1. Week 8 Day 1 Announcements
Grades
First iClicker scores have posted (from 4 classes)
Participation scores will be up to date after Spring Break
Homework:
Homework assignments for Chs 4 & 5 will post tomorrow
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Extensions
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2. Question 1
a) wavelength
b) frequency
c) period
d) amplitude
e) energy
The distance between successive wave crests defines the ________ of a wave.
Answer: a

3. Question 1
a) wavelength
b) frequency
c) period
d) amplitude
e) energy
The distance between successive wave crests defines the ________ of a wave.
Light can range from short-wavelength gamma rays to long-wavelength radio waves.

4. Question 2
a) radius.
b) mass.
c) magnetic field.
d) temperature.
e) direction of motion.
The frequency at which a star’s intensity is greatest depends directly on its
Answer: d

5. Question 2
a) radius.
b) mass.
c) magnetic field.
d) temperature.
e) direction of motion.
The frequency at which a star’s intensity is greatest depends directly on its
Wien’s Law means that hotter stars produce much more high- frequency light.

6. Question 3
Rigel appears as a bright bluish star, whereas Betelgeuse appears as a bright reddish star.
Rigel is ______ Betelgeuse.
a) cooler than
b) the same temperature as
c) older than
d) hotter than
e) more massive than
Betelgeuse
Answer: d
The constellation ORION
Rigel

7. Question 3
Rigel appears as a bright bluish star, whereas Betelgeuse appears as a bright reddish star.
Rigel is ______ Betelgeuse.
a) cooler than
b) the same temperature as
c) older than
d) hotter than
e) more massive than
Betelgeuse
Hotter stars look bluer in color; cooler stars look redder.
The constellation ORION
Rigel

8. Question 4 If a light source is approaching you, you will observe
a) its spectral lines are redshifted.
b) the light is much brighter.
c) its spectral lines are shorter in wavelength.
d) the amplitude of its waves has increased.
e) its photons have increased in speed.
If a light source is approaching you, you will observe
Answer: c

9. Question 4 If a light source is approaching you, you will observe
a) its spectral lines are redshifted.
b) the light is much brighter.
c) its spectral lines are shorter in wavelength.
d) the amplitude of its waves has increased.
e) its photons have increased in speed.
If a light source is approaching you, you will observe
The Doppler Shift explains that wavelengths from sources approaching us are blueshifted.

10. Clicker Question: Why is the sky blue?
A: Molecules in the atmosphere scatter red light more than blue light.
B: Molecules in the atmosphere scatter blue light more than red light.
C: Molecules in the atmosphere absorb the red light
D: The sky reflects the color of the oceans.

11. Types of Spectra
1. "Continuous" spectrum - radiation over a broad range of wavelengths
(light: bright at every color).
2. "Emission line" spectrum - bright at specific wavelengths only.
3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines.

12. Kirchhoff's Laws
1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum.
2. A transparent hot gas produces an emission line spectrum.
3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum.

13. Sodium emission and absorption spectra
The pattern of emission (or absorption) lines is a fingerprint of the element in the gas (such as hydrogen, neon, etc.)
For a given element, emission and absorption lines occur at the same wavlengths.
Sodium emission and absorption spectra

14. The Particle Nature of Light
On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons. c
photon energy is proportional toradiation frequency:
E  (or E   )
example: ultraviolet photons are more harmful than visible photons. 1 

15. The Nature of Atoms The Bohr model of the Hydrogen atom:
electron _ _ + +
proton
"ground state"
an "excited state"
Ground state is the lowest energy state. Atom must gain energy to move to an excited state. It must absorb a photon or collide with another atom.

16. But, only certain energies (or orbits) are allowed:_ _ _
a few energy levels of H atom +
The atom can only absorb photons with exactly the right energy to boost the electron to one of its higher levels.
(photon energy α frequency)

17. When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits a photon - in a random direction

18. Other elements Helium Carbon
neutron proton
Atoms have equal positive and negative charge. Each element has its own allowed energy levels and thus its own spectrum.

19. So why absorption lines?. . . . .
cloud of gas . . . . . .
The green photons (say) get absorbed by the atoms. They are emitted again in random directions. Photons of other wavelengths go through. Get dark absorption line at green part of spectrum.

20. Why emission lines?
hot cloud of gas . . . . . .
- Collisions excite atoms: an electron moves into a higher energy level
- Then electron drops back to lower level
- Photons at specific frequencies emitted.

21. Ionization Two atoms colliding can also lead to ionization. Hydrogen _+
Energetic UV Photon _
Helium + +
Energetic UV Photon _
Atom
"Ion"
Two atoms colliding can also lead to ionization.

22. Clicker Question:
Astronomers analyze spectra from astrophysical objects to learn about:
A: Composition (what they are made of)
B: Temperature
C: line-of-sight velocity
D: Gas pressures
E: All of the above

23. Clicker Question: Ionized Helium consists of two neutrons and:
A: two protons in the nucleus and 1 orbiting electron
B: two protons in the nucleus and 2 orbiting electrons
C: one proton in the nucleus and 1 orbiting electron
D: one proton in the nucleus and 2 orbiting electrons
E: two protons in the nucleus and 3 orbiting electrons

24. Clicker Question: Why is the sky blue?
A: Molecules in the atmosphere scatter red light more than blue light.
B: Molecules in the atmosphere scatter blue light more than red light.
C: Molecules in the atmosphere absorb the red light
D: The sky reflects the color of the oceans.

25. Stellar Spectra
Spectra of stars are different mainly due to temperature and composition differences.
'Atmosphere', atoms and ions absorb specific wavelengths of the black-body spectrum
Interior, hot and dense, fusion generates radiation with black-body spectrum
Star

26. We've used spectra to find planets around other stars.

27. Star wobbling due to gravity of planet causes small Doppler shift of its absorption lines.
Amount of shift depends on velocity of wobble. Also know period of wobble. This is enough to constrain the mass and orbit of the planet.

28. As of today ~400 extrasolar planets known
As of today ~400 extrasolar planets known. Here are the first few discovered.

30. Molecules Two or more atoms joined together.
They occur in atmospheres of cooler stars, cold clouds of gas, planets.
Examples
H2 = H + H
CO = C + O
CO2 = C + O + O
NH3 = N + H + H + H (ammonia)
CH4 = C + H + H + H + H (methane)
They have
- electron energy levels (like atoms)
- rotational energy levels
- vibrational energy levels

31. Searching for Habitable planets around other stars

32. Molecule vibration and rotation

33. The Solar System Chapter 4 Ingredients? The Sun Planets
Moons and Rings
Comets
Asteroids (size > 100 m)
Meteoroids (size < 100 m)
Kuiper Belt
Oort cloud
Zodiacal dust
A lot of nearly empty space

34. Solar System Perspective
 ,000 light-years
< >
 light-hours
< >
Zoom out 220 million times --->
Artist’s view or our Galaxy

35. Orbits of Planets All orbit in same direction.
Most orbit in same plane.
Elliptical orbits, but low eccentricity for most, so nearly circular.

36. Exceptions:
Mercury Pluto
(no longer a planet)
orbital tilt 7o orbital tilt o
eccentricity eccentricity 0.25
(Earth: orbit eccentricity 0.016:
nearly circular orbit)

37. Sun, Planets,our Moon and Pluto to scale (mostly)
Mistakes: Jupiter should have rings
Pluto should be smaller than Moon