revision_edexcel astrophysics - the student room
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Revision:Edexcelastrophysics
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Contents1Recordingstarimages
1.1CCDChargedCoupleDevices1.2Film
2BenefitsofobservingfromabovetheEarth'satmosphere2.1Telescopes2.2Informationfromwavelengthsofdifferentstars
3Wien'slaw4Luminosityofstars5MeasuringDistances
5.1ParallaxMethod6HertzprungRusseldiagram7CepheidVariables8Stars=
8.1MainSequencestars8.2WhiteDwarves8.3RedGiants8.4Supernovae8.5NeutronStars8.6BlackHoles
9Comments
Recordingstarimages
CCDChargedCoupleDevices
AdvantagesOverFilm:
Higherquantumefficency~70%(numberofbitsofinforecorded/numberofphotonsx100)Morelinearresponse(outputismoredirectlyproportionaltolightintensityreceived)NoneedtoreplaceCanbeusedrepeatedlyFasterOperation
Disadvantages
SmallerfieldofviewNeedtobekeptcool(about120K)NotportableResolutionisnotasgood
Madefromslicesofsiliconthatstoreelectonsfromphotonsandbuildanimage(inpixels)fromthis.Pixelsarefrom550micrometers,andpixelswherelargerquantitiesoflightland(higherlightintensity)havemoreelectons.
Avoltageisappliestopixelssothatthecomputercan"read"theimage.
Film
AdvantagesOverCCD:
C4HelpNeeded.
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6/15/2015 Revision:EdexcelastrophysicsTheStudentRoom
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CheapGoodresolutionPermanentrecordEasilytransported
Disadvantages
NotasefficentNotaslinearHavetobereplacedSlowertouse(upto2hourswhileCCDisabout1minute)Becauseofthis,alsomoreexpensivebecausetelescopetimeisexpensive.GrainSizeis0.11micrometer(largegrainsaresensitivetolightbutproduceagrainyimagewhilesmallgrainsarelesssensitivebutproduceasharperimage)
BenefitsofobservingfromabovetheEarth'satmosphereLightPollutionreflectsoffgaseousatmosphere(knownasscattering)PollutionreflectsandabsorbslightLightpassingthrhoughatmosphereexperiencesdifferentrefractiveindecesDiffractionlimitsresolvingpoweroftelescopesSomewavelengthsareabsorbedbyatmosphere
Thereare2"windows"inatmosphere.Theyletradioandopticalwavesthrough.
Wheredisdiameterofparticleinatmosphere,andwiswavelength.
d>wparticlesactlikemirrorsd=wscatteringisproportionalto1/w(bluemorescatteredthanred)d
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StefanBoltzmanfoundthattheluminosityisdirectilyproportionaltothetemperature(k)tothepowerof4.
Where isaconstantcalledtheStefanconstantandhasavalueof:
MeasuringDistances
Forlargedistances:
1Lightyear=9.46x10^15m(Speedoflightis3x10^8m/s)
1AUtheaveragedistancebetweentheearthandthesun.(maynotbeinsyllabus)
ParallaxMethod
Onlysuitableforrelativelyclosestars
ok...soEisearthatoppositetimesintheyear(6monthsapart),SisSunandTisthestaryouarelookingat...
....ESE
.....x.........|.........x
.......x.......|.......x
.........x.....|.....x
...........x...|...x
.............x.|.x
...............T
ImagineanangleatE(T)S..callitd
becausetheangleissmall,(lessthan1degree)ETcanbeassumedtobethesameasST.
So:tand=~sind=1AU/D.
Methodforannualparallaxmethod:
Fromapositiononearthlineupthenearbystarwithamoredistantstarobservedfromearth.Exactly6monthslaterrepeattheprocessinnumber1,byvaryingtheangle.Measurethedistancebetweentheangleof1and2.Knowingtheaveragediameterfromtheearth'sorbitradius,theDistanceD=1AU/tand
HertzprungRusseldiagram
ForastarLANDERmaxisproportionalto1/Temperature(fromWien'slawwhcihisLANDERmax.T=constant.
Thefurtherthestarisawayfroearth,theloweritsintensity(I).
Intensityisdirectlyproportionaltoluminosity
Intensityonearthcanbemeasured,andthenLuminositycanbecalculatedbyusingthefollowingequation:
,whereDisthedistancefromearth,adncanbecalculatedusingthe
parallaxmethod.
UsingthisvalueofL,thestarcannowbeputontheHRdiagram.
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TheHRdiagramcanalsobeusedtomeasuedistances(fornearbyorfarawaystars?):
MeasurethetemperatureTbyusingWien'slaw,thenusetheHRdiagramtoestimatealuminosity.Thentheluminositycanbeusedtocalculatethedistance.[
]
CepheidVariables
Thesearestarsthatpulsate.Theyhaveuseduptheirmainsupplyofhydrogen.
