Starlight: An Introduction to Stellar Physics for Amateurs

274 Pages·2009·5.04 MB·English

by  Keith Robinson (auth.)

Patrick Moore’s Practical Astronomy Series
Forothertitlespublishedintheseries,goto
http://www.springer.com/series/3192
Starlight
An Introduction to Stellar Physics
for Amateurs
Keith Robinson
123
KeithRobinson
4BedfordPlace
Scotforth,Lancaster
UnitedKingdom
[email protected]
ISSN1431-9756
ISBN978-1-4419-0707-3 e-ISBN978-1-4419-0708-0
DOI10.1007/978-1-4419-0708-0
SpringerNewYorkDordrechtHeidelbergLondon
LibraryofCongressControlNumber:2009931800
©SpringerScience+BusinessMedia,LLC2009
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Printedonacid-freepaper
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Acknowledgements
MyverygratefulthanksgotoJean-FrancoisLeBorgneoftheLaboratoire
Astrophysique de Toulouse, for permission to reproduce spectra from
their STELIB library. Many thanks also to Gary Billings, Jeff Hopkins,
Robin Leadbeater, and Daniel Majaess of the American Association of
VariableStarsPhotometryDiscussionGroupforkindlyprovidinginfor-
mationregardingonlinesourcesofspectra.
Introducing a little bit of mathematics into a book like this can be a
risky business, so it’s always good to have someone who can offer crit-
ical appraisal. My sincere thanks go to my great friend and amateur
astronomer par excellence, Denis Buczynski, for doing this in respect of
chapter Starlight by Numbers and offering some helpful criticisms and
suggestions.
ManythankstoMaurySolomon,HarryBlom,andallatSpringer,New
York, for their help and enthusiastic support during the writing of the
book and also to John Watson for his great enthusiasm and encourage-
mentovertheoriginalideaforthebook.
FinallymyheartfeltgratitudegoestomywifeElizabethforreadingthe
manuscriptandofferingmanyhelpfulcommentsandsuggestions.
Alldiagramswerepreparedbyme.
v
Contents
ARiverofStarlight......................................... 1
StarlightbyNumbers....................................... 7
FromLighttoStarlight ..................................... 23
Space–TheGreatRadiationField............................ 59
AMultitudeofMagnitudesfortheColorsofStarlight .......... 87
ThePhotonsMustGetThrough–RadiativeTransfer ........... 119
FirstLookInsideaStar–TheAtmosphere .................... 145
DeepInsideaStar.......................................... 175
IntheSpaceBetweenStars .................................. 215
AStarStory–10BillionYearsintheMaking................... 235
Appendix1:TheGreekAlphabet............................. 265
Appendix2:AstronomicalandPhysicalUnitsandConstants .... 267
Appendix3:TheDopplerEffect.............................. 271
Index .................................................... 273
vii
About the Author
Keith Robinson became interested in astronomy at the age of eight after
seeing the TV documentary “Universe” produced by the National Film
BoardofCanada.HegraduatedwithadegreeinphysicsattheUniversity
ofLancaster(UK)intheearly1970sandsubsequentlyworkedasobserva-
torysuperintendentattheJeremiahHorrocksObservatoryintheCentre
forAstrophysicsattheUniversityofCentralLancashire.Hereheobtained
hisPhDforresearchonmodelingtheBalmeremissionlineprofilesinthe
spectraofsymbioticstars.
Resident in his native northwest of England and married, with one
daughter, he is very much involved with his local astronomy club and
works as a full-time writer and educator. He became a member of the
AmericanAssociationofVariableStarObserversin2001.
ix
A River of Starlight
Backintheearly1980s,Iswappedaprettygoodquality3inchrefractor
for a “so so” 81/ inch Newtonian reflector on a fairly rough and ready
2
Germanequatorialmounting(tobefairitdidhavemanualslowmotion
driveswhich,afterabitofpractice,Igottobefairlygoodatusing,andit
didhavequitelargeandeasytoreadsettingcircles).IguessIwasgreedy
forthatextraaperture,whichwouldenablemetoseedeeperandfainter,
andindeedIhadathoroughlyenjoyablecoupleofyearsobservingdeep-
skyobjects.
The fact is, as a teenage amateur astronomer in the late 1960s (yes, I
did manage somehow to find the time), I had read that once one had
progressedbeyondthebeginnerlevel,oneshouldseriouslyconsiderspe-
cializing in some specific area of observational astronomy. To be hon-
est I didn’t really like the idea of, for example, spending the rest of my
life just observing Jupiter (no disrespect to Jupiter observers). The trou-
ble was that the books of the time didn’t seem to make any mention of
the fact that there was no “law” that said that if you specialized in one
area of astronomy you were not allowed to investigate other areas. On
the contrary, there was this sense that you were strongly encouraged to
specializeinonething.Ididatthetimeratherliketheideaofobserving
whatwerereferredtoas“nebulaeandgalaxies,”etc.(Inevercameacross
the phrase “deep-sky object” until the 1970s.) However I remember
someone–probablyanolderkidatmyschool–tellingmethattherewas
no useful work that could be done in this area by amateurs (try telling
thattosupernovahunters),sotherewasnopointtoit.
K.Robinson,Starlight,PatrickMoore’sPracticalAstronomySeries, 1
DOI10.1007/978-1-4419-0708-0_1,©SpringerScience+BusinessMedia,LLC2009
2 Starlight
Now of course, deep-sky observing is quite rightly one of the most
popular areas of amateur astronomy, whether or not it is scientifically
