Spectroscopy: The Key to the Stars: Reading the Lines in Stellar Spectra (Patrick Moore's Practical Astronomy Series)

164 Pages·2007·1.4 MB·English

by  Keith Robinson

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Patrick Moore’s Practical Astronomy Series
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OtherTitlesinThisSeries
NavigatingtheNightSky HowtoObservetheSunSafely
HowtoIdentifytheStarsandConstellations LeeMacdonald
GuilhermedeAlmeida TheSuninEclipse
ObservingandMeasuringVisualDoubleStars SirPatrickMooreandMichaelMaunder
BobArgyle(Ed.) Transit
ObservingMeteors,Comets,Supernovaeandother WhenPlanetsCrosstheSun
transientPhenomena SirPatrickMooreandMichaelMaunder
NeilBone LightPollution
HumanVisionandTheNightSky ResponsesandRemedies
HowtoImproveYourObservingSkills BobMizon
MichaelP.Borgia AstronomicalEquipmentforAmateurs
HowtoPhotographtheMoonandPlanetswithYour MartinMobberley
DigitalCamera TheNewAmateurAstronomer
TonyBuick MartinMobberley
PracticalAstrophotography LunarandPlanetaryWebcamUser’sGuide
JeffreyR.Charles MartinMobberley
PatternAsterisms ChoosingandUsingaSchmidt-CassegrainTelescope
ANewWaytoCharttheStars AGuidetoCommercialSCT’sandMaksutovs
JohnChiravalle RodMollise
DeepSkyObserving TheUrbanAstronomer’sGuide
TheAstronomicalTourist AWalkingTouroftheCosmosforCitySkyWatchers
SteveR.Coe RodMollise
VisualAstronomyintheSuburbs AstronomywithaHomeComputer
AGuidetoSpectacularViewing NealeMonks
AntonyCooke
MoreSmallAstronomicalObservatories
VisualAstronomyUnderDarkSkies SirPatrickMoore(Ed.)
ANewApproachtoObservingDeepSpace
TheObserver’sYear
AntonyCooke
366NightsintheUniverse
RealAstronomywithSmallTelescopes SirPatrickMoore(Ed.)
Step-by-StepActivitiesforDiscovery
CareofAstronomicalTelescopesandAccessories
MichaelK.Gainer
AManualfortheAstronomicalObserverandAmateur
ThePracticalAstronomer’sDeep-skyCompanion TelescopeMaker
JessK.Gilmour M.BarlowPepin
ObservingVariableStars TheDeep-SkyObserver’sYear
GerryA.Good AGuidetoObservingDeep-SkyObjectsThroughout
Observer’sGuidetoStellarEvolution theYear
TheBirth,LifeandDeathofStars GrantPrivettandPaulParsons
MikeInglis SoftwareandDataforPracticalAstronomers
FieldGuidetotheDeepSkyObjects TheBestoftheInternet
MikeInglis DavidRatledge
AstronomyoftheMilkyWay DigitalAstrophotography:TheStateoftheArt
TheObserver’sGuidetotheSouthern/NorthernSky DavidRatledge(Ed.)
Parts1and2hardcoverset CCDAstrophotography:High-QualityImagingfrom
MikeInglis theSuburbs
AstronomyoftheMilkyWay AdamStuart
Part1:Observer’sGuidetotheNorthernSky TheNexStarUser’sGuide
MikeInglis MichaelSwanson
AstronomyoftheMilkyWay AstronomywithSmallTelescopes
Part2:Observer’sGuidetotheSouthernSky Upto5-inch,125mm
MikeInglis StephenF.Tonkin(Ed.)
ObservingComets AstroFAQs
NickJamesandGeraldNorth QuestionsAmateurAstronomersFrequentlyAsk
TelescopesandTechniques StephenF.Tonkin
AnIntroductiontoPracticalAstronomy BinocularAstronomy
ChrisKitchin StephenF.Tonkin
SeeingStars PracticalAmateurSpectroscopy
TheNightSkyThroughSmallTelescopes StephenF.Tonkin(Ed.)
ChrisKitchinandRobertW.Forrest AmateurTelescopeMaking
Photo-guidetotheConstellations StephenF.Tonkin(Ed.)
ASelf-TeachingGuidetoFindingYourWayAround UsingtheMeadeETX
theHeavens 100ObjectsYouCanReallySeewiththeMightyETX
ChrisKitchin MikeWeasner
SolarObservingTechniques ObservingtheMoon
ChrisKitchin PeterT.Wlasuk
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Spectroscopy:
The Key to
the Stars
Reading the Lines in Stellar Spectra
Keith Robinson B.A., Ph.D., F.R.A.S.
