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Light and Matter

Light and Matter. Astronomy 315 Professor Lee Carkner Lecture 6. Using Light. We want to know something about the properties of the material that makes up the star Such as: Motion. How Do Light and Matter Interact?. The properties of the photons change as this happens How?

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Light and Matter

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  1. Light and Matter Astronomy 315 Professor Lee Carkner Lecture 6

  2. Using Light • We want to know something about the properties of the material that makes up the star • Such as: • Motion

  3. How Do Light and Matter Interact? • The properties of the photons change as this happens • How? • We need to know something about atoms

  4. The Nature of Matter and its Antecedents • Protons and neutrons form the nucleus • Electrons are in orbits (or shells or levels or states) surrounding the nucleus • In a neutral atom the number of protons and electrons are equal

  5. Atoms • Atoms interact with each other (and light) through the electron shells • The most common atoms are: • Helium (2 protons, 2 neutrons, 2 electrons) • An atom can become ionized by losing one or more electrons

  6. Electron States • Each orbit has a very specific energy • e.g. An electron in a hydrogen atom cannot be anywhere, only in the permitted state

  7. Energy Levels

  8. Electron Transitions • Moving an electron from one state to another involves energy • An atom will only absorb a photon if it is at the exact energy for a level transition • Thus, any one type of atom is able to absorb photons at a only a few specific energies

  9. Absorption and Emission

  10. Absorption and Emission • Again, any atom will only emit at certain specific energies • If we examine a spectrum of emitting or absorbing atoms, we see absorption and emission lines • Emission lines are bright

  11. Emission and Absorption Lines

  12. Identifying Atoms • Atoms can be excited by radiation or collision • Each atom has its own distinct emission spectrum and can be thus identified

  13. Kirchhoff’s Laws • For a dense gas (or a solid or liquid) the atoms collide so much that they blur the lines into a continuous blackbody spectrum • e.g. a light bulb • A low density gas excited by collisions or radiation will produce an emission spectrum • e.g., an emission nebula • A low density gas in front of a source of continuous radiation will produce an absorption spectrum • e.g., a star (due to its cool outer atmosphere)

  14. Absorption + Continuum

  15. Pure Emission Spectrum

  16. Kirchhoff’s Laws

  17. The Doppler Effect • When you observe a moving object, the wavelengths of light you observe change • Moving towards -- wavelength decreases -- blue shift • The faster the motion the larger the change • By measuring the shift of lines in a spectrum, you can determine how fast the object is moving

  18. Doppler Effect

  19. Stellar Doppler Shift

  20. Spectral Line Shifts • Look at a spectral line at rest in the lab • Look a moving star and measure the shifted wavelength • The ratio of the wavelengths is the ratio of the velocity of the star (v) to the speed of light (c=3X108 m/s)) (lobs – lrest)/lrest = v/c • n.b., in calculator 3X108 is 3E8 or 3EE8

  21. Line Broadening • Doppler broadening results from the atoms being in motion so some photons are a little red shifted and some a little blue • Collisional broadening results from atom-atom collisions in the gas • A larger temperature and larger density produces more broadening

  22. Doppler Broadening

  23. How Do We Use Light to Find Stellar Properties? • Temperature: • From the Doppler broadening • Composition: • From the spectral lines compared to standards • Motions:

  24. Next Time • Read Chapter 4.5, Chapter 17.2-17.3

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