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Chapter 3: Spectral lines in stars

Chapter 3: Spectral lines in stars. Emission and absorption of light. Continuous spectrum (thermal, blackbody). Emission line spectrum. Independent of composition. Dependent on composition. Each element has its own unique spectrum. Absorption lines in the Sun’s spectrum.

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Chapter 3: Spectral lines in stars

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  1. Chapter 3: Spectral lines in stars

  2. Emission and absorption of light Continuous spectrum (thermal, blackbody) Emission line spectrum Independent of composition Dependent on composition

  3. Each element has its own unique spectrum

  4. Absorption lines in the Sun’s spectrum

  5. Gustav Kirchhoff (1824-1887) Absorption Line Spectrum

  6. absorption lines of hydrogen

  7. Emission Line Spectrum • Produced by a low-density gas • depends on composition and temperature

  8. Emission lines

  9. The Balmer series for hydrogen: Visible light electrons falling to n=2 Rydberg formula (Balmer for nf = 2) : 1 /  = R (1/nf2 - 1/ni2) R = Rydberg constant = 1.097 x 107 m-1

  10. The hydrogen atom

  11. Energy levels and transitions of the many-electron atom: Sodium Quantum states of the valence electron

  12. The Bohr Model • Classical physics predicts that the electron should spiral into the nucleus • Cannot explain emission spectra

  13. The Bohr model: • The e- stays in certain stable orbits, emits no radiation unless it jumps to a lower level • The angular momentum of the e- is quantized • the attaction between p and e- provides the centripetal acceleration n = principal quantum number

  14. 1 q1 q2 F = r2 40 From Coulomb’s law, the force between the proton and electron is Where q1 = q2 = e for the hydrogen atom This is the centripetal force, mv2 / r

  15. -1 e2 r 40 PE of the electron in the nth level: Un = So when the electron is in any energy level n: Bohr radius a0 = 0h2 / me2 = 5.29 x 10-11 m KE of the electron in the nth level: Kn = 1/2 mv2 Total energy En = Kn + Un = ??? Compare to Rydberg formula!

  16. m1 m2 mr = m1 + m2 Reduced mass: the nucleus is not infinite in mass, Bohr model is off by 0.1% isotopes

  17. Ionized Helium is also a 1-electron atom

  18. Why is the emission spectrum of ionized helium similar to that of hydrogen? Because hydrogen and helium are similar chemically Because several of the energy levels of hydrogen and helium are the same Because hydrogen and helium have similar atomic masses It is a total coincidence

  19. Energy Level Transitions • Continuum • Ionization • Differences between elements • isotopes Not Allowed Allowed

  20. Stellar classification scheme

  21. Is this star hotter or cooler than the Sun?

  22. Spectrum of Arcturus

  23. The Hertzsprung-Russell diagram plots the luminosity vs. temperature of stars Luminosity Temperature

  24. B - V is a measure of color: the smaller B-V, the hotter the star (magnitudes, remember!)

  25. Lines in a star’s spectrum correspond to a spectral type that reveals its temperature (Hottest) O B A F G K M (Coolest)

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