Welcome to Astronomy 117B !
Dr. Monika Kress
Science 262 mkress@science.sjsu.edu
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*, c
 
• Their only property is their energy,
E
 h
  hc



See Table 2.1 for wavelength and frequency of EM radiation

Blackbody (thermal) radiation
• Hotter objects emit more photons at all wavelengths (per unit area)
• Hotter objects emit photons with a higher average energy
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Stefan-Boltzmann equation:
L

A

T
4

= 5.670 x 10 -5 erg s -1 cm -2 K -1


Wien’s Displacement Law  max
T = 0.290 cm-K



Planck’s Law for emission of blackbody radiation:
Quantization of energy!!!
I (

, T )

2 h

3 c
2
1 e h

/ kT 
1
I (

, T )

2 hc
2

5
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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Distance in astrophysics
1 AU = 149.6 million km
1 LY = 9.46 x 10 12 km
1 pc = 3.26 LY
Earth’s motion around Sun
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Distant stars


p

d
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Not to scale!


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, m
2
- m
1 star 1 appears 100 times brighter than star 2:
= 5 b b
1
2

100
( m
2
 m
1
)/5 m
2
 m
1


 b
1 b
2




Compare apparent magnitude of the Sun to that of the faintest object observable by HST: m sun
= -26.7
m
HST
= +23.7
Compare apparent magnitude of Jupiter to its absolute magnitude: m
J
= -2 M
J
= +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).
m

M

10

 d


Distance modulus

Quantifying stellar colors m
2
 m
1

2.5log
10

 b ( b (


2
1
)
)


= “color”
 Suppose





As T increases, b ( b (

1
)

21
) increases
So m
2
- m
1 increases

1st typo of the book: 3 paragraph under 2.5 ‘Stellar Colors’ decreases should be increases