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

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

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

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

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)

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

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

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

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

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 #3.6 Ref: Giancoli Ch.32

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

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