February 7, 2006 Astronomy 2010 1 Chapter 16: Analyzing Starlight.

February 7, 2006February 7, 2006 Astronomy 2010Astronomy 2010 11

Chapter 16:Chapter 16:Analyzing StarlightAnalyzing Starlight



StarlightStarlightstars: bright lights in the skystars: bright lights in the sky

like our sun? different? in what ways? why?like our sun? different? in what ways? why?

Nearest: Proxima Centauri 4.2 LYNearest: Proxima Centauri 4.2 LY

100,000 years for fastest spacecraft100,000 years for fastest spacecraft

red dwarf, 7% of Sun's diameterred dwarf, 7% of Sun's diameter

1/18,000 as bright as Sun1/18,000 as bright as Sun

nearest star of a triple system nearest star of a triple system

observable from southern hemisphereobservable from southern hemisphere

Alpha Centauri 4.3 LYAlpha Centauri 4.3 LY

40% brighter than Sun (G2 V spectral class)40% brighter than Sun (G2 V spectral class)

yellow orange (main sequence)yellow orange (main sequence)

magnitude -0.3 apparent, +4.4 absolutemagnitude -0.3 apparent, +4.4 absolute


All Stars are DifferentAll Stars are Differentcolors: blue-white to redcolors: blue-white to red

brightness: bright to very faint brightness: bright to very faint

Orion: Constellation with many Orion: Constellation with many different star typesdifferent star types

Betelgeuse: orange-red Betelgeuse: orange-red supergiantsupergiant

Rigel: blue-white supergiantRigel: blue-white supergiant


BetelgeuseA red supergiant star


Key Properties of StarsKey Properties of StarsSun's key propertiesSun's key properties

mass mass = 333,400 x Earth mass= 333,400 x Earth masssurface temperature:surface temperature: color yellow color yellow 5860 K 5860 Kcomposition:composition: spectrum spectrum mostly hydrogen mostly hydrogensize size = 110 x Earth diameter= 110 x Earth diameterluminosityluminosity = 3.8 x 10 = 3.8 x 102626 watts (spectral type G2V) watts (spectral type G2V)

Magnitude – 26.7 apparent, +4.8 absoluteMagnitude – 26.7 apparent, +4.8 absolute

Deduce: core “burns” hydrogen, converting it to Deduce: core “burns” hydrogen, converting it to helium by thermonuclear fusion.helium by thermonuclear fusion.Stars: how do we infer mass, temperature, Stars: how do we infer mass, temperature, chemical composition, size from observations?chemical composition, size from observations?


Key Properties of StarsKey Properties of StarsStars: how do we infer mass, temperature, chemical Stars: how do we infer mass, temperature, chemical composition, size from observations?composition, size from observations?

once we know distance, we can say a lotonce we know distance, we can say a lotChapter 18: how we measure distanceChapter 18: how we measure distanceassume for now we know the distance assume for now we know the distance

Distance Units:Distance Units:1 AU handy unit for distances in Solar system1 AU handy unit for distances in Solar systemlight year: distance light travels in one year, 9.46 x 10light year: distance light travels in one year, 9.46 x 101717 m m

New unit: New unit: parsecparsec (pc) (pc)1 pc = 3.26 LY is roughly the average distance between 1 pc = 3.26 LY is roughly the average distance between starsstars1 kiloparsec = kpc = 1,000 parsecs is roughly the size of 1 kiloparsec = kpc = 1,000 parsecs is roughly the size of galaxygalaxynatural unit for measuring distances – see Ch. 18natural unit for measuring distances – see Ch. 18


Star BrightnessStar BrightnessStar brightness specified with the Star brightness specified with the magnitudemagnitude system. system.

Devised by the Greek astronomer Hipparchus around Devised by the Greek astronomer Hipparchus around 150 B.C.E. 150 B.C.E.

brightest stars into the first magnitude class, brightest stars into the first magnitude class,

next brightest stars into second magnitude class, next brightest stars into second magnitude class,

and so on, until he had all of the visible stars grouped into and so on, until he had all of the visible stars grouped into six magnitude classes. six magnitude classes.

dimmest stars were of sixth magnitude. dimmest stars were of sixth magnitude.

brighter objects have brighter objects have smallersmaller magnitudes than fainter magnitudes than fainter objects!objects!

magnitude system was based on how bright a star magnitude system was based on how bright a star appeared to the unaided eye. appeared to the unaided eye.


