2. The Cosmic Messenger

3. Electric and magnetic fields oscillate together, but perpendicular to each other and the electromagnetic wave moves in a direction perpendicular to both of the fields. Light is a electromagnetic wave.

4. Light as a Wave Wavelengths of light are measured in units of nanometers (nm) 1 nm = 10 -9 m or Ångström (Å): 1 Å = 10 -10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å or 400 nm and 700 nm. The abbreviation used for wavelength is the Greek letter lambda.

5. Light as a Wave Light waves are characterized by a wavelength and a frequency f. f and are related through f = c/ c = 300,000 km/s = 3*10 8 m/s

6. Properties of Light –However, the speed of light is reduced as it traverses transparent materials and the speed is also dependent on color Introduction Light is radiant energy: it does not require a medium for travel Light is an electromagnetic wave –Light travels at 299,792,458 km/s in a vacuum (fast enough to circle the Earth 7.5 times in one second)

7. The Nature of Light –Newton Early discoveries –Newton discovered that white light passing through a prism is comprised of a spectrum of colors. Newton said, “Light is made of very small particles” Huygens: “It’s wave-like”

8. Visible light is composed of the colors of the rainbow. Each color is a different wavelength of light. Red is at the long wavelength end and violet is at the short wavelength end. THE VISIBLE ELECTROMAGNETIC SPECTRUM

9. Photon energy Note the trends: bluer light has shorter, higher f, and more energy. Redder light has longer, lower f, and less energy.

10. The electromagnetic spectrum. 4000 A 5000 A 6000 A 7000 A

11. Which Rays get through our atmosphere?

12. The Nature of Light –Light travels at a finite speed, c, not instantaneously. In 1675 Romer measured the delay in Jupiter’s moon eclipses It takes 8 minutes for Sunlight to reach Earth, and 0.13 seconds for light to go around the world Or

13. h = 6.626x10 -34 Js h is Planck constant. The energy of a photon does not depend on the intensity of the light!!! Energy depends on its frequency (color) Light can also particles as particles, called photons. A photon has a specific energy E, proportional to the frequency f:

14. Light can also behave as a particle and a wave at the same time. An example of light acting as both a particle and a wave is the digital camera---the lens refracts (bends and focuses) waves of light that hit a charge- coupled device (CCD). The photons kick electrons out of the silicon in the CCD.

15. Young’s Double slit experiment, showing light acts as waves.

16. –Waves compressed with source moving toward you; s ound pitch is higher. –Waves are stretched with source moving away from you; sound pitch is lower. The Doppler Effect (Sound)

17. Doppler effect similar in light and sound Knowing the rest position and the position due to motion of a spectral line allows us to calculate the speed of the object. The size of the shift gives the speed. and so, Z is the Doppler shift, sometimes called the red shift.

18. Four Ways in Which Light can Interact with Matter 1.Emission – matter releases energy as light 2.Absorption – matter takes energy from light 3.Transmission – matter allows light to pass through it 4.Reflection – matter repels light in another direction Light is a form of energy!

19. Spectra Lines

20. There are three laws, known as Kirchhoff's laws, that govern the spectra and allows us interpret the spectra we observe. 1.A hot solid, liquid or gas at high pressure has a continuous spectrum. Example : the Sun and a light bulb There is energy at all wavelengths

21. 2. A gas at low pressure and high temperature will produce emission lines. There is energy only at specific wavelengths. A closed tube containing a gas heated to a high temperature

22. 3. A gas at low pressure in front of a hot continuum causes absorption lines. Dark lines appear on the continuum.

23. As illustrated below, Kirchhoff's laws refer to three types of spectra: continuum, emission line, and absorption line. Thus when we see a spectrum we can tell what type of source we are seeing. Low Pressure

24. Types of spectra: (A) continuous, (B) emission-line, and (C) absorption-line.

25. Stars have a continuous and absorption spectrum Gases around the Sun absorb photons

26. Hydrogen Helium Oxygen Neon Iron Emission Spectrum caused by heating up a gas under low pressure to a high temperature.

27. Astronomers have to find elements among the spectrum of many elements combined.

28. 7000 A 4000 A 700 nm400 nm

30. Red Shift

31. Compare these spectra. Spectrum of Hydrogen in Lab Spectrum a Star What do these spectra tell us about the star?

32. Compare these spectra. Spectrum of Hydrogen in Lab Spectrum a Star’s spectral Hydrogen What do these spectra tell us about the star?

33. Compare these spectra. What do these spectra tell us about the object? Hydrogen Star

34. Compare these spectra. Spectrum of Hydrogen in Lab Spectrum a Star…..Day 1 What do these spectra tell us about the star? Spectrum a Star…..Day 2 Spectrum a Star…..Day 3 Spectrum a Star…..Day 4

35. Stars are different colors due to their temperature. Composition has no effect on the color because the stars are all made up mostly of the same elements with a small amount of difference. Blue white are the hottest and red stars are the coolest. The temperature of a star can be determined by the highest point in its trace spectrum. ORION

36. Hydrogen Continuum Absorption Lines AA spectrum can be converted to a trace spectrum.

37. Continuum & Lines Real stars usually have a blackbody-like continuous spectrum, upon which absorption lines are superimposed

38. Spectral Types Annie Cannon classified stars according to the appearance of their spectra. The star were classified according to the strength of the hydrogen absorption lines in the sequence A, B, C….P. These spectral classes were changed to a temperature ordered sequence and some were discarded. This left the following O-B-A- F-G-K-M. Subclasses: …O8, O9, B0, B1, B2…

39. Oh, Be A Fine Girl (Guy) Kiss Me O B A F G K M 50,000 K 3,000 K Sun(G2) A5K7 Stellar Spectroscopy is the study of the properties of stars by measuring absorption line strengths. O B A F G K M Hottest Coolest

40. The Spectral Sequence BluestReddest Spectral Sequence is a Temperature Sequence HottestCoolest 50,000K1300K O B A F G K M L

41. O Stars B Stars T = 11,000 - 30,000 K; Strong He lines; very weak H lines A Stars Hottest Stars: T>30,000 K; Strong He + lines; no H lines T = 7500 - 11,000 K; Strongest H lines, Weak Ca + lines. F Stars T = 5900 - 7500 K; H grows weaker Ca + grows stronger, weak metals begin to emerge.

42. G Stars T = 5200 - 5900 K; Strong Ca +, Fe + and other metals dominate, K Stars T = 3900 - 5200 K; Strong metal lines, molecular bands begin to appear M Stars T = 2500 - 3900 K; strong molecular absorption bands particularly of TiO