1. Dust in the Orion nebula: Opaque to visible light, dust is created in the outer atmosphere of massive cool stars and expelled by a strong outer wind of particles. Dust reflects starlight and emits continuous radiation in the far infrared.

2. Exam #1 Wednesday, February 15
Homework #3
Multiple choice questions due Friday, February 10, 5:00 pm.
Short answer questions due Monday, February 13, 2:30 pm
Exam #1 Wednesday, February 15

3. Radiative Energy: E = hc /  or E = hf
 is the wavelength f is the frequency c is the speed of light h is the Planck Constant
Shorter wavelength light carries more energy per photon.

4. Light as Information Bearer
Spectrum: light separated into its different wavelengths.
Spectroscopy: The quantitative analysis of spectra
The spectrum of an object can reveal the object’s:
Composition
Temperature
Velocity

5. Four Ways in Which Light can Interact with Matter
emission – matter releases energy as light
absorption – matter takes energy from light
transmission – matter allows light to pass through it
reflection – matter repels light in another direction
The type of interaction is determined by characteristics of the “matter” and the wavelength of light.

6. Three ways in which spectra manifest themselves:
Continuous spectra
Absorption spectra
Emission line spectra

7. Continuous spectra are usually related to the temperature of an object that is emitting radiation.
Absorption & emission line spectra are related to the composition of the material absorbing or emitting radiation.

8. Kirchhoff’s Laws
1. A hot, dense glowing object (solid or gas) emits a continuous spectrum.

9. Kirchhoff’s Laws
2. A hot, low density gas emits light of only certain wavelengths – an emission line spectrum.

10. Kirchhoff’s Laws
3. When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum – an absorption line spectrum.

12. Rules for Thermal Emission by Opaque Objects
Hotter objects emit more total radiation per unit surface area.
Hotter objects have their peak radiation at shorter wavelengths (they will appear “bluer”)

13. The sun emits its peak radiation in the yellow portion of the visible spectrum At “room temperature”, or “body-temperature”, an object emits its peak radiation in the infrared.

14. Now, let’s apply these rules.
Reiterating…
Hotter objects emit more total radiation per unit surface area.
Hotter objects have their peak radiation at shorter wavelengths (they will appear “bluer”)
Now, let’s apply these rules.

15. Which of the two stars (A or B) emits light that has a peak emission with the longer wavelength?
(red) Star A
(blue) Star B
(green) The stars’ peak emissions are at the same wavelength
(yellow) None of the above
visible range A
Energy output per second B
VIBGYOR
Wavelength

16. Which of the two stars (A or B) emits light that has a peak emission with the longer wavelength?
(red) Star A
(blue) Star B
(green) The stars’ peak emissions are at the same wavelength
(yellow) None of the above
visible range A
Energy output per second B
VIBGYOR
Wavelength

17. Which of the two stars (A or B) would appear red?
(red) Star A
(blue) Star B
(green) Neither would appear red
(yellow) There is insufficient information to determine the star’s color
visible range A
Energy output per second B
VIBGYOR
Wavelength

18. Which of the two stars (A or B) would appear red?
(red) Star A
(blue) Star B
(green) Neither would appear red
(yellow) There is insufficient information to determine the star’s color
visible range A
Energy output per second B
VIBGYOR
Wavelength

19. The figure shows the spectra of two stars. Which star is hotter
The figure shows the spectra of two stars. Which star is hotter? (red) A (blue) C (yellow) neither
visible range A
Energy output per second C
VIBGYOR
Wavelength

20. The figure shows the spectra of two stars. Which star is hotter
The figure shows the spectra of two stars. Which star is hotter? (red) A (blue) C (yellow) neither
visible range A
Energy output per second C
VIBGYOR
Wavelength

21. Which of the following is possible to infer about stars A and C based upon the information provided in the graph?
(red) Star A is smaller than star C
(blue) Star A is larger than star C
(green) The stars are the same size
(yellow) It is not possible to infer any of these relationships
visible range A
Energy output per second C
VIBGYOR
Wavelength

22. Which of the following is possible to infer about stars A and C based upon the information provided in the graph?
(red) Star A is smaller than star C
(blue) Star A is larger than star C
(green) The stars are the same size
(yellow) It is not possible to infer any of these relationships
visible range A
Energy output per second C
VIBGYOR
Wavelength

23. Electron Energy Levels
Electrons cannot have just any energy while orbiting the nucleus.
Only certain energy values are allowed (like the floors of an aprtment building).
Electrons may only gain or lose certain specific amounts of energy (equal to differences in energy levels).

24. This diagram depicts the energy levels of Hydrogen.
Each element (atom and ion) has its own distinctive set or pattern of energy levels.
This diagram depicts the energy levels of Hydrogen.
1 eV = 1.60 x joules

25. How can electrons absorb or emit energy?
By absorbing or emitting light
Through collisions of parent atom with another atom

26. Emission/Absorption Spectra
Each electron is only allowed to have certain energies in an atom.
Electrons can absorb light and gain energy or emit light when they lose energy.
Hydrogen
Only photons whose energies (colors) match the “jump” in electron energy levels can be emitted or absorbed.

28. Absorption Spectra
If light shines through a gas, each element will absorb those photons whose energy match their electron energy levels.
The resulting absorption line spectrum has all colors minus those that were absorbed.
We can determine which elements are present in an object by identifying emission & absorption lines.

29. Group Activity
A look at different types of spectra, as predicted by Kirchhoff’s Laws
Be sure to put your group name on the paper!!!

30. A. The white dwarf star (a thermal radiator) in the center of the nebula.
B. A distant star (that is much hotter than the gas) viewed through the cold gas expelled by the dying star.
C. An empty, dark region of space.
D. The diffuse gas expelled by the dying star seen against the dark background of space.
E. What type of element(s) do you expect to see in some of these spectra? Why?
What kind of spectrum is seen at each location depicted below? Explain.