Spectroscopy: The Key to the Stars: Reading the Lines in Stellar Spectra

164 Pages·2007·2.732 MB·English

by  Keith Robinson (auth.)

Keith Robinson
Spectroscopy:
The Key
to the Stars
Reading the Lines in Stellar Spectra
Patrick Moore’s
Practical
Astronomy
Series
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Patrick Moore’s Practical Astronomy Series
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GuilhermedeAlmeida TheSuninEclipse
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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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transmitted,inanyformorbyanymeans,withthepriorpermissioninwritingofthepublishers,orin
thecaseofreprographicreproductioninaccordancewiththetermsoflicencesissuedbytheCopyright
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Theuseofregisterednames,trademarks,etc.inthispublicationdoesnotimply,evenintheabsenceofa
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forgeneraluse.
Thepublishermakesnorepresentation,expressorimplied,withregardtotheaccuracyoftheinformation
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thatmaybemade.
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SpringerScience+BusinessMedia
springer.com
iv
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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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