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ASTR 5100 Stellar Astrophysics
Fall 2017
Time: Mondays 2:20 - 5:20 PM Venus: 501, 2nd General Building Webpage: http://orion.astr.nthu.edu.tw/star/
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Stars - Course Plan
Instructor & TAVivien Chen ��� Office: 513 2nd General Building Email: [email protected] Office hour: 10:30-11:30, Wednesdays
He-Feng Hsieh ��� Office: 520 2nd General Building Email: [email protected] Office hour: 14:00-15:00, Wednesdays
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Stars - Course Plan
Class PolicyGrading policy Problem sets: 70% Final examination: 30%
Problem sets are due 5PM Thursdays of the following week unless otherwise instructed Late problems will not be accepted unless there is a valid excuse approved by the instructor prior to the deadline
14 lectures in total National holidays on October 9 and January 1 Vivien away on October 2, 16, and 23. Make-up class on October 11, 9:00-11:50
Final examination on January 8, 2018
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Stars - Course Plan
Textbook and ReferencesTextbook The Physics of Stars (2nd edition) by A. C. Phillips (1999, John Wiley), listed price NT$1,200 from 苉蝢 (the 1st edition is fine, too)
In MKS units, inconvenient later to connect with professional journal papers in CGS units
References The Physical Universe by Frank H. Shu (1982, University Science Books)Radiative Processes in Astrophysics by G. B. Rybicki & A. P. Lightman (1979, Wiley), electronic version available on the campus network
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Stars - Course Plan
Course OutlineBasic concepts in Astrophysics Properties of matter and radiation Heat transfer in stars Thermonuclear fusion in stars Stellar structure The end-points of stellar evolution Radiative transfer
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Stars - Course Plan
Astronomical Measurements
Astronomical measurements A quick overview of stars
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Chapter 0 Astronomical M
easurements
Parallax Angles7
Trigonometric parallax p
Stellar parallax (http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html)
parsec (parallax-second, pc)
1 pc = 2.06264806� 105 AU= 3.0856776� 1016 m= 3.2615638 ly
d (pc) =1 (AU)
p��
d =B
tan p
=B
p
Astronomical M
easurements
Luminosity and FluxLuminosity L (ergs s-1)
Intrinsic radiant energy per second from a star
Flux f (ergs s-1 cm-2) Measured radiant energy per second per unit area by an observer at a distance d
Inverse square law
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F =L
4� d2
Astronomical M
easurements
MagnitudesApparent magnitude m (mag)
Relative brightness between starsDifference of 5 magnitudes = difference of 100 times in flux
Absolute magnitude M (mag) Apparent magnitude of a star would have if it were located at 10 pc
Distance Modulus
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100(m�M)/5 =F10 pc
F=
�d
10 pc
⇥2
m�M = 5 log d� 5 = 5 log�
d
10 pc
⇥
F2
F1= 100(m1�m2)/5
m1 �m2 = �2.5 log10
✓F1
F2
◆
Astronomical M
easurements
Planck FunctionA description of blackbody radiation
Emerging flux density per unit solid angle per unit frequency
Emerging flux density per unit solid angle per unit wavelength
Note that
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B⇥d⇥ = B�d�
d⇥ =c
�2d�
n̂
dE = B�(T ) dt d� dA n̂·d�
B�(T ) =2h�3
c2
1eh�/kT � 1
dE = B� dt d� dA n̂·d�
B�(T ) =2hc2
�5
1ehc/�kT � 1
Astronomical M
easurements
Blackbody RadiationWien’s displacement law (hν ≫ kT)
Rayleigh-Jeans law (hν ≪ kT)
Stefan-Boltzmann equation
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Astronomical M
easurements
�maxT = 0.2897755 cmK
B⌫(T ) =2kT⌫2
c2=
2kT
�2
L = 4⇡R2�SBT4e ,
where Te is the e↵ective tempera-ture. In other words, the surfaceflux is
F = �SBT4e .
