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Light and Atoms

Light and Atoms. 0. Light and Matter. Spectra of stars are more complicated than pure blackbody spectra . → characteristic lines , called Fraunhofer ( absorption) lines . → atomic structure and the interactions between light and atoms . 0. Atomic Structure.

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Light and Atoms

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  1. Light and Atoms

  2. 0 Light and Matter Spectra of stars are more complicated than pure blackbody spectra. → characteristic lines, called Fraunhofer (absorption) lines. → atomic structure and the interactions between light and atoms.

  3. 0 Atomic Structure • An atom consists of an atomic nucleus(protons and neutrons) and a cloud of electrons surrounding it. • Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud.

  4. 0 If you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, how much would you guess this would weigh? • 2 kg • 2 tons • 2,000 tons • 2 million tons • 2 billion tons

  5. 0 Different Kinds of Atoms • The kind of atom depends on the number of protonsin the nucleus. Different numbers of neutrons↔ different isotopes • Most abundant:Hydrogen (H), with one proton (+ 1 electron). • Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.). Helium 4

  6. 0 Electron Orbits • Electron orbits in the electron cloud are restricted to very specific radii and energies. • These characteristic electron energies are different for each individual element. Larger orbital radus r  Higher electron energy r3, E3 r2, E2 => E3 > E2 > E1 Orbit 1: “Ground State” r1, E1

  7. 0 Atomic Transitions • An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy. Eph = E3 – E1 Eph = E4 – E1 Wrong energy • The photon is absorbed, and the electron is in an excited state. (Remember that Eph = h*f) • All other photonspass by the atomunabsorbed.

  8. 0 Which one of the three photons has the highest frequency (i.e., the highest energy)? D: They all have the same frequency. C A B

  9. 0 For veryhigh photon energy( high frequency; short wavelength), anelectron can bekicked outof the atom completely. => Photoionization

  10. 0 Absorption spectra • Only light atvery specific frequencies (energies) is absorbed. • Light at allother frequenciespasses through. Animation

  11. 0 This is causing the typicalabsorption spectraof stars.

  12. 0 Analyzing absorption spectra • Each element produces a specific set of absorption(and emission)lines. • Comparing the relative strengths of these sets of lines, we can study the compositionof gases. By far the most abundant elements in the Universe animation

  13. 0 The Balmer Lines Transitions from 2nd to higher levels of hydrogen n = 1 n = 4 n = 5 n = 3 n = 2 Ha Hb Hg The only hydrogen lines in the visible wavelength range. 2nd to 3rd level = Ha (Balmer alpha line) 2nd to 4th level = Hb (Balmer beta line) …

  14. 0 The Cocoon Nebula (Ha emission)

  15. 0 Knowing that theHaline is red, whatcolorwould you expect the Hb line to have? • Infrared • Red • Blue/Green • Violet • Ultraviolet

  16. 0 Spectral Classification of Stars (I) Temperature

  17. 0 Spectral Classification of Stars (II) Mnemonics to remember the spectral sequence:

  18. 0 If a star is moving towards us with a velocity of 30,000 km/s, we will see its light approaching us with a velocity of … and its color … • 330,000 km/s; unchanged. • 300,000 km/s; unchanged. • 330,000 km/s; shifted towards the blue end of the spectrum. • 300,000 km/s; shifted towards the blue end of the spectrum. • 300,000 km/s; shifted towards the red end of the spectrum.

  19. 0 The Doppler Effect The light of a moving source is blue/redshifted by Dl/l0 = vr/c l0 = actual wavelength emitted by the source Dl = Wavelength change due to Doppler effect vr = radial velocity Blue Shift (to higher frequencies) Red Shift (to lower frequencies) vr animation

  20. 0 The Doppler effect allows us to measure the source’s radial velocity. vr

  21. 0 Example: Take l0 of the Ha (Balmer alpha) line: l0 = 658 nm Assume, we observe a star’s spectrum with the Ha line at l = 660 nm. Then, Dl = 2 nm. We find Dl/l0 = 0.003 = 3*10-3 Thus, vr/c = 0.003, or vr = 0.003*300,000 km/s = 900 km/s. The line is red shifted, so the star is receding from us with a radial velocity of 900 km/s.

  22. 0 Doppler Broadening In principle, line absorption should only affect a very unique wavelength. In reality, also slightly different wavelengths are absorbed. ↔ Lines have afinite width; we say: they arebroadened. Blue shifted abs. Red shifted abs. One reason for broadening: The Doppler effect! vr Observer vr Atoms in random thermal motion

  23. 0 Line Broadening Higher Temperatures • Higher thermal velocities  broader lines Doppler Broadening is usually the most important broadening mechanism. Other line broadening mechanisms: • Pressure Broadening (density diagnostic) • Natural Broadening

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