1	The Nature of Light Lecture 6 Ay-1 continued
2	Electromagnetic radiation emitted by astronomical objects
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5	Kirchhoff’s Laws of Radiation Relate continuous spectra, emission line spectra, & absorption line spectra Hot opaque solid (dense gas) emits light of all wavelengths ® continuous spectrum of radiation Low density (transparent) hot gas emits spectrum of bright emission lines Low density (transparent) cool gas in front of a blackbody absorbs wavelengths from the continuous spectrum ® dark absorption line spectrum on continuous spectrum (wavelengths same as emission lines from hot gas) Spectroscopy enables determinations of chemical structure of stars
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8	e-m radiation: formation of spectral lines Quantum transitions energy levels within atomic and molecular systems quantized only discrete orbits/energies permitted when atom makes a transition between 2 states, gives up energy corresponding to the difference discrete amounts of energy only (photons) DE = E_final - E_initial = hn (h = Planck’s constant, 6.6 x 10^-²⁷ erg sec) types of transition collisional excitation/de-excitation DE = ½mv² radiative absorption + spontaneous emission; stimulated emission photon energy µ radiation frequency (color) DE = hn = hc/l e.g. for l = 7000Å, E_photon ~ 2 eV (1eV = 1.6 x 10^-12 ergs) - very low energy
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14	Measuring the temperatures of stars Continuous spectrum B-B radiation ® T* Spectral classification ® T* tightly bound atoms (e.g. H and He) can survive to 20,000K weakly bound atoms are ionized above ~ 8000K molecules survive only up to ~3000K e.g. if O, Ca, Mg lines observed, but no molecules T ~ 5000-8000K
15	Estimating the sizes of stars Recall Lµ R² x T⁴– knowing L, T ® R For Mira, L = 400 L_¤(L_¤= 3.8 x 10³³ ergs/sec) T= 2700K = 0.5T_¤ (T_¤ = 5800K) so, R = 20/0.25 = 80 R_¤ (R_¤ =6.9 x 10¹⁰ cm For Sirius B, L = 0.04 L_¤ T= 23,200K = 4T_¤ so, R = 0.2/16 = 0.01 R_¤
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17	Estimating stellar masses
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19	Estimating stellar masses cont’d
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