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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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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

Whathappensafter...justgetsmessyandcomplicatedinmyopinion,butI'lltry...

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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