Extra magnitude?Extra magnitude?Some objects go beyond Hipparchus' original bounds of Some objects go beyond Hipparchus' original bounds of magnitude 1 to 6. magnitude 1 to 6.

Very bright objects can have magnitudes of 0 or even negative Very bright objects can have magnitudes of 0 or even negative numbers.numbers.

Very faint objects have magnitudes greater than +6. Very faint objects have magnitudes greater than +6.

Remember: brighter objects have Remember: brighter objects have smallersmaller magnitudes than magnitudes than fainter objects!fainter objects!


Apparent MagnitudeApparent MagnitudeApparent brightness of a star observed from the Apparent brightness of a star observed from the Earth is called the Earth is called the apparent magnitudeapparent magnitude. .

The apparent magnitude is a measure of the The apparent magnitude is a measure of the star's star's fluxflux received by us. received by us.

Examples of apparent magnitudes: Examples of apparent magnitudes: Sun = -26.7,Sun = -26.7, Moon = -12.6, Moon = -12.6, Venus = -4.4, Venus = -4.4, Sirius = -1.4, Sirius = -1.4, Vega = 0.00, Vega = 0.00, faintest naked eye star = +6.5, faintest naked eye star = +6.5, brightest quasar = +12.8, brightest quasar = +12.8, faintest object = +27 to +28. faintest object = +27 to +28.


Absolute MagnitudeAbsolute MagnitudeMeasure of star Measure of star luminosity.luminosity.

Luminosity is the total amount of energy radiated by the star every secondLuminosity is the total amount of energy radiated by the star every second

If you measure a star's apparent magnitude and know If you measure a star's apparent magnitude and know its absolute magnitude, you can find the star's its absolute magnitude, you can find the star's distance distance

If you know a star's apparent magnitude and distance, If you know a star's apparent magnitude and distance, you can find the star's luminosityyou can find the star's luminosity

A quantity that depends on the star itself, not A quantity that depends on the star itself, not on how far away it ison how far away it is

Provides information about the structure of the Provides information about the structure of the star – this is the star – this is the real real luminosityluminosity

More important quantity than the apparent More important quantity than the apparent brightnessbrightness

need the distance to determine the absolute need the distance to determine the absolute magnitude magnitude


Star Luminosity vs Star Luminosity vs TemperatureTemperature

stars are luminousstars are luminousbecause because

they are hotthey are hot

they are large they are large

or both! or both!

Luminosity of an object = the amount of energy every Luminosity of an object = the amount of energy every square meter produces multiplied by its surface area. square meter produces multiplied by its surface area.

Luminosity = Luminosity = x T x T44, ,

Luminosity of a star increases very quickly with even Luminosity of a star increases very quickly with even slight increases in the temperature. slight increases in the temperature.


Star Luminosity vs SizeStar Luminosity vs SizeLuminosity ~ surface area.Luminosity ~ surface area.

1,000 watt bulb has same luminosity as a row of ten 100 watt bulbs1,000 watt bulb has same luminosity as a row of ten 100 watt bulbs

Luminosity of a bigger star larger than a smaller star at the Luminosity of a bigger star larger than a smaller star at the same temperature. same temperature.

From the apparent brightness, temperature, and distance of a From the apparent brightness, temperature, and distance of a star, one can determine its size. star, one can determine its size.


Absolute vs Absolute vs ApparentApparent

Star brighter if Star brighter if closercloserbrightness fades brightness fades with distancewith distance

inverse square inverse square lawlaw

if stars were all if stars were all the same the same brightness than brightness than apparent apparent luminosity would luminosity would measure distancemeasure distance


Famous starsFamous starsMost famous Most famous apparently bright stars apparently bright stars are also intrinsically are also intrinsically bright (luminous). bright (luminous).

Can be seen from great Can be seen from great distances away. distances away.

Most nearby stars are Most nearby stars are intrinsically faint. intrinsically faint.

