Welcome to Astronomy 117B ! Dr. Monika Kress Science 262

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)

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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