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TheSolarspectrum

Thehotdensecentreofthesunemitsacon1nuousspectrum Thecoolerouterlayerofthesun,theHeliosphere,absorbs

someofthelightatspecificenergies

Lookingattheabsorp1onspectrumwecanfindoutwhattheSunismadeof

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TheSolarspectrum

TheSunemitsprimarilyinthevisiblespectrumbutalsoatotherwavelengths

MuchoftheUVradia1onfromtheSunisabsorbedbytheatmosphere

TheSunatX-ray

wavelengths

TheSunatUV

wavelengths

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Starsandplanets

3

Starsaremassiveenoughtotriggernuclearfusionattheircorestheyproducetheirownlight

Planetsarenotmassiveenoughtogeneratetheirownlightbutcanreflectthelightoftheirhoststars

Minimummassrequiredtostartnuclearreac1onsatcore: 8.3%M

(Solarmasses,about871mesthemassofJupiter)

BrownDwarfstarsarefailedstars theycannotgenerateenoughpressureattheircorestostart

fusion (Between3and80Jupitermasses)

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Redshift

Lightwavesfromamovingsourceexperiencearela1vis1cDopplershiinfrequency

Thespeedoflightisalwaysconstantbutthefrequency(andwavelength)changes

ClassicalDopplerequa1onsdonotapply Redshiformula:

4

z =

1+v

c

1v

c

1

z =

0

0

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Blueshift

5

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Distancestostars

Parallax:differenceintheapparentposi1onofanobjectviewedalongtwodifferentlinesofsight

6

AstheEarthorbitstheSun,theposi1onofnearbystars

shisagainstthebackground

Distancetostar=(Earth-Sundistance)/(parallaxangle)=

AU/p

parsec=3.26ly=3.0903km=.9203miles

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Brightnessofstars

Parallax:differenceintheapparentposi1onofanobjectviewedalongtwodifferentlinesofsight

Mainsequencefi7ng Apparentbrightness:howbrightastarappearstobe

Absolutebrightness:howbrightastaractuallyis

(calibratedforadistanceof

0parsec)

7

Hetrtzsprung-Russeldiagram

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Brightness,Luminosity,

Magnitudes Magnitudesmeasurehowbrightstarsare

logarithmicscale thelowerthemagnitude,thebrighterthestar apparentmagnitude:themagnitudethestarappearstohave absolutemagnitude:themagnitudethestarwouldhaveata

distance0parsecawayfromtheEarth

Luminosity:theamountofenergyescapingthestarssurfacepersecond

(measuredinerg/sec) erg=0-7J=624.5GeV

8

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ApparentMagnitude CelesalObject

-26.7 Sun

-2.6 FullMoon

-4.4 Venus(atbrightest)

-3.0 Mars(atbrightest)-.6 Sirius(brighteststar)

+3.0 Nakedeyelimitfromcity

+5.5 Uranus(atbrightest)

+6.0 Nakedeyelimit+9.5 Faintestobjectsvisiblewithbinoculars

+3.7 Pluto(atbrightest)

30 FaintestobjectsobservablebyHST

9

Fainter

Brighter

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Example

Atitsbrightest,Venushasamagnitudeof-4.Howmuch

brighterdoesVenusappeartousincomparisonwiththe

dimmeststarsvisibletothenakedeye?

[nakedeyemagnitudelimit:+6]

0

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Comparingthebrightnessof

twostars differenceinapparentmagnitudesoftwostars:

Lightintensityobeysaninversesquarelaw

m2 m1 = 2.5logI1

I2

"

#$

%

&'

I 1

r2

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Comparingthebrightnessof

twostars Assumeastarhas

apparentmagnitudeandbrightnessm,iatadistancedfromtheEarth(inparsec)

absolutemagnitudeandbrightnessM,Iatdistance0pc

m-Miscalledthedistancemodulusofthestar2

mM = 2.5logI

i

"

#$

%

&' mM= 2.5log

1102

1d

2

#

$

%%

&

'

((= 2.5log

d

10

#

$%

&

'(

2

logxp= p log(x)

m M= 5logd

10

"

#$

%

&' M=m 5log

d

10

"

#$

%

&'

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Example

TheapparentmagnitudeoftheSunis-26.73.Calculateits

absolutemagnitude.

[distanceoftheSunfromtheEarth=.49608km=4.848

0-6pc]

3

m M= 5logd

10

"

#$

%

&'

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StellarClassiAication

4

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StellarClassiAication

WiensLaw: max=peakintensity(inm) T=temperature(inK)

5

max

T= 3103

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TheHertzsprung-Russel(H-R)

diagram

6

Sol

L = 4R2T

4

StefansLaw:

LuminosityL(W)

R=starradius

= 5.6710-8 Wm-2K-4T= temperature (K)

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Example

TheSunsluminosity(poweroutput)is3.94026W.Antares,in

theconstella1onofScorpio,isaredsupergiantstarthatis

57,5001mesmoreluminousthantheSunanditsradiusis883

1mestheradiusoftheSun.Whatisitssurfacetemperature(in

K)?

StefansLaw

SurfacetemperatureofSun=5778K

7

L = 4R2T

4

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Stellarevolution

Starsarebornininterstellargasclouds Thecloudscollapseundergravityandformprotostars Protostarsaccretemoregasfromthecloudandeventually

switchon(fusionbeginsattheircores)

Howlongastarlastsfordependsonitsini1almass Thatdetermineshowfastitburnsupitsnuclearfuel Starlife1me=(ini1alamountoffuel)/(consump1onrate)

8

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Stellarevolution

9

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Whitedwarfs,neutronstars,

blackholes Whitedwarfsarestarremnantsthatcannolongersupport

fusionreac1ons

Theyradiateawayremnantheatandcooloffwith1mebecomingfainter

Neutronstarsarewhatsleaerasupernovaexplosion verydenseandcompact theycannotcollapsefurther(neutrondegeneracypressure) pulsarsareneutronstars

Blackholesareformedwhenverymassivestarscollapseattheendoftheirlifecycle

notevenlightcanescapetheirgravita1onalpull wecaninfertheirpresencefromtheirgravita1onalinterac1on

withotherstars20

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ToDo

Readsec1on2fromthebook[AstronomyandCosmology] HomeworkAssignmentwk:revisionproblems(seeblog) Handitinnolaterthan4:00pmWednesday3rdApr-LATE

WORKWILLNOTBEACCEPTED

2