Not necessarily Not necessarily representative of all representative of all stars…stars…

Magnitudes and Distances of some stars(from the precise measurements of the Hipparcos mission)

StarApparentMagnitude

Distance(pc)

AbsoluteMagnitude

Luminosity(relative to Sun)

Sun -26.74 4.84813×10-6 4.83 1

Sirius -1.44 2.6371 1.45 22.5

Arcturus -0.05 11.25 -0.31 114

Vega 0.03 7.7561 0.58 50.1

Spica 0.98 80.39 -3.55 2250

Barnard's Star

9.54 1.8215 13.24 1/2310

Proxima Centauri

11.01 1.2948 15.45 1/17700


Color and TemperatureColor and TemperatureStars are dense hot balls of gas Stars are dense hot balls of gas

Their spectrum is close to that of Their spectrum is close to that of a perfect thermal radiatora perfect thermal radiatorWhich produces a smooth continuous Which produces a smooth continuous spectrumspectrum

So called So called blackbody spectrumblackbody spectrum..

Color of stars depends on their Color of stars depends on their temperature: temperature: hotter stars are bluer hotter stars are bluer

cooler stars are redder. cooler stars are redder.


Color and TemperatureColor and TemperatureOne can observe the stars through One can observe the stars through different different filtersfilters to get an approximate to get an approximate temperature. temperature. Filter allows only a narrow range of Filter allows only a narrow range of wavelengths (colors) through. wavelengths (colors) through. By sampling the star's spectrum at two By sampling the star's spectrum at two different wavelength ranges (“bands”), different wavelength ranges (“bands”), one can determine if the spectrum is one can determine if the spectrum is that a hot, warm, cool, or cold star. that a hot, warm, cool, or cold star. Hot stars have surface temperatures Hot stars have surface temperatures around 60,000 K while cold stars have around 60,000 K while cold stars have surface temperatures around 3,000 K.surface temperatures around 3,000 K.


Star’s Color Star’s Color TemperatureTemperature


B-V Color IndexB-V Color IndexMeasure of the temperature based Measure of the temperature based on apparent color.on apparent color.

Based on two different filters.Based on two different filters.

A blue (B) filter that only lets a A blue (B) filter that only lets a narrow range of colors or narrow range of colors or wavelengths through centered on the wavelengths through centered on the blue colors.blue colors.

A “visible”' (V) filter that only lets the A “visible”' (V) filter that only lets the wavelengths close to the green-wavelengths close to the green-yellow band through. yellow band through.

A hot star has a B-V color index close to A hot star has a B-V color index close to 0 or negative, while a cool star has a B-V 0 or negative, while a cool star has a B-V color index close to 2.0. Other stars are color index close to 2.0. Other stars are somewhere in between. somewhere in between.

Defined as the difference in magnitude Defined as the difference in magnitude between the B and V bands.between the B and V bands.


Spectra of StarsSpectra of Starsprimary reason stellar spectra look different is primary reason stellar spectra look different is stars have different temperatures stars have different temperatures

hydrogen most abundant element – most stars hydrogen most abundant element – most stars show hydrogen absorption linesshow hydrogen absorption lines

hottest may not hottest may not

so hot that hydrogen is completely ionizedso hot that hydrogen is completely ionized

coolest stars coolest stars

hydrogen atoms are all in lowest state hydrogen atoms are all in lowest state

no hydrogen transitions seen no hydrogen transitions seen


Spectral ClassesSpectral Classes

O

B

A

F

G

K

M


Spectral Class

Characteristics

OIonized Helium and metals; weak

Hydrogen

BNeutral Helium, ionized metals,

stronger Hydrogen

ABalmer Hydrogen lines dominant, singly-

ionized metals

FHydrogen weaker, neutral and singly-

ionized metals

GSingly-ionized Calcium most prominent,

Hydrogen weaker, neutral metals

KNeutral metals, molecular lines begin

to appear

MTitanium Oxide molecular lines

dominant, neutral metals


Doppler EffectDoppler EffectCase (a)Case (a)

Object (source) moving Object (source) moving towards observer A at velocity towards observer A at velocity “v”“v”

Observer “A” sees compressed Observer “A” sees compressed wave, I.e. wave, I.e. shorter wavelength, shorter wavelength, higher frequencyhigher frequency..

Observer “B” see stretched Observer “B” see stretched wave, I.e. longer wavelength, wave, I.e. longer wavelength, lower frequency.lower frequency.