Cepheidvariablesgofrombeingverydim,tobeingverybrightinatimerangeofanythingbetweenabout150days.
TheperiodofaCepheidisrelatedtoitsluminosity:
........I....................x10^4..I..................x.........I...............x.........I.............x10^3..I...........x.........I.........x.........I.......x10^2..I.....x.........I...x.........I_____________________..........1....3....10....30....100
AstonomershavefoundthatoveraperiodofpulsationsthatthetimeperiodofpulsationdependsonLuminosity.(Almostadirectproportionalitybasedonarelativeluminosity)
SoastronomersmeasurethetimeperiodTanduseittofindtheluminosity.thentheycanuseI=L/(4piD^2),toworkoutthedistance.
TherearealsoRRLyraevariables,buttheirperiodislessthan1day,andtheydonotfollowthesamepattern.
Stars=
Starsarebornasinterstellargascloudscollapseundertheirowngravitationalcontraction.
TheyaremadeupofHydgrogenasmallamountofHeliumandabout12%ofheavierelements.Ifthemassofthecloudislargeenough(100xthatofthesunsupposedly?)Hydogennucleiallmovetowardsthecentre,andtheirgravitaionalpotentialenergyisconvertedintokineticenergy.Thiscausestheprotostartoheatupandbeginfusionwhereitthenbecomesastarandsettlesintoamainsequencestar.Pressureoutwards(radioactiveandcausedbyheatIthink)matchespressureinwardsofgravity.
FusionppchainiswhereHydrogenisconvertedtoHelium(Ithink).Butforedexceldetailsofppchainarenotneccessary.
Massofsunis2x1030kg
Forenergyconversionsinstars .
MainSequencestars
Largestarshavelargegravitationalforcesactingonthem.Theythereforehavealargeinwardspull.Theyreachhighertemperaturesfaster,andhavehigherluminosity.TopleftoftheHRdiagram.SHORTLIFETIMES
Smallstars:havesmallgravitationalforces,solowertempsandluminosity.BottomrightofHR.LONGLIFETIMES
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29,000
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Ms=Massrelativetosun.
WhiteDwarves
Lessthan1.4SolarMassesWhenthemainsequencestarhasfinisheditshydrogenburning[whichisveryslowduetothelowmass]itisnotlargeenoughtohavethegravitationalforcesrequiresforouterlayerstobedrawninwardandimplode.TheouterlayersarecastoffslowlyleavingamoltencoreofwhatismainlyIron,andstillretainsalotofheat.Noburningisoccurringsothestarbeginstocool,andwillcontinuetodosountilitbecomesablackdwarf.
RedGiants
Thestarsbetween1.4and8MsbecomeRedGiantswhenmostoftheHisconvertedtoHe.Thisisbecausetheoutwardspressureisnolongerbalancedwiththeinwardspressure.Thiscausesthecoretoheatup,andincreasesenergyflowtogasesaroundtheoutsideofthestar.Theoutsidegasesbeginfusion,whichmakestheoutwardforcesunbalancedagain,meaningthatthesunbeginstoexpand(byabout~100x)tobecomeaRedGiantoraSuperGiant.(Redgiantsthengoontobecomewhitedwarfs)
Nowmoreandmorefusionofheavierandheavierelementsoccursatthecore(inlayersaroundthecentre).
WhenthefusionreachesIron,theredgiantcannotfuseanymore(becausethestarhasrunoutoflotsofenergy,andIronisveryhardtofuse)
Supernovae
Therapidimplosionofstarsgreaterthan8solarmasses,[supergiants]Causedmainlyduetothemassivegravitationalinwardpullofthecorepullingtheouterlayersinwardwhenhydrogenburningstopssonooutwardforceiscreated.
Alltheouterlayersaredrawninwardandenergyisreleasedasashockwaveastheouterlayersarethenrapidlycastoff.
NeutronStars
IfMsofcoreofasuperovaeis>1.4,thecentralcoreofneutronswillformaneutronstar.Theyarehighlydense(10^15xnormalmatter).Areabout20kmindiameterandhaveamassof10^30kg.
AStheneutronstarrotatesititemitshighfrquencyradiosignals.(Isthisapulsar?)
BlackHoles
Ifthecoreremnantsofasupernovaearegreaterthan2.5Msablackholeisformed.Thesupernovacollapsestoaninfinitedensitycalledasingularity.
CanbedectedbynoticingstrongXraysourcesnearstars
BygravitationallensingBydopplershiftsfromstarsnearby.
CommentsSuitablefor:ALevelphysics/astrophysics,especiallyEdexcelexamboard.Writtenby:Swordsikan.Fromthisthread.
Categories:ALevelPhysicsRevisionNotes|PhysicsRevisionNotes
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