2. Light & Origin of the Solar System
Read: Chapters 5: “Light” Reading.
Chapter 8: First 10 pages only.
Homework Due Tomorrow (Friday).
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Last Time:
Light
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Last Time:
Light as a Wave
f is frequency
 is wavelength
For light: f  = c
c = 300,000 km/s
Our eyes recognize f (or ) as color .
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Last Time:
Light as a particle
Light as photons
Each Photon
Has an Energy:
E = hf
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6. Emission of Light by Atoms or Molecules
Last Time:
Emission of Light by Atoms or Molecules
Atoms and Molecules
have
Distinct Energy Levels
Excited atoms & molecules
change from high to low,
photons emitted
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Energy Levels of Atoms
Electron is allowed to have certain energies in an atom.
Electrons can absorb light and gain energy or emit light when they lose energy.
Hydrogen
Atom
Consider light as a photon when discussing its interaction with matter.
Only photons whose energies (colors) equal the difference in electron energy levels can be emitted or absorbed.
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8. Absorption of Light by Atoms & Molecules
Atoms absorb photons whose energies (i.e. wavelengths) match the energy difference between two levels in an atom.
The resulting spectrum has all wavelengths (all colors), but is missing wavelengths that were absorbed.
You can determine which atoms are in an object by the emission & absorption lines in the spectrum.
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9. Warm, Opaque Objects Glow by Thermal Emission of Light
Cool Warmer Hot Hotter
Red & Faint White & Bright
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10. Warm, Solid Objects Glow by Thermal Emission of Light
Cool Warmer Hot Hotter
Red & Faint White & Bright
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11. At Infrared Wavelengths
Thermal Emission
At Infrared Wavelengths
Dog
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12. Warm, Solid Objects Glow by IR Thermal Emission of Light
Brighter ==> Warmer
Human skin is an emitter of infrared “thermal” radiation with wavelengths greater 3 microns. This energy may be recorded to yield a quantitative temperature map of the skin. The skin temperatures are determined mostly by the flow of blood nearby and by the heat conducted from within the body. An image in the infrared yields information about pathological conditions within the body.
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13. Thermal Emission from the Earth
6 x 10-8 x T4
(T in degrees Kelvin)
Flux =
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14. “Thermal Emission” from Warm, Opaque Objects
1. Warm objects emit Infrared light and radio waves
Examples: Warm embers of fire, electric stove.
2. Hotter objects emit more light energy per unit surface area (per second).
(Energy emitted per sec increases as Temp4 )
3. Hotter objects emit bluer photons (with a higher average energy.)
average increases as 1/ T (using kelvin Temp scale)
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15. Warm, Solid Objects Emit Light: “Thermal Emission”
Examples:
Electric Stove
Filaments
Fireplace Coals
Light bulb filament
Warm human body
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16. “Thermal Emission” from Opaque Objects
Hotter objects emit more light energy per unit surface area per second.
Energy emitted = 6x10-8 T (Joules per m2 per sec)
Hotter objects emit bluer photons (with a higher average energy.) “Wien Law”
max = 2900 m / T (T in degrees Kelvin)
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17. Light carries information about the Planets and Stars
Key: Separate light into its different wavelengths (spectrum).
By studying the spectrum of an object, we can learn its:
Composition
Temperature
Velocity
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18. Spectrum from a Typical Planet, Comet, or Asteroid
Reflected visible light from Sun
Thermal Emission (IR)
Absorption by molecules
in gases in atmosphere
Spectrum reveals:
Chemical Composition
Temperature
Velocity
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19. . Formation of the Solar System
Section 1
Clouds of Gas and Dust in the Milky Way Galaxy
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Overall Properties of our Solar System
Circular Orbits (elliptical, but nearly circles)
All planets lie in one flat plane (the Ecliptic).
They orbit & spin in same direction (counter clockwise)
Inner Planets: small, rocky
Outer Planets: large, made of gas and ice
How did our Solar System Form ? ? ?
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21. The Origin of the Solar System
Four characteristics of our Solar System must be explained by a formation theory.
What is the basic idea behind the theory?
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Interstellar Gas and Dust in
our Milky Way Galaxy
Light absorbed
from distant stars
along mid-plane.
Dust and Gas In
Interstellar
Clouds !
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The Dark Clouds in the Milky Way
Milky Way
Centaurus A
HST
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24. The Interstellar Medium (ISM)
Dust &
Gas
The Interstellar Medium (ISM)
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25. Dark Clouds in our Galaxy
Dense gas and dust. 1% (by mass) is “rocky/icy” dust particles that could eventually make terrestrial planets.
