1. A105 Stars and Galaxies  Homework 4 due Sept. 21  Telescopes  Read units 26, 27  News Quiz Tuesday Today’s APODAPOD ROOFTOP TONIGHT 8:30

2. OBSERVING OPPORTUNITIES Rooftop session TONIGHT at 8:30 PM –SWAIN WEST roof – take the elevator to the 3 rd floor and follow the signs Remote Observing Friday and Saturday evenings at 10 PM –Swain West 403 – take the elevator to the 3 rd floor and follow the signs.

3. REVIEW THE E.M. SPECTRUM

4. Where does light come from?  Thermal radiators emit light at all wavelengths  Atomic emission occurs only at particular wavelengths THERMAL EMISSION ATOMIC EMISSION

5. Understanding Atomic Emission Atom Nucleus Electron Cloud What is the structure of matter?

6. The Nucleus Electrons

7. Atomic Terminology Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons

8. Atomic Structure  Electrons may have only certain, discrete energies  We can think of those energies as only certain allowed distances from the nucleus

9. Atomic Emission (and Absorption) Light is emitted or absorbed when electrons jump between energy levels in an atom

10. How is energy stored in atoms? Electrons in atoms are restricted to particular energy levels Ground State Excited States

11. Energy Level Transitions The only allowed changes in energy are those corresponding to a transition between energy levels AllowedNot Allowed

12. Atoms and Light Hands-On Atom

13. Three types of spectra  Continuous – Thermal Radiator  Emission – from hot gas  Absorption – continuous spectrum passes through cooler gas

14. Continuous Spectrum The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption

15. Emission Line Spectrum A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines

16. Absorption Line Spectrum A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum

17. Chemical Fingerprints Each type of atom has a unique set of energy levels Each transition corresponds to a unique photon energy, frequency, and wavelength Energy levels of Hydrogen

18. Chemical Fingerprints Each type of atom has a unique spectral fingerprint

19. Is It Hydrogen or Helium? Hydrogen Helium

20. Which letter(s) labels absorption lines? ABCDE Reviewing Spectra

21. Which letter(s) labels emission lines? ABCDE Reviewing Spectra

22. How do light and matter interact?  Emission  Absorption  Transmission  Transparent objects transmit light  Opaque objects block (absorb) light  Reflection or Scattering

23. Reflection and Scattering in Nature The moon reflects sunlight. Moonlight scatters off the fog and reflects off the lake Dust near the stars of the Pleiades scatters starlight

24. Why is a rose red? a)The rose absorbs red light. b)The rose transmits red light. c)The rose emits red light. d)The rose reflects red light. Checking Understanding….

25. The Doppler Shift Police Siren Doppler Pumpkin Johan Christian Doppler

26. The Doppler Effect How does light tell us the speed of a distant object? How does light tell us the rotation rate of an object? Doppler Principles (Police Traffic Radar Handbook) The Doppler Shift of Sound Waves What's Happening?

27. The Doppler Shift in Astrophysical Situations Emission from a moving cloud of gas Absorption by a moving cloud of gas Identifying shifted spectral lines

28. The Doppler Shift for Light Astronomers us the Doppler effect to measure the “radial” velocities of astronomical objects Radial velocities are motions toward or away from us

29. Measuring the Shift Stationary Moving Away Away Faster Moving Toward Toward Faster Doppler shift tells us ONLY about the part of an object’s motion toward or away from us The amount of blue or red shift tells us an object’s speed toward or away from us:

30. The Doppler Shift in YOUR Life

31.  Read units 26, 27  News Quiz on Tuesday  Telescope homework (#4) worksheet due Thursday