The Stars and the Sun I. Colors of stars ttp://www.phy.cuhk.edu.hk/gee/mctalks/mcpdp.html Chu Ming-chung 朱朱朱 Department of Physics The Chinese University of Hong Kong [email protected]
The Stars and the Sun I.Colors of stars
http://www.phy.cuhk.edu.hk/gee/mctalks/mcpdp.html
Chu Ming-chung 朱明中
Department of Physics
The Chinese University of Hong Kong
M37
Capella 五車二
Binary stars in Cygnus
M20
M42
M8M57All taken in CUHK
What information are carried in star light?
Colors of Stars
1.1 Starlight1.2* Light-matter interaction1.3* Stellar spectrum1.4 Doppler effect1.5 Stellar luminosity1.6 H-R Diagram 1.7 H-R Diagrams for star clusters
1.1 Starlight What is light?
When the velocities of moving charged
particles are changed, electromagnetic
radiation (EM radiation) 電磁輻射 (light
is a kind of EM radiation) is emitted in the
form of waves ( EM waves 電磁波 ).
Thermal motion of particles in a star → light
+
+
http://www.colorado.edu/physics/2000/applets/fieldwaves.html
Different bands of EM waves
3,000oC6,000oC Higher temperature
Why do hot materials give out light?
What happens if temperature rises further?
Charged particles in a hot gas (e.g. inside a star) move
around rapidly and undergo many collisions
their velocities are changed in the collisions
light (EM waves in general) is emitted
Why are the colors of light different for different
temperature?
Violent collisions high-energy light (short wavelength,
high frequency, e.g. violetviolet)
Gentle collisions low-energy light (long wavelength, low
frequency, e.g. redred)
In a star, the temperature is very high, both violent and gentle collisions occur it gives out electromagnetic radiation of all
wavelengths
starlight can be decomposed into a continuous spectrum 連續光譜 (like a rainbow)
Spectrum 光譜 : decomposing light into different colors
Red: lower frequency, longer wavelength
blue: higher frequency, shorter wavelength
Sun’s spectrum
As temperature rises: 1. light intensity increases, 2. the color of light shifts towards high frequency (blue) side
Blue stars are hotter than red stars
1)exp(
125
2
kThc
hcI
= Energy of EM wave with wavelength per unit time per unit area emitted by a body at T. Boltzmanns constant Plancks constant
Intensity peak (Wien’s law)Blue stars are hotter than red stars (higher surface T)
Planck’s distribution
-123 JK1038.1 kJs1063.6 34h
mK109.2 3max
T
for blackbody radiations 黑體輻射
Just for reference!
Cooler stars are dimmer and redderTotal radiation power for all ’s = = energy per unit time per unit
area emitted by a body at temperature TStefan-Boltzmanns const.
Luminosity of a blackbody sphere
Color:
4
0TdI
-4-283245 KWm1067.5152 hck
424 TRL
max 1/T
‘colors’ emitted by different ‘stars’. Eg. Sun’s radiation peaks at ~ 0.5 microns
You emit light too! What ‘color’ is the light you emitted?
Ans.: Body temperature ~ 300 K ~ 1/20 Sun’s surface temperature. Therefore, human’s radiation peaks at 20x 0.5=10 microns.
Spectral classification ( 光譜分類 )
spectral classes Approximate surface
temperature (K) O 40,000 B 20,000 A 10,000 F 7,500 G 5,500 K 4,500 M 3,000
Eg.: Sun: G Vega ( 織女星 ): A
OOh! BBe AA FFine GGirl (Guy)! KKiss MMe!
Group stars with similar spectra (temperature, elements) into same classes.
M20
M42
M8M57
All taken in CUHKWhat information are carried in star light?http://apod.nasa.gov/apod/ap010729.html
http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/ssc2005-09b.shtmlIllustration courtesy NASA/Spitzer Infrared Space Telescope
Use infrared telescopes to detect planets directly – 2 found already so far!
Examples of using non-visible light
http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/ssc2005-09b.shtmlIllustration courtesy NASA/Spitzer Infrared Space Telescope
Planetary Eclipses in Infrared
http://www.spitzer.caltech.edu/Media/releases/ssc2005-10/ssc2005-10b.shtmlIllustration courtesy NASA/Spitzer Space Telescope
Found even asteroid belt around HD 69830 using Infrared telescope
Examples of using non-visible light
Positron Clouds near the galactic center
How do we know there are positrons ?