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26. Absorptionof Light by Dust
Dust clouds: Opaque in visible (“Optical”) light.
Lower opacity in infrared.
Dust scatters visible light more efficiently than infrared ==>
To Study the Milky Way Galaxy: use IR !
Visible Light Infrared Light
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Gas Clouds contain hydrogen, helium, carbon,nitrogen, oxygen and complex molecules
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Dust is Made of Atoms
Small Dust particle:
Only a few thousand atoms
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29. Large Dust Particle: 10,000’s of Atoms!
Interstellar Dust Grain:
C, O, Si, H20 ice, Si-O.
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Basic Observation
Stars are continuously forming in the galaxy.
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31. Dense Clouds Floating in our Milky Way Galaxy
Gravity pulls atoms closer together
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32. Collapse of the Solar Nebula
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33. A Young Star Forming, surrounded by a protplanetary disk
Artists Rendering
A Young Star Forming, surrounded by a protplanetary disk
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Cloud Contracting
Due to Self-Gravity
As the nebula contracts due to its self-gravity, it heats up, spins faster, and flattens.
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35. Flattening of the Solar Nebula
As the nebula collapses, clumps of gas collide & merge.
Their random velocities average out into the nebula’s direction of rotation.
The spinning nebula assumes the shape of a disk.
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37. Protoplanetary Disks…
Star and planet formation
Protoplanetary Disks…
Solar System size
Measured Sizes: AU
Masses: 10-3 – 10-1 Msun
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38. Formation of Planetary Systems
Observations  Models of Planet Formation
Protoplanetary Disks
of Gas & Dust
Observations
Thermal Emission (Infrared) from Dust
Hubble Space Telescope
Pictures of protoplanetary disks.
 Mass of Disk = MJUP
Disk Lifetime ~ 3 Million years
Theory of
Planet Formation:
Dust collides, sticks
and grows 
pebbles/rocks
Gravity helps attract
more rocks
Gravity attracts gas
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39. Theory of Rocky Planet Formation Inward of 2 AU
Planetesimals (km-sized comets & asteroids)
Growth of rocks (planetesimals) by collisions and sticking together
Friction circularizes orbits
Big planetesimals gravitationally stir small rocks
Mergers among planetesimals: They grow to Earth-Size
Analytical and N-body:
Lissauer 1987
Rafikov 2003
Chambers, Thommes 2002
Goldreich, Lithwick, Sari 2004
Safronov 1969
Greenberg et al 1978
Wetherill & Stewart 1993
Kokubo & Ida 2000
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40. Building the Planets Inward of 3 AU
At 3 AU is “Snow Line” : Hotter than 0C
Only rocks & metals condensed inward.
Too hot for gases (H, He) to stick to rocks.
Hydrogen compounds (H2O, NH3, CH4 ) are gases.
 Only rocky planets inward of 3 AU.
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41. Building the Planets Beyond 3 AU
- Cold!
- Hydrogen compounds (ices, H2O, NH3, CH4) condensed.
- Planetesimals made of ROCK and ICE !
Gases (H, He, hydrogen compounds) gravitate to rocks:
 Form Planets made of rock, ices, and gases! Jupiter, Saturn, Uranus, Neptune
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Building the Planets
accretion -- small grains stick to one another via electromagnetic force until they are massive enough to attract via gravity to form...
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43. Building the Giant Planets
Gas-giant planets form by gravitationally attracting H and He gas. More gas acquired: More gravity. Attraction of Gas is a “runaway” ! Jupiters form their own “miniature” solar nebula.
Moons formed out of the mini-nebula.
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44. Summary: Origin of the Solar System
Theory – our Solar System formed from a giant, swirling cloud of gas & dust.
Depends on simple principles of Physics:
Dust particles collide, stick together & grow.
Law of Gravity:
gravitational attraction of particles and gas
Conservation of angular momentum to
flatten the protoplanetary disk.
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45. Theory Explains: Orderly Motions in the Solar System
The Sun formed in the center of the gas-dust protoplanetary disk.
The planets formed in the protoplanetary disk.
This explains:
all planets lie along one plane (in the disk)
all planets orbit in one direction (the spin direction of the disk)
the Sun rotates in the same direction
the planets would tend to rotate in this same direction
most moons orbit in this direction
most planetary orbits are near circular (viscous smoothing of orbits)
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46. Origin of the Solar System
Our theory explains the properties
of the Solar System
Planets in orderly motions:
circular orbits, flat plane, orbit same direction.
There are two types of planets.
small, rocky terrestrial planets
large, hydrogen-rich Jovian planets
Asteroids & comets exist in certain regions of the Solar System
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