1	Starlight and Atoms Chapter 6
2	Chapter 6: Starlight and Atoms Some Good Star Quotes “Be humble, for the worst thing in the world is of the same stuff as you; be confident, for the stars are of the same stuff as you.” –Nicholai Velimirovic “No pessimist ever discovered the secrets of the stars or sailed to an uncharted land or opened a new heaven to the human spirit.” - Helen Keller
3	The Amazing Power of Starlight
4	Light and Matter
5	Atomic Structure An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it.
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7	Different Kinds of Atoms The kind of atom depends on the number of protons in the nucleus.
8	Electron Orbits Electron orbits in the electron cloud are restricted to very specific radii and energies.
9	Atomic Transitions An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy.
10	Color and Temperature
11	Black Body Radiation (I)
12	Two Laws of Black Body Radiation
13	Planck and other Formulae Planck formula gives intensity of light at each wavelength It is complicated. We’ll use two simpler formulae which can be derived from it. Wien’s law tells us what wavelength has maximum intensity Stefan-Boltzmann law tells us total radiated energy per unit area
14	Example of Wien’s law What is wavelength at which you glow? Room T = 300 K so This wavelength is about 20 times longer than what your eye can see. Camera in class operated at 7-14 μm. What is temperature of the sun – which has maximum intensity at roughly 0.5 mm?
15	Example of the Stefan-Boltzmann law Suppose a brown-out causes the temperature of a lamp filament to drop to 0.9 of its original value. By what factor does the light output of the lamp drop? Using the Stefan-Boltzmann law (with the numerical value of s) we could have calculated how big (in m²) a light filament would have to be to emit 100 W of light, at any given temperature. We could also use it to find the size of a star, if we know how much light energy that star emitted
16	The Color Index (I)
17	The Color Index (II) We define the Color Index B – V (i.e., B magnitude – V magnitude) The bluer a star appears, the smaller the color index B – V. The hotter a star is, the smaller its color index B – V.
18	Kirchhoff’s Laws of Radiation (I) A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.
19	Kirchhoff’s Laws of Radiation (II) 2. If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.
20	Kirchhoff’s Laws of Radiation (III) 3. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.
21	The Spectra of Stars
22	Lines of Hydrogen
23	The Balmer Lines
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26	The Balmer Thermometer Balmer line strength is sensitive to temperature:
27	Measuring the Temperatures of Stars
28	Spectral Classification of Stars (I)
29	Spectral Classification of Stars (II)
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32	The Composition of Stars
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34	The Doppler Effect
35	Example (I):
36	Example (II): Take l₀ of the H_a (Balmer alpha) line: l₀ = 656 nm