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
DouglasAdams
Hitchhiker’sGuidetotheGalaxy
Cosmology:ProgressReportafterNewton
Recall:Cosmologyisamysterystory
Basedonevidencewecanobserve,wanttounderstand:
◦ structureofUniverse:howbig?mapofcontentsacrossspace?
◦ contentsofUniverse:whatingredientsisitmadeof?
◦ origin&evolutionofUniverse:birth?past?future?
◦ rulesofthegame:whatmakestheUniversethisway?
Cosmologicalprogresssofar:
◦ Copernicus’Sun-centered“Universe”
◦ Keplerpreciselydescribedplanetarymotion
◦ Newtonexplainedplanetarymotion,agreesw/observations
LessonsfromNewtonianTriumph
LessonfromNewtoniansuccess:insightintorulesofthegame
◦ powerfullawsexistexplainingallmotionduetoforces
◦ planetarymotionisduetoforceofgravity
⇒ gravitydeterminesbehaviorofthecosmos!
AlsoimportantimplicationsforstructureofUniverse
LessonsfromNewtonianTriumph
Recall:tonakedeye,stellarparallaxisundetectablestarsdon’tseem
to“wiggle”annuallyonsky
soifparallaxexists,wigglemustbesmall
⇒ starsmustbeveryfaraway!
Butifstarshugelyfaraway,andcanstillbeseentheymustemitalot
oflight
→perhapsstarsareliketheSun!
Wheredowegofromhere?
Light
Lightdeeplyconnectedtoelectricchargeand
electricandmagneticforcefields
Experimentsshow:
◦ ChangingEforcegeneratesMforce
◦ ChangingMforcegeneratesEforce
1.
E&Mlinked:“electromagneticforce”
2.
EMdisturbancescantravelthroughspace:
eachregeneratestheother:
E→M→E→M→...
Electromagneticwaves=“EMradiation”
Maxwell’sEquations
Light
Lightdeeplyconnectedtoelectricchargeand
electricandmagneticforcefields
Experimentsshow:
◦ ChangingEforcegeneratesMforce
◦ ChangingMforcegeneratesEforce
1.
E&Mlinked:“electromagneticforce”
2.
EMdisturbancescantravelthroughspace:
eachregeneratestheother:
E→M→E→M→...
Electromagneticwaves=“EMradiation”
Maxwell’sEquations
LightasElectromagneticWaves
lightisanelectromagneticphenomenon:
◦ createdbymotions(infact:acceleration)ofelectriccharges
◦ thatis:lightmustbecreatedbychargedparticlese.g.,inlightbulb,hotfilamentdrives
electronmotion
butlightitselfismadeofelectromagneticfields
◦ whichexertforcesonchargedparticles
◦ thatis:lightinteractswithchargedparicles e.g.,inantenna,electronsdrivenbackandforth
note:ifanelementaryparticlehaszeroelectriccharge(neutral):
◦ itcannotemitlight
◦ itcannotabsorborscatterlight
LightasElectromagneticWaves
lightisatypeofwave:
awaveisoscillatingdisturbanceinamediumwavecantravel,mediumdoesnot
◦ Demo:thewave!
snapshotinspace,takenatoneinstantoftime:
◦ diagramwavelengthλ sizeofonecycle→wave“IDnumber”
intensityI“strength”ofwave=“heightofpeaks”
◦ Demo:slinky:samewavelength,diff’tintensities
Q:Soundwaves:howdoweexperienceλ ?I?
Q:Lightwaves:howdoweexperienceλ ?I?
LightandSoundWaves
SOUND
LIGHT
λ ↔pitch
λ ↔color
highpitch(treble):smallλ
visiblelight:
lowpitch(bass):largeλ
intensity=loudness
◦ largerλ:morered
◦ smallerλ:moreblue
intensity=apparentbrightness
Noteforexperts:inmoredetail,wedistinguish
flux=apparentbrightnessofunresolved(point-like)sourcee.g.,stars,distantquasars
intensity=surfacebrightnessofresolved(extended)sourcee.g.,Moon,planets,nearbygalaxies
TheSpeedofLight
Lightisveryfast!
Sofastthatitwasafeattomeasurethespeedinlabnowknownquitewell
c=constant=299,792,458m/s=3.0× 108m/s=186,000miles/s=6.7×108 miles/hr
enormous–butnotinfinite!
◦ →finitespeedoflighthugelyimportantforastronomy
◦ →telescopesaretimemachinesQ:how?
note:lightspeedcissameforallλ
Q:whatwouldhappenifthiswerenottrue?
TheElectromagneticSpectrum
EMwavescanhaveλ outsideofvisiblerange
Generally,lightiscombinationofpurewaveswithdifferentλ
distributionofintensities:
differentbrightnessatdifferentλ:spectrum
diagram:sketchspectrumaxes
Q:spectrumoflaserpointer?
Q:spectrumofwhitelight?
