Download Ch.2 lecture

Welcome to Astronomy 117B !
Dr. Monika Kress
Science 262
[email protected]
Office hours: MW 10:30-noon
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Chapter 2: Continuous radiation from stars
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Homework problems to do for Wednesday:
Page 22-23, # 2, 3, 4, 10, 13, 16, 17, 24, 30
The electromagnetic spectrum
optical
Photons: carriers of the electromagnetic force
•
•
All photons travel at the speed of light*,
Their only property is their energy,
c  
E  h 
hc



See Table 2.1 for wavelength and frequency of EM radiation
Blackbody (thermal) radiation
•
BB thermal emission intensity
•
Hotter objects emit more photons at all wavelengths (per
unit area)
Hotter objects emit photons with a higher average energy
Stefan-Boltzmann equation:
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L  AT
4
 = 5.670 x 10-5 erg s-1 cm-2 K-1
Wien’s Displacement Law
maxT = 0.290 cm-K
Planck’s Law for emission of blackbody radiation:
Quantization of energy!!!
2h
1
I( ,T)  2 h /kT
c e
1
3
I( ,T) 
2hc

5
2
1
e
hc/ kT
1
** This is not a simple substitution of c = . Why not?
High Resolution Solar Spectrum
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Solar radiation
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The solar radiation that reaches the surface
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Distant stars
Distance in astrophysics
1 AU = 149.6 million km
1 LY = 9.46 x 1012 km
1 pc = 3.26 LY
Earth’s motion
around Sun
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1 AU
tan p (in arcsec) 
d (in pc)

Not to scale!
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The magnitude scale
m = apparent magnitude (how bright a star appears to us)
M = absolute magnitude (how bright it would be if it were
10 pc away)
Brightest stars have apparent magnitude m = 1
Faintest visible stars have magnitude m = 6
Calibration:
When the difference between 2 stars, m2 - m1 = 5
star 1 appears 100 times brighter than star 2:
b1
(m 2 m1 )/5
 100
b2
b1 
m2  m1  2.5log  
b2 
Compare apparent magnitude of the Sun to that
of the faintest object observable by HST:
msun = -26.7
mHST = +23.7
Compare apparent magnitude of Jupiter to
its absolute magnitude:
mJ = -2
MJ = +27
Absolute magnitude and stellar distances
m = apparent magnitude (how bright a star appears to us)
M = absolute magnitude (how bright it would be if it were
10 pc away)
M is a measure of the star’s luminosity (total energy output).
 d 
m  M  5log 10

10 pc 
Distance modulus
Quantifying stellar colors
b(1) 
m2  m1  2.5log 10
 = “color”
b(2 ) 
Suppose As T increases,
b( 1 )
increases
b( 21)

So m2 - m1
increases

1st typo of the book: 3 paragraph under 2.5 ‘Stellar Colors’
decreases should be increases