1. © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Lecture PowerPoint Chapter 3 Astronomy Today, 5 th edition Chaisson McMillan Last revised: 2-Feb-10

2. Chapter 3 Radiation

3. Units of Chapter 3 Information from the Skies: - We can’t travel far away yet to sample stars directly - We can study mainly light waves (photons) sent to us - We can collect some particles (cosmic rays, neutrinos) What is Light? The Wave Nature of Radiation The Electromagnetic Spectrum - Discovery of the regions beyond the visible - Atmospheric windows (visible, radio, a little UV & IR)

4. Interactions of Radiation & Matter Reflection, refraction, transmission, scattering, absorption, emission Thermal Radiation The Kelvin Temperature Scale Radiation Laws Planck equations Wien’s Law Stefan-Boltzmann Law The Doppler Effect Determining speeds of various physical motions by observed wavelength shifts

5. 3.1 Information from the Skies Electromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields Example: Light Andromeda Galaxy (M31) 2.2 million ly away

6. Interactions of Radiation & Matter Reflection, refraction, scattering, transmission, absorption, emission

7. 3.1 Information from the Skies Wave motion: transmits energy without the physical transport of material Wavelength: distance between peaks

8. 3.1 Information from the Skies Example: water wave (mechanical waves) Water just moves up and down Wave travels and can transmit energy

9. 3.1 Information from the Skies Frequency: number of wave crests that pass a given point per second Period: time between passage of successive crests Relationships: Frequency (  = 1 / Period (T)  c

10. 3.1 Information from the Skies Wavelength: distance between successive crests Velocity: speed at which crests move Relationship: Velocity = Wavelength / Period v = wavelength x frequency v =

11. 3.1 Information from the Skies Visible spectrum:

12. 3.2 Waves in What? Mechanical waves: water waves, sound waves, and so on, must travel in a medium (water, air, …) Electromagnetic waves: need no medium to travel Created by accelerating charged particles:

13. 3.2 Waves in What? Electromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field, and vice versa (J. C. Maxwell)J. C. Maxwell

14. 3.2 Waves in What? What is the wave speed of electromagnetic waves? c = 3.0 × 10 8 m/s This speed is very large, but still finite; it can take light millions or even billions of years to traverse astronomical distances Sun: 8.3 light minutes Nearest stars: (  Centauri system): 25 trillion miles = 4.2 light years (ly) Milky Way: 100,000 ly across

15. The wave nature of radiation: radiation can diffract and interfere, which are purely wave phenomena Diffraction by a circular aperture YoungYoung’s double slit experiment:experiment __________ Interference fringes Particle model

16. Discovery of the EM Spectrum William Herschel (1800) – Infrared & UltravioletHerschel Heinrich Hertz (1887-8) – radio wavesHertzwaves Wilhelm Roentgen (1895) – x-raysRoentgen Ernest Rutherford (1900) – gamma (γ) raysRutherford Cleeton & Williams (1933) - microwavesCleeton

17. 3.3 The Electromagnetic Spectrum No limit on wavelengths Different ranges have different names: Gamma X-ray Ultraviolet (UV) Visible Infrared (IR) Microwave (MW) Radio λ

18. The Atmospheric Windows rockets satellites balloons airplanes ground based For most of the time since the scientific revolution started we could only use ground based instruments in the visible region of the EM-spectrum (a little bit of IR & UV in 20 th century) Airborne, balloon & rocket astronomy became important after WW II (allowed x-ray, gamma, more IR & UV) Space astronomy began with Sputnik I in 1957 (all regions)

19. 3.4 Thermal Radiation Kelvin Temperature scale: All thermal motion ceases at 0 K Water freezes at 273 K and boils at 373 K Absolute zero °F°C °K

20. 3.4 Thermal (Blackbody) RadiationBlackbody Blackbody Spectrum: radiation emitted by an object depending only on its temperature λ max Planck’s BB Radiation Eqt. ( ) I( )

21. 3.4 Thermal Radiation Radiation Laws 1. Wien’s Law: Peak wavelength is inversely proportional to temperature max  = constant = 2.9 x 10 -3 m So, T = 2.9 x 10 -3 m λ max

22. 3.4 Thermal Radiation Radiation Laws 2. Total energy emitted is proportional to fourth power of temperature (note intensity-the area under the curves) Stefan-Boltzmann Law: E =  T 4

23. Solar-Terrestrial Spectrum Solar BB radiation Earth’s surface BB radiation 5800 K BB peak is here Atmospheric (telluric) absorptions

24. 3.5 The Doppler Effect If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer

25. 3.5 The Doppler Effect Relationship between frequency and speed:  = v    c  ( o  ( o ) ( ) (c) (v)

26. 3.5 The Doppler Effect (anim. ) Depends only on the relative motion of source and observer:

27. You’ve heard the Doppler effect …and you’ve gotten a speeding ticket from it as well as seen the weather reports using it (animation)animation

28. 3.5 The Doppler Effect Equation Relationship between frequency and speed:  = v    c  ( o  ( o ) ( ) (c) (v)

29. Doppler Shift Calculation rearrange to If we detect a Hydrogen alpha spectral line from a star or galaxy that is shifted 0.01 nm from its wavelength in the lab (656.3 nm) then we can determine how fast that object is moving towards or away from us: This is how the police determine your car speed. They bounce microwaves off your car and measure the wavelength shift & calculate your velocity.

30. Summary of Chapter 3 Waves: period, wavelength, amplitude Electromagnetic waves created by accelerating charges Visible spectrum is different wavelengths of light (ROY G BIV) Entire electromagnetic spectrum: radio waves, infrared, visible light, ultraviolet, X rays, gamma rays (in order of increasing frequency/energy) Atmospheric windows

31. Can tell the temperature of an object by measuring its blackbody radiation (Wien’s Law) Doppler effect can change perceived frequency of radiation Doppler effect depends on relative speed of source and observer Doppler effect can be used to determine speeds of rotation, stellar and galactic motions, detect extrasolar planets and more Summary of Chapter 3, cont.