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Lecture 6

# AS101 Lecture Notes - Lecture 6: Apparent Magnitude, Hypothetical Star, ParsecPremium

Department
Astronomy
Course Code
AS101
Professor
Ioannis John Haranas
Lecture
6

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AS101Lecture6TheFamilyofStars
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StarDistances
Distanceisthemostimportantandthemostdifficultmeasurementinastronomy
TheAstronomer’sTriangulationMethod
Stellarparallax:thesmallapparentshiftinpositionofanearbystarrelativetodistant
backgroundobjectsduetoEarth’sorbitalmotion
Lightyear:thedistancelighttravelsinoneyear
Parsec:thedistancetoahypotheticalstarwhoseparallaxis1secondofarc
 Aparsecis206.265AU,whichequalsaround3.26ly
indiameter,whichmakesitdifficulttomeasureparallax

BrightnessandDistance
Intrinsicbrightness:ameasureoftheamountoflightastarproduces
Flux:ameasureoftheflowofenergyoutofasurface;usuallyappliedtolight
Specifically,theenergyinjoulespersecondfallingon1squaremetre
Afluxof1joulepersecondisknownas1watt
Inversesquarelaw:Thebrightnessofthelightsourceisinverselyproportionaltothesquareof
thedistance
Absolutevisualmagnitude:Intrinsicbrightnessofastar.Theapparentvisualmagnitudethestar
wouldhaveifitwere32.6lyaway,or10parsecs.

StarTemperatures
ThehydrogenBalmerlinescanbeusedtoinferthesurfacetemperatureofastar
WeakhydrogenBalmerlines:
oCoolstars(atomsarenotexcitedoutofthegroundstate)
oHotstars(atomsareexcitedtohigherorbitsorareionized)
StronghydrogenBalmerlines:Intermediatetemperaturestars

TemperatureSpectralClassification
OriginallydevisedatHarvardUniversity
Thestarclassificationsystemnowusedbyastronomers
Consistsofsevenmainspectralclasses:O,B,A,F,G,K,M
Theseclassesarearrangedinaspectralsequence,whichisatemperaturesequence(hottest
tocoolest)
Forbetterprecision,eachspectralclassisfurtherdividedinto10subclasses

StarSizes
Astar’sluminosityisproportionaltoitssurfacearea
HertzsprungRusseldiagram:aplotoftheintrinsicbrightnessversusthesurfacetemperatureof
stars.Itseparatestheeffectsoftemperatureandsurfaceareaonstellarluminosityandis