Monochromatic QuantitiesMonochromatic luminosity of a blackbody
Emergent energy per second between ν and dν+ν
Monochromatic flux of a blackbody Emergent flux per second between ν and dν+ν
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Astronomical M
easurements
L⌫d⌫ = B⌫d⌫
Z 2⇡
�=0d�
Z ⇡/2
✓=0cos ✓ sin ✓d✓
Z
AdA
= 4⇡2R2⇤B⌫d⌫
= 4⇡2R2⇤2h⌫3
c21
eh⌫/kT � 1d⌫
F⌫d⌫ =L⌫
4⇡d2d⌫
=2⇡h⌫3
c21
eh⌫/kT � 1
✓R
d
◆2
d⌫
= ⇡B⌫
✓R
d
◆2
d⌫
Colors and FiltersUBV system: wavelength filter
U band: 68 nm bandwidth centered at 365 nmB band: 98 nm bandwidth centered at 440 nmV band: 89 nm bandwidth centered at 550 nm
Standard filter system
UBVRIJHKLM
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NGC 290 (open cluster) Credit: ESA, NASA, E. Olszewski (U. Arizona) HST
Astronomical M
easurements
Credit: Sky & Telescope
Color IndicesColor index
Brightness difference between two filter bands Independent of the star’s distance
Bolometric correction BC
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Astronomical M
easurements
U �B = mU �mB = MU �MB
B � V = mB �mV = MB �MV
BC = mbol � V
= Mbol �MV
The Sun
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Astronomical M
easurements
The Sun as a Reference16
Property Value
Mass M� = 1.99 × 1033 gRadius R� = 6.96 × 1010 cmLuminosity L� = 3.90 × 1033 erg s-1 Effective temperature Teff = 5780 KAge t� = 4.55 × 109 yrCentral density ρc = 1.48 × 102 g cm-3 Central temperature Tc = 15.6 × 106 KCentral pressure Pc = 2.29 × 1017 dyn cm-2
Astronomical M
easurements
Stellar Spectral Types17
(early) O B A F G K M L T Y (late) Stars (a.k.a. dwarfs): OBAFGKMBrown dwarfs: LTYFollowed by subclass 0, 1, ..., 9
Credit: KPNO 0.9-m Telescope, AURA, NOAO, NSF
Astronomical M
easurements
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Astronomical M
easurements
Hertzsprung-Russell Diagram
The Interiors of Stars19
Astronomical M
easurements
Stars of different mass have different interior structures
Stellar Evolution20
http://chandra.harvard.edu/photo/2007/sn2006gy/more.htmlAstronom
ical Measurem
ents
Generations of Stars21
Important role of stars: enrich the interstellar medium (ISM) with heavy elements (metals)
Credit: Wolfgang Brandner (JPL/IPAC), Eva K. Grebel (U of Washington), You-
Hua Chu (UIUC), ѿ NASA
Astronomical M
easurements
Interstellar Medium
Display of ISM Absorption: dark clouds Scattering: reflection nebulae Emission: emission nebulae
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Credit: Daniel Verschatse (Antilhue Observatory)
Astronomical M
easurements
AGB Stars (Evolved Stars)Asymptotic giant branch (AGB) stars
Explosive phase in late stages of stars with M < 8 M� Large mass-loss rate Ṁ ~ 10-4 M� yr-1 Enrich ISM with metals
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Astronomical M
easurements
Crab Nebula (SN 1054)Expansion velocity ~ 1450 km s-1 Luminosity ~ 8×104 L⊙, mostly highly polarized synchrotron radiation
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Credit: NASA - X-ray: CXC, J.Hester (ASU) et al.; Optical: ESA, J.Hester and A.Loll (ASU); Infrared: JPL-Caltech, R.Gehrz (U. Minn)
http://antwrp.gsfc.nasa.gov/apod/ap061026.html
Astronomical M
easurements
