1. Physics of Astronomy Thursday, winter week 7 (23 Feb.06)
Quiz til 1:30, then grade together
Phys.B students - please do the whole quiz
Discuss research
Math-A: Giancoli Ch. 8 – Energy
2:30 Seminar
4:00 Math-B: outstanding questions?
C&O Ch.3 – Light

2. Phys-A: Giancoli Ch.8: Energy
Review: Kinetic energy =
Potential energy
Work done = change in potential or kinetic energy
Conservation of mechanical energy: E = K + U = constant
Conservative forces have associated potential energy: F=-dU/dx

3. Gravitational potential energy and force
Near earth far from Earth
Force F
Potential energy U

4. Ch.8-8,9: Energy diagrams and Power
Power = rate of change of Energy
P = dE/dt
Minimum energy = stable state (F=0)
Ch.8 (Power, 203) #57, 59, 62, 65, 67, (Diag) 68-71, 94-97

5. Phys-B: C&O Ch.3 – Light tells us everything* about stars
Color (wavelength)  temperature, power output, absolute brightness…
Apparent vs absolute brightness  distance
Spectral lines  composition & atmosphere, stellar type and age,
Shifts in spectral lines  proper motion, rotation, magnetic fields, oscillations  internal structure, internal rotation, planets…
* (Light, plus neutrinos & gravity waves, if we’re lucky)

6. Parallax  distance  brightness
Ch.19 # 59: Animation 19.1, parallax
Starry Night: Ch.19 #61 (colored pairs)
#63: Use Starry Night to investigate the brightest stars. Turn on constellations. Which are most luminous? Which are most distant? What about six months later?
Do CO.3 # 1

7. Color  temperature, power, brightness
Color  temperature: l(m) = 3x10-3/T(K)
Temperature  Power output per unit area:
flux = intensity of radiation = F=sT4
where s = 5.67 X 10-8 W / (m2 K4)
Power output = Luminosity = L
Intensity = power / area: F= L/4pR2
CO 3.2

8. Luminosity  Magnitude  distance
Intensity = power / area: F= L/4pR2
Greater radiation flux  brighter star: F ~ b
Brightness is perceived on a logarithmic scale. Apparent magnitude difference m2-m1=Dm= 1  brightness ratio b1/b2 = 100 1/5 = 2.512
Convention: absolute magnitude M is what a star would have if it stood at a distance of d=10 pc from Earth.
CO 3.5: Find relation between distance & magnitude.
Set up CO # 3.5, 7, 8, 13

9. Light as particles and waves
E = hc/l = hn = hf = pc
Interference + diffraction: Light = wave (l,f)
Photoelectric effect: Light = particles
photons carry momentum p= h/l
Maxwell’s theory + Hertz’s experiment: EM waves

10. Energy of EM wave Electric field E has energy density uE=e0 E2/2
Magnetic field B has energy density uB=B2/2 m0
E = c B so total u = uE+ uB = e0 E2
Power/area = (Energy/volume)* speed
Intensity of EM radiation: S = cu = e0 E2 =EB/ m0
Radiation travels perpendicular to both E and B:
Do CO # Ref: Giancoli Ch.32

11. Spectra  composition, motion
Spectra can have emission and absorption lines due to quantum transitions in atoms
Spectra from moving sources are Doppler-shifted

12. Summary from Universe Ch.19: The Nature of Stars
Hotter stars are bluer
Luminosity = power emitted by star
Power = Energy/time
Energy = mc2 is released by fusion reactions
Flux = intensity of emission = power/area
F = sT4 = L/4pR2, so L~ T4R2
Hotter and bigger stars are more luminous
Brightness depends on luminosity and distance
Magnitude measures brightness logarithmically
CO.3 # 1,6,8,13