Light and Matter

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