 1911:E.Rutherforddiscoversatomicnucleus ParticleDiscoveries TheAtom Thomson ◦ Electronsuniformlydistributedthrough-outatom ◦ Likeraisonsinplumpudding Rutherford ◦ Foundoccasionalback-scattering ◦ Aconcentratednucleusofpositivecharge MassSprectrometer Candeterminemass-to-chargeratioof atoms FrancisWilliamAstonfoundthatthere existsatomswiththesameatomiccharge, butwithdifferentatomicmasses. Hefoundthatthesemassesvariedby integeramountsanddevelopedthewhole numberrule. Chadwick’sdiscoveryoftheneutron explainedthisobservation. Anucleuscontainsprotonandneutrons, bothwithroughlythesamemass,butonly protonscarryingelectriccharge. TheAtom Nucleustinycomparedtoatom Nucleusmadeofneutronsandprotons Cloudofelectronsaroundnucleus ◦ Occupyingspecificorbitals Allthatpositivechargeboundinsuchatiny space…... What’supwiththat???? PeriodicTablenotadequateforisotopes ChartofIsotopes Eachisotopehasaspecificnumberofprotonsand neutrons Anelementisdefinedbythenumberofprotons ◦ It’satomicnumber ◦ Isotopesallhavethesamenumberofprotons ◦ Theyareallofthesameelement ◦ Theyonlydifferintotalmassornumberorneutrons Notallisotopesexistinnature! Why??? ForcesofNature Gravity:Allmassiveobjectsareattractedtoeachandeveryothermassiveobject Electromagnetism:Chargedparticlesattract/repeleachother Strongforce:NeededtoholdALLnucleitogether,otherwiseEMrepulsionwouldexplodenuclei Weakforce:Neededtoexplainstableandunstablenuclei&radioactivedecay Howdoweunderstandthesenuclideproperties? TotalEnergyofsystem Takeplanetsorasimplehydrogenatom: ◦ Examthetotalenergyandaskwhatitmeanstobebound TotalEnergy=KineticEnergy+PotentialEnergy Whatdoesescapespeedtellus? BorrowfromEinstein E=mc2 Energyandmassarethesamething,interchangeable ActualMassofSystem=MassofConstituents+KineticEnergy+PotentialEnergy Ifasystemofconstituentobjectsisbound,whatdoesthatmeanforthemassofthesystem? BorrowfromEinstein E=mc2 Energyandmassarethesamething,interchangeable ActualMassofSystem=MassofConstituents+KineticEnergy+PotentialEnergy Ifasystemofconstituentobjectsisbound,whatdoesthatmeanforthemassofthesystem? ActualMassofSystem<MassofConstituents BindingEnergy=MassofConstituents– ActualMassofSystem BindingEnergyofNuclei Nucleicanbepicturedasidealizedspheres VolumeofNuclei=CountingNucleons (neutronsandprotons) ◦ Volume∝ R3 ∝ A=N+Z ◦ NuclearRadius,R=r0A1/3 SurfaceArea∝ R2 ∝ ◦ Payapenaltyforbeingnotequal ◦ Penalty∝ A2/3 CoulombEnergyofProtons∝ Absenceofallotherforces,neutronsand protonsarethesame (NucleistrivetowardsN=Z) $% & $% ∝ '(/* +,$ ' Semi-EmpiricalMassFormula BindingEnergy=VolumeEnergy– SurfaceEnergy– CoulombEnergy– AsymmetryPenalty /0 1 = 𝑎456789 − ;<=>?@AB ( 1* − ;CD=EDFG H % I 1* − N,H 𝑎1JK889LMK 1 ChartofIsotopes Eachisotopehasaspecificnumberofprotonsand neutrons Anelementisdefinedbythenumberofprotons ◦ It’satomicnumber ◦ Isotopesallhavethesamenumberofprotons ◦ Theyareallofthesameelement ◦ Theyonlydifferintotalmassornumberorneutrons Notallisotopesexistinnature! Why??? Nucleienjoybeinginlowestenergystate StrongforcetellsuswhichNuclideismoststableforagivenA Weakforcetellsushowwedecay NuclearDecays(Here,theaxeshaveflipped) Whatishappeningattheedges? ◦ WhenAisverybig? ◦ WhenNisverybig? ◦ WhenZisverybig? BindingEnergyofNuclei WhatistheUniversemadeof? 1 Hydrogen Carbon Helium Relative abundance 10–2 Oxygen Neon 10–4 Nitrogen Magnesium Iron Silicon 10–6 Sulfur 10–8 Boron 10–10 Lithium Beryllium 10–12 1 10 20 30 40 Atomic number 50 NuclidePropertiesCosmicComposition 1 Hydrogen Carbon Helium Relative abundance 10–2 Oxygen Neon 10–4 Nitrogen Magnesium Iron Silicon 10–6 Sulfur 10–8 Boron 10–10 Lithium Beryllium 10–12 1 10 20 30 40 Atomic number 50