TheElectromagneticSpectrum
Matter,TemperatureandLight
hotmatterglows(thinkstoveburner)
◦ temperature– radiationconnection
microscopicpicture:
◦ temperature→atommotion
butatomsmadeofchargedparticles
◦ motion→changingEMforces→light
thermalbody(atsometempT)emitsEMradiation:whichλ emitted?
Matter,TemperatureandLight
spectrumoflight;dependsonT
perfectabsorberoflight:“blackbody”
absorbedenergy→heatsup
re-emitsaccordingtoT
“blackbodyradiation”=thermalradiation
ThermalSpectrum:Lightasa
Thermometer
peakλ iscolorseen:λpeak ∝ 1/T
hotter→moreblue→shorterλ “Wien’sLaw”
Turntheequationaround:T∝ 1/λpeak,namely
𝑻 = 𝟑𝟎𝟎𝟎𝑲
𝟏𝟎(𝟔 𝒎
λpeak
so:canfindTjustfromlight!
⇒ spectrumasthermometer
colormeasurestemperature
Intensity (arbitrary units)
whereTisabsolutetemperatureinKelvin recall:TKelvin =273+TCelsius
Visible spectrum
Infrared
Ultraviolet
106
103
4000 K
7000 K
1000 K
1
300 K
1012 1013 1014 1015 1016
Frequency (Hz)
105
104 1000 100
Wavelength (nm)
10
SolarSpectrum
Sun’sspectrumpeaksinmiddlevisiblewavelengths
Q:buttheSunisnotallatonetemperature–whathasthisT?
Q:doesWien’sLawapplytopeople?
ThermalSpectrum
note:sunlightcomesfromSunsurface(“photosphere”)
→wehavefoundTSun,surfac →evenhotterinside!
Humansandthermalradiation:
wearenotatzerotemperature–cananddoradiate!
◦ butinfrared–invisibletonakedeye,veryvisibletoIRsensors
Butnote:T-Shirtcolor≠thermalradiation(goodthing!)
◦ reflectedlight,notglowfromheat!
Separatingoutthespectrumofwhitelight
SeparatingoutthespectrumoftheSun
Whatdoyouthink?
Temperature,Atoms,andStatesofMatter
atomsalwaysinrandommotion→collidewitheachother!imaginestartingcold,
andturningupT
◦ atlowestT:coldmatterlowspeeds→gentlecollisionsifdenseenough:atomslockedtogether→
solid!random“jiggle”tooweaktodislodgeatoms
◦ ifwarmer:atomsfreedto“roll”pasteachothermelt→liquid!atomsmove(“flow”)butstill
touch
◦ ifwarmerstill:atomscanflyfreeevaporate→gas!andatoms“bounceback”whencollide
◦ ifreallyhot:gasatomscollideathighspeedatomstornapart=ionized→freeeandnuclei:
plasma
hotgasingalaxyclusters:ionizedplasma,noboundatoms
coldgasclouds:allatomsarebound,noneionized
TheQuantom Atom
atsmalldistances(sizeofatoms)Newton’slawsfail!
atoms,lightobeynew&differentrules:quantummechanics
electronorbits
nucleus+e:likesolarsystem?
No!QM→enotlikeplanet
inatom,actslikewave!?!
◦ mostorbitsforbidden!
◦ onlyspecialorbitdistancesallowed→“quantized”insteps
allowedorbits→energylevels
lowestenergy→stableorbit,closesttonucleus
“groundstate”
Photons
justasmatter(likee)cansometimesactlikewaveslightcansometimesactlike
particles...
onsmalllength-scalesorlowintensitieslightactslikeparticle:“photon,”symbolγ
discrete“lump”or“packet”ofenergy
differentcolors↔differentenergiessmallerλ →higherE:
𝑬𝒑𝒉𝒐𝒕𝒐𝒏 ∝
𝟏
𝝀
TheElectromagneticSpectrum
Light-AtomInteraction
Iflighthitsatomandphotonenergy=atomenergylevel
1. atomabsorbsphoton
2. ejumpstohigherlevel
3. atomin“excited”state
butexcited=unstableandaftersometime,
1. ejumpsbacktogroundstate
2. emitsphotonwhoseenergy=excited−groundstatedifference
Atomsabsorb/emitlight
atomstructuresetsenergies,andλ ∝ 1/E
...whichisdifferentfordifferentatoms
soenergylevelspacings differentfordifferentatoms
Intensity
Light-AtomInteraction
650
500
Wavelength (nm)
400
550 500 450
Wavelength (nm)
400
Hydrogen
Sodium
Helium
Neon
Mercury
650
600
lightspectrumgivesatom“fingerprint”or“barcode”
spectrum→composition
350
MeasuringtheCompositionoftheCosmos
Example:TheSun
Sun,starshotter,denserincentercooler,lessdenseatsurface
so:sunlight/starlightshows Q:whatkindofspectrum?
amountabsorbedineachline→amountofatoms→compositionofSun;worksforotherstarstoo!