The Nature of Light

Lecture 6 Ay-1

continued

Electromagnetic radiation emitted by astronomical objects

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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Slide 7

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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Slide 13

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

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_¤


	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


	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



	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


	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_¤