Case (b)Case (b)Stationary sourceStationary source

Observer “A” and “B” see Observer “A” and “B” see same same wavelengthwavelength..

SourceObserver A Observer B

v

(a)

SourceObserver A Observer B

(b)


Doppler Effect with StarsDoppler Effect with StarsMotion of the light source (star) causes the Motion of the light source (star) causes the spectral lines to shift positions. spectral lines to shift positions. An object's motion causes a wavelength shiftAn object's motion causes a wavelength shift

= = newnew - - restrest

Depends on speed and direction of moving Depends on speed and direction of moving object. object. Shift given by:Shift given by:

= = restrest × × VVradialradial / / cc, , cc is the speed of light, is the speed of light,

restrest is the wavelength measured if object is at rest. is the wavelength measured if object is at rest.

VVradialradial is object speed along the line of sight. is object speed along the line of sight.


Red and Blue ShiftRed and Blue Shift

If object is moving If object is moving towardtoward you, the waves are compressed, you, the waves are compressed,

So their wavelength is So their wavelength is shorter.shorter. Lines are shifted to shorter (bluer) wavelengths.Lines are shifted to shorter (bluer) wavelengths.This is called a This is called a blueshiftblueshift. .

If the object is moving If the object is moving awayaway from you, the waves from you, the waves are stretched out, are stretched out,

So their wavelength is So their wavelength is longer.longer. The lines are shifted to longer (redder) wavelengths.The lines are shifted to longer (redder) wavelengths.This is called a This is called a redshiftredshift. .


Spectral ShiftsSpectral ShiftsDoppler effect doesn’t affect overall color of an Doppler effect doesn’t affect overall color of an object unless it is moving at a significant object unless it is moving at a significant fraction of the speed of light (VERY fast!). fraction of the speed of light (VERY fast!).

For an object moving toward us, the red colors For an object moving toward us, the red colors will be shifted to the orange and the near-will be shifted to the orange and the near-infrared will be shifted to the red, etc. All of the infrared will be shifted to the red, etc. All of the colors shift. colors shift.

The overall color of the object depends on the The overall color of the object depends on the combined intensities of all of the wavelengths combined intensities of all of the wavelengths (colors). (colors).


Spectral ShiftsSpectral ShiftsSun spectrum at 3 Sun spectrum at 3 speeds (0, 0.01speeds (0, 0.01c,c, 0.1 0.1c).c). Hydrogen-alpha line (at Hydrogen-alpha line (at 656.3nm) is shown.656.3nm) is shown.Objects in our galaxy Objects in our galaxy move at speeds much move at speeds much less than 0.01less than 0.01c.c. Doppler-shifted Doppler-shifted continuous spectrum for continuous spectrum for the Sun moving at 0.01the Sun moving at 0.01cc almost indistinguishable almost indistinguishable from the Sun at rest.from the Sun at rest.


Spectral Shifts (cont’d)Spectral Shifts (cont’d)Doppler shift of spectral Doppler shift of spectral lines measurable even lines measurable even for slow speed. for slow speed.

Astronomers can Astronomers can detect spectral line detect spectral line doppler shifts for doppler shifts for speeds as small as speeds as small as 1 km/sec or lower (less /sec or lower (less than 3.334than 3.3341010-6-6 c).c).


Proper MotionProper MotionDoppler effect Doppler effect provides speed provides speed along along the line of sight.the line of sight.

Most stars move at Most stars move at an angle to our line of an angle to our line of sight. sight.

We measure this by We measure this by watching stars move watching stars move over time. over time.


Doppler ShiftDoppler Shift


SummarySummaryA wealth of information is contained in the A wealth of information is contained in the spectra of stars.spectra of stars.

Astronomers can learn about:Astronomers can learn about:luminosityluminosity

surface temperaturesurface temperature

compositioncomposition

radial motion from the doppler shiftradial motion from the doppler shift

rotation from broadening of spectral linesrotation from broadening of spectral lines

February 7, 2006 Astronomy 2010 1 Chapter 16: Analyzing Starlight.

Documents

magnitude system

key properties of stars

brightest stars

starlight stars

magnitude classes

extra magnitude

visible stars

dimmest stars