useful;hencetheacquisitionoftheNewtonianand“Yah!Boo!Sucks!”to
thatlongforgottenschoolkid.However,oneclearAugusteveningthings
changed.
I had taken the trouble to polar align my equatorial mount, so that I
could use the scope’s setting circles, and I remember quite a feeling of
satisfaction at being able to locate the Dumbbell Nebula, M27, without
even looking at the sky. I also remember that upon enjoying the view
of what I reckon is a more impressive planetary nebula than the Ring
Nebula,M57,Ifeltasenseofwantingtodosomekindofobservingthat
involvedmorethanjustlooking.
Astrophotographywasoutofthequestionwithmyscope,soithadto
besomekindofvisualobserving.Onlookingupatthesky,Ithennoticed
thatAlgol(thefamouseclipsingvariablestarinPerseus)lookeddistinctly
dimmer than its nearby neighbor Mirfak (Alpha Persei). I happened to
have at hand the Handbook of the British Astronomical Association, and
sureenough,Algolwasaroundhalfanhourfromminimummagnitude.
ThiswasthefirsttimeIhadeverseenavariablestar“inaction.”
Some years previously, when considering my “choices” for a special-
ized area of amateur astronomy, I had been distinctly put off the idea of
observing variable stars, simply because those aforementioned books of
thetimeseemedtosuggestthat,notonlywasitpossiblebymakingvisual
observations to estimate the magnitude of a variable star to an accuracy
ofonetenthofamagnitude,butthatthiswasactuallysomekindof“stan-
dard” that was expected. Maybe I misinterpreted what I’d read, but one
thing’sforsure.Idon’trecallcomingacrossanybookthatgaveanillus-
trationofareal(ratherthanastylized)lightcurveofavariablestar,which
showed the obvious scatter that you get when pooling the observations
of a group of people. Such light curves clearly show that the 0.1 magni-
tudeaccuracythingisakindofidealizedlimit,whichcanmorelikelybe
approached, but not very often actually achieved. I thus arrived a little
late at considering the possibility of becoming a variable star observer,
anditturnedouttobequiteanadventure.
I managed to get hold of one or two amateur books on variable stars
andvariablestarobserving,andtheveryfirstthingthatstruckmeabout
thisareaofamateurastronomywasthatitisn’tjust“amateurastronomy,”
it is amateur astrophysics. The observations made by amateur variable
observers are real data in the truest scientific meaning of the word, and
to be honest, I found it astounding that such simple observations could
revealthingsgoingoninsidedistantsunsthataresofarawaythattheycan
only be seen as points of light. When you have this kind of “revelation,”
A River of Starlight 3
theoftenspokenof“addictive”qualityofvariablestarobservingcomesas
no surprise, but in addition to being able to make scientifically valuable
observations on a regular basis, I know that in my own case I wanted to
knowmoreaboutwhatreallydoesgoonwithinstars,tomakethemvary
astheydo–orindeed,notvaryatall.
After leaving high school I got a degree in physics at my local univer-
sity,andwhilethishelpedinmywishtoknowmoreaboutthephysicsof
stars,thefactisthatmuchofstellarastrophysicsisaspecialtyuntoitself
and not the kind of stuff that you are likely to come across in a straight
physics degree course. However, as a result of my new found interest in
variable stars, I got to know a much more experienced amateur variable
star observer who did photoelectric photometry with a real live photo-
multiplier (“live” being very appropriate here, because the high-voltage
power supply that ran his photomultiplier was housed in a washing-up
bowl, which sat on his rather damp lawn). This guy also did some of
his own data analysis, and he was certainly the kind of person that any
novice variable star observer was truly privileged to know. I remember
him, though, complaining on more than one occasion about the lack of
decent books on both variable star physics and for that matter on just
stellarphysicsitself,whichweresuitableforamateurs.Hehimselfhadto
pick up what effectively were disjointed fragments of information from
professional research papers, specialist monographs, and the occasional
textbook.Hejusthappenedtobeasciencelibrarian,whichwasveryfor-
tunateatatimewhentherewasnoInternet.Eventhesedays,muchofthis
kindofinformationstillveryoftencomesintheformofarticles–online
or otherwise, which just don’t have the space to be able to deal with a
subjectinthekindofdepththatitmaybedeserves,orevenworse,itgets
theoddparagraphortwoineitheramoregeneralastronomybookorin
booksthatarespecificallywrittenaspracticalobservingguides.Thereare
stillalso,ofcourse,thestudenttextbooksandtheresearchpapers.
It goes without saying that textbooks on astronomy are not written
with amateur astronomers in mind. The fact is that any student who
wishes to become a professional observational astronomer has to learn
a lot of background theory – “required reading,” as they say, and there
has always been a plentiful supply of textbooks, some of them veritable
classicworkstogivestudentswhattheyneed.Wheredoesaseriousama-
teurastronomergethisorherbackgroundtheoryfrom,though?I’msure
that many amateur astronomers probably have a sufficient background
in mathematics and physics to be able to tackle at least the more basic
leveltextbooks–butthenagain,therewillbemanythatdon’t.Therewill
surelyalsobemanywhowouldsaythatfortheworktheydo,theysimply
have no need for this kind of theoretical background – but wouldn’t it