The continuous spectrum of light Astrophysics Ch.3 Physics of Astronomy, winter week 5 Star Date Light tells us everything about stars Parallax Magnitude, luminosity, flux (3.2, 3.5) Light as particles and waves Blackbody radiation (3.11) Spectra workshop: Ferguson Ex.19

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?

Luminosity  Magnitude  distance Color  temperature: (m) = 3x10 -3 /T(K) Temperature  Power output per unit area: flux = intensity of radiation = F=  T 4 Power output = Luminosity = L Intensity = power / area: F= L/4  R 2 Greater radiation flux  brighter star: F ~ b Brightness is perceived on a logarithmic scale. Apparent magnitude difference m 2 -m 1 =  m= 1  brightness ratio b 1 /b 2 = 100 1/5 = 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.

Light as particles and waves E = hc/ = h  pc Interference + diffraction: light = wave Photoelectric effect: Light particles (photons) each carry momentum p= hc/ (Giancoli Ch.38) Maxwell’s theory + Hertz’s experiment: EM waves

Energy of EM wave Electric field E has energy density u E =  0 E 2 /2 Magnetic field B has energy density u B =B 2 /2  0 E = c B so total u = u E + u B =  0 E 2 Power/area = (Energy/volume)* speed Intensity of EM radiation: S = cu =  0 E 2 =EB/  0 Radiation travels perpendicular to both E and B: Ref: Giancoli Ch.32

Blackbody radiation (CO #3.11, Giancoli Ch.38) Blackbodies were carefully studied in the lab in the late 1800s Rayleigh-Jeans theory explained long-wavelength tail: ~ 1/T (Wien’s law) Ultraviolet catastrophe at short l! Planck’s phenomenological relation fit, but why? Three weeks later, Planck’s revolutionary explanation

Spectra workshop: Ferguson Ex.19 Emission and Absorption lines can modify smooth (continuum) blackbody spectra