With78Figures
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KeithRobinson
RoyalAstronomicalSociety,UK
4BedfordPlace
Scotforth,Lancaster,UK
BritishLibraryCataloguinginPublicationData
AcataloguerecordforthisbookisavailablefromtheBritishLibrary
LibraryofCongressControlNumber:2006930106
PatrickMoore’sPracticalAstronomySeriesISSN1617-7185
ISBN-10:0-387-36786-1 eISBN-10:0-387-68288-0
ISBN-13:978-0-387-36786-6 eISBN-13:978-0-387-68288-4
Printedonacid-freepaper
(cid:2)C Springer-VerlagLondonLimited2007
Apartfromanyfairdealingforthepurposesofresearchorprivatestudy,orcriticismorreview,aspermitted
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Acknowledgements
MygratefulthanksgotoDr.HarryBlomandallatSpringer,NewYork,fortheirhelp
andenthusiasticsupportduringthewritingofthebook.Manythanksarealsodue
toJohnWatson,particularlyforhisenthusiasmovertheoriginalideaforthebook.
Finally,myheartfeltgratitude(andsympathy)goestomywifeElizabethforreading
themanuscriptandofferingmanyhelpfulcommentsandsuggestions.
Diagrams
Alldiagramswerepreparedbytheauthor.
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Contents
Introduction .................................................... xi
Spectroscopy—ANewGoldenAgeforAmateurAstronomy ............... 1
TheBasicStuff—LightRadiationandAtoms ........................... 5
Light ........................................................... 5
ElectromagneticRadiation.......................................... 11
Atoms .......................................................... 19
Summary........................................................ 21
BehindtheLines—TheMagnificentEnergyLevelStructureofanAtom...... 23
EnergyLevels..................................................... 23
ElectronTransitions ............................................... 26
ItAllComesDowntothe(Quantum)Numbers ......................... 31
TheRulesoftheGame—SelectionRules ............................... 34
OrderfromChaos—SpectralSeries ................................... 35
DancingElectrons—ItTakesTwo(orMore)toTango..................... 38
Ions............................................................ 41
AFinalbutVeryImportantNote ..................................... 42
Summary........................................................ 42
OurOldFriendtheDopplerEffect ................................... 45
WavesandMovement.............................................. 45
HowItWorks .................................................... 46
TheRelativisticDopplerShift........................................ 47
AVeryImportantPoint ............................................ 48
Summary........................................................ 50
WhenIsaSpectralLineNotaSpectralLine? ........................... 51
LineProfiles ..................................................... 51
EquivalentWidth ................................................. 52
PopulationsofAtoms.............................................. 54
ShiveringEnergyLevels............................................. 55
EntertheDopplerEffect............................................ 57
Turbulence ...................................................... 60
PilingthePressureOn.............................................. 61
Convolutions..................................................... 61
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viii Contents
HowBroadIsaLineProfile?......................................... 63
Summary........................................................ 68
StellarSpectraandThatFamousMnemonic............................ 69
StellarAtmospheres ............................................... 69
ContinuousAbsorption ............................................ 70
LineAbsorption .................................................. 72
TheSpectralSequence ............................................. 75
LineBroadening.................................................. 78
SpectralSnapshots ................................................ 79
AWordorTwoAbouttheHerzsprung–RussellDiagram................... 80
Summary........................................................ 81
CoolbutnotSmooth—TheMolecularSpectraofRedStars................ 83
StellarAtmosphereVersustheChemistryLab ........................... 84
TheThingsThatMoleculesDo....................................... 84
Summary........................................................ 91
GlowsintheDark—EmissionLinesandNebulae........................ 93
WhatComesDownMustFirstGoUp ................................. 93
Recombination................................................... 95
PhotonDegradingandRecycling..................................... 96
ThickandThinNebulae............................................ 97
YetMorePhotonRecycling—Fluorescence.............................. 100
ForbiddenRadiation............................................... 100
TheEdgeofaNebula .............................................. 102
Summary........................................................ 102
GlowingVortices—AccretionDisks................................... 105
AstrophysicalModelling............................................ 105
AnatomyofanAccretionDisk ....................................... 106
BuildingtheModel................................................ 107
ABetterModel ................................................... 113