Examples of using non-visible light
2 ,
511 keV.
e e
E
Gamma ray telescope
1.2* Light-matter interaction
http://www.colorado.edu/physics/2000/index.pl
Bohr’s model of atomsElectrons have wave properties (de Broglie)de Broglie wavelength Bohr: circular orbits
only standing wave orbits are stable
Only discrete energies allowed radius = n²ao ao= 5x10-11 m Bohr’s radius
/h p
2n r 2 2 2/ /mv r e r
21 /nE E n
http://id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html
E1 →E2E1 →E3
E1 →E4
E1
E2
E3
E4Hydrogen atom:En= -13.6eV/n²Transitions: emission or absorption of light at specific energiesAbsorption spectrum
吸收光譜
Different elements emit different spectral lines
Emission spectrum 放射光譜
E2 →E1E3 →E1
E4 →E1 E1
E2
E3
E4
http://www.colorado.edu/physics/2000/index.pl
1.3* Stellar Spectrum
Light source emitting a continuous spectrum
Atoms in the atmosphere absorb light of particular frequencies
Dark linesAbsorption spectrum 吸收光譜
Grating and CCD C11 + spectrograph + CCD
Stellar Spectrum
Photos taken by Lee Wing Kit and Chan Wing Hang
Stellar atmosphere colder than interior
Stellar light absorbed
selectively by atoms in
stellar atmosphere
http://apwww.smu.ca/~ishort/Astro/
Spectrum of Vega 織女星
Photos taken by Lee Wing Kit and Chan Wing Hang in CUHK
Hydrogen Alpha line (6563Å)All are H lines !!
1Å=10-10m
red
H lines
violet
Spectrum of Sirius 天狼星光譜
Both are Type A Stars
Compared with Vega’s Violet
RED
紅
Photos taken by Lee Wing Kit and Chan Wing Hang
Spectral Class 恆星光譜型Type Color
Surface Temperature
O Blue > 25,000 K
B Blue 11,000 - 25,000
A Blue 7,500 - 11,000
F Blue/White 6,000 - 7,500
G White/Yellow 5,000 - 6,000
K Orange/Red 3,500 - 5,000
M Red < 3,500
Spectrum of Betelgeuse 參宿四光譜violet
Hydrogen Alpha line (6563Å) No H lines??
red
A typical Type M star (Red Giants)
Metal lines TiO
Photos taken by Lee Wing Kit and Chan Wing Hang
Orion 獵戶座
參宿四 Betelgeuse
獵戶座大星雲 Orion Nebula
Photos taken by Lee Wing Kit and Chan Wing Hang
Spectrum of Orion Nebula
emission spectrum
H lines
What are these?~ 5890Å
violetred
Photos taken by Lee Wing Kit and Chan Wing Hang
O2+ (Earth’s atmosphere)
4959Å, 5007Å
violet
red
Light pollution!!
Street lamp (sodium)
Photos taken by Lee Wing Kit and Chan Wing Hang
Spectra of Planets 金星 Venus
red
violet
土星 Saturn
Why are they so similar?
H
different
Photos taken by Lee Wing Kit and Chan Wing Hang
Atmospheric Absorption大角 Aldebaran
心宿二 Antares
參宿四Betelgeuse
天狼 Sirius
織女 Vega
金星 Venus
土星 Saturn
七姊妹星團Pleiades
獵戶座大星雲
Telluric Lines H HPhotos taken by Lee Wing Kit and Chan Wing Hang
Absorption lines => elementsIntensity peak position => surface temperatureStrengths of absorption lines => also surface
temperature Hydrogen as an example: Very high temperatures
=> electrons leave the atoms (ionized) ; low temperatures, electrons stay at the ground state.
Low High Very High
Measure the absorption line intensities of Balmer lines ( 巴耳末線 ) [electrons transit from the 2nd level to higher levels]
We can know the number of atoms in which the electrons are at the 2nd level
Hence get an estimate of the surface temperature
2nd level
Intensities depend on the number of electrons at the 2nd level
Taken from NOAO/AURA/NSF webpage http://www.noao.edu/image_gallery/html/im0649.html
Spectrum of Sun
Transit method: observe the planetary transit →small periodic dimming of star light, new absorption lines →elements in the planetary atmosphere
Photo and animation courtesy NASA/STScI
Eg. HD209458: Na detected in planetary atmosphere
Examples of Spectral Method
1.4 Doppler effect ( 多普勒效應 )
v=0 v=0.4 v=1
stationary source
moving source
moving source
http://www.tmeg.com/esp/p_doppler/doppler.htm
Light emitted by the source will have wavelength decreased (blue shifted) in front of its motion and increased (red shifted) behind it.