ThinkingUpanEvenBetterModel ................................... 115
Summary........................................................ 117
ThePCygniProfileandFriends ..................................... 119
TheClassicPCygniProfile.......................................... 119
WindOutflowGeometry ........................................... 120
PCygniProfilesfromCoolStars...................................... 121
APCygniProfileMystery—SymbioticStars ............................ 122
Summary........................................................ 125
SpectralMagnetism—TheZeemanEffect .............................. 127
HowStrongIsaMagneticField?...................................... 127
MoreonElectronsinAtoms......................................... 128
Momentum...................................................... 128
AngularMomentum............................................... 129
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Contents ix
TheWonderfulWorldofxyz........................................ 130
EntertheMagneticField............................................ 131
ElectronTransitionsinaMagneticField ............................... 134
LookingStraightDowntheMagneticField ............................. 136
HowWideDotheLinesGetSplit?.................................... 138
ComplexAtoms .................................................. 139
VeryStrongMagneticFields......................................... 139
Summary........................................................ 140
‘HowMuchGoldinThemThereStars?’—TheCurveofGrowth ............ 141
Abundances...................................................... 141
ALaboratoryExperiment........................................... 142
ABitofTheory................................................... 143
AnotherBitofTheory.............................................. 145
DeterminingAbundances........................................... 147
Summary........................................................ 148
Conclusion ...................................................... 149
AppendixA—PowersofTen ........................................ 151
AppendixB—ConstantsandFormulae................................ 155
PhysicalConstants ................................................ 155
AstronomicalConstants............................................ 156
Formulae........................................................ 156
Index........................................................... 159
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Introduction
Recently,Iattendedmyusuallocalastronomyclubmeeting;ourspeakerwasalocal
amateurwhotalkedaboutCCDastronomy.Listeningtohistalkitseemedtomethat
herewastheveryguyforwhomI’dwrittenthisbook.He’dstartedoutbydoingCCD
photography and had produced images that most of us in the audience could only
envy.Laterhe’dmovedontovariablestarCCDphotometry;itwasclearthathewanted
to combine his hobby with doing scientifically useful observations. Finally, he said
thathismostrecentadventurewastogetintoCCDspectroscopy,atthispointhistalk
tookonasignificantlyelevatedsenseofimportance.Iaskedhimaboutthekindof
spectralresolutionwhichamateurscouldachieveandherepliedenthusiasticallythat
verysoon,sub-angstromresolutionwouldbefairlynormal.IadmitIwassomewhat
relieved,becauseitmeantthatmuchofwhatI’vewritteninthisbookwouldnotbe
merelyofacademicinterest.
Myowninterestinastronomystartedinthe1960s;Iwasjustakidandcouldn’t
get enough of the stuff, but there were two areas which I seemed to avoid like the
plague.Oneoftheseareaswasradioastronomy;inthosedaysitseemedtobemore
to do with electronics than stars and of course we didn’t have the wonderful radio
imagesoftoday,justwigglylinesonapieceofchartrecorderpaper.Theotherareato
beavoidedwasspectroscopy;thephysicsinvolvedseemedjusttooadvanced.Iknew
that spectral lines told us about different chemical elements in stars and that red-
orblue-shiftedlineswerecausedbythemotionofastronomicalbodiesbutthatwas
aboutit.Inanycase,spectroscopywasdefinitelynotontheobservingagendaofmost
amateurastronomers.
Technologyhaschangedallthat;nowamateurastronomerscanuseCCDcameras
togetherwithbackyardspectroscopestodoastronomicalspectroscopy.Unfortunately,
thephysicshasn’tchangedandtobehonest,muchofitisn’tthekindofphysicsyou’re
likely to do in high school. Virtually all books on astronomical spectroscopy are
textbooks which as they say ‘don’t take prisoners’; they’re the kind of books which
wouldn’tevenbeusedbymostundergraduateastronomystudents.Asforbookson
spectroscopy aimed at amateur astronomers, aside from Stephen Tonkin’s Practical
AmateurSpectroscopyalsopublishedbySpringer,they’reprettythinontheground.
A.D.Thackeraywroteasemi-popularbook AstronomicalSpectroscopybackin1961
butthisislongoutofprint.
Tonkin’sbookisthefirstofIhopemanyinthisnewareaofamateurastronomical
researchandasitstitlesuggests,itisapracticalbook.Thereclearlyisaneedforatheory
bookonastronomicalspectroscopywhichdoesn’texpectthereadertobeascience
graduateandIhopethisbookwillatleastmakeastartinfulfillingthatneed.This
is a book about physical processes and physics processes which make astronomical
spectratheywaytheyare,sothere’squiteabitofphysicsinit.Theonlyassumption
I’vemadehereisthatyoudidsomephysicsinhighschool;evenso,everythingisdone
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