Blue shifted藍移
Red shifted紅移
v
Spectrum of object at rest
Spectrum taken for approaching object
Spectrum taken for receding object
/ / / for ,
wavelength, frequency,
speed of object, speed of light.
f f v c v c
f
v c
http://sci.esa.int/content/doc/16/28950_.htm
Doppler effect demonstration
Width of a spectral line may be affected byNatural broadening - quantum effect, very smallDoppler broadening - Doppler shifts due to random
thermal motions of atoms.
For
Total width
cv
c
vr
m
kTvv
32rms
m
kT
c
32
E.g., H line of the sun
1000 times > natural broadeningK,5770T ,A6563
o
kg,1067.1 27mo
A52.0
Rotational broadening - light coming from a rotating star is Doppler shifted
Eg. see different shifts on different sides of Saturn’s ring: rotation speed
1.5 Stellar luminosity ( 恆星光度 )
Magnitudes and LuminosityApparent magnitude m ( 視星等 ): measures the
luminosity (B) of starlight received on earth.5 magnitudes = 100 timesAbsolute magnitude M ( 絕對星等 ): measures
the luminosity a star would have if it was placed at a distance of 10 pc (~33 light years) away.
Luminosity ( 光度 ) B: Total amount of energy that the star radiates in one second. It is determined by a combination of two factors: Surface area Surface temperature
424 TRL
1 2 1 22.5log /m m B B
Convention: mVega= 0
Luminosity ~ 1/r2
Distance modulusComparing apparent and absolute magnitudes gives distance r
Vega Vega2.5log / 2.5log , 2.5log .m B B k B k B
2
2
( ) 10 ( )log 2 2log ,
(10) (10)
2.5log ( ) 2.5log (10)
( ) 2.5log 5 5log .
(10)
B r B rr
B r B
m M k B r k B
B rr
B
Mm
a hot star with a large surface
area must be luminous
a cool star with a small surface
area must be dim
a cool star could be luminous
if it is very large (not much
radiation is emitted per unit
area, but the total radiation
rate is large because its has a
large surface area for light
emission)
Stefan-Boltzmanns law
y = m x + b
Lines of constant R
424 TRL 2 4
L R T
L R T
log 4log
2log
L T
L T
R
R
2logb R R
1.6 Hertzsprung-Russell diagram, H-R diagram ( 赫羅圖 )
Cooler, NOTNOT hotter
主序星
白矮星
超巨星
Main sequence: A belt from upper left to lower right, 90% of all stars Cool stars are faint and small; hot stars are bright
and largeGiants at the upper right corner, they are cool but
luminous must have large surface area ~10-100 supergiants have ~100-1000
White dwarfs lie in the lower left, they are hot but faint must be very small (~ size of Earth)
RR
RR
Figures courtesy HST/NASA
Red Giant
1.7 H-R Diagrams of Star Clusters
Two kinds of clusters: Open clusters ( 疏散星團 ) : contain 10-1000
stars; open, less densely populated, younger, mostly distributed close to the plane of our Galaxy
Star clusters ( 星團 )
M45 Pleiades Open Cluster M37 taken in CUHK七姊妹
M13 taken in NAM3 taken in CC by Delphi
Globular Clusters 球狀星團Globular Clusters 球狀星團
Star clusters Stars in the same star cluster are formed from the
same cloud
they have similar ages and initial chemical
compositions
But the stars differ in luminosity (mass) and surface temperature (color) They are located at different positions on the
H-R diagram
The Pleiades M45七姊妹星團The Pleiades M45七姊妹星團
From CUHKFrom CUHKHow do you explain the turnoff point?How do you explain the turnoff point?
The Pleiades
More massive members
(more luminous) leave the
main sequence and become
giants, while lower mass
members still lie on the
main sequence
confirms the evolution
picture that more massive
stars have shorter lives on
the main sequence
massive stars
Locating the turnoff point of a cluster’s H-R diagram
determines the cluster’s age by stellar evolution
theory; the lower the turnoff point, the older it is
SummaryLight = EM waves, emitted when charges change speed;
frequency, wavelengthStarlight: continuous spectrum, higher temperature
→higher intensity, bluer spectral lines (absorption or emission)
Bohr model: spectral lines correspond to energy level separations
Doppler effect: red/blue shifts, broadening (T, rotation)Spectral classes: OBAFGKMAbsolute/apparent magnitudes, luminosityH-R Diagram: luminosity vs. surface temperature
424 TRL max 1/T
Colors of Stars
1.1 Starlight1.2* Light-matter interaction1.3* Stellar spectrum1.4 Doppler effect1.5 Stellar luminosity1.6 H-R Diagram 1.7 H-R Diagrams for star clusters
The Stars and the Sun I.Colors of stars
http://www.phy.cuhk.edu.hk/gee/mctalks/mcpdp.html
Chu Ming-chung 朱明中
Department of Physics
The Chinese University of Hong Kong