Transcript 6. Light: The Cosmic Messenger

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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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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Light as a particle
Light as photons
Each Photon
Has an Energy:
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E = hf
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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
Hydrogen
Atom
• Electron is allowed to have
certain energies in an atom.
• Electrons can absorb light
and gain energy or emit
light when they lose energy.
• 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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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.© 2005 Pearson
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Warm, Opaque Objects Glow by
Thermal Emission of Light
Cool
Red & Faint
Warmer
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Hot
Hotter
White & Bright
Warm, Solid Objects Glow by
Thermal Emission of Light
Cool
Red & Faint
Warmer
Hot
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Hotter
White & Bright
Thermal Emission
At Infrared Wavelengths
Dog
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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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Thermal Emission from the Earth
-8
4
Flux = 6 x 10 x T
(T in degrees Kelvin)
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“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
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(using kelvin Temp scale)
Warm, Solid Objects Emit Light:
“Thermal Emission”
Examples:
•Electric Stove
Filaments
•Fireplace Coals
•Light bulb filament
•Warm human body
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“Thermal Emission” from
Opaque Objects
1. Hotter objects emit more light energy per
unit surface area per second.
Energy emitted =
4
-8
6x10 T
(Joules per m2 per sec)
2. Hotter objects emit bluer photons (with a higher
average energy.) “Wien Law”
max = 2900 m / T
(T in degrees Kelvin)
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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:
1 Composition
2 Temperature
3 Velocity
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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:
1 Chemical Composition
2 Temperature
3 Velocity
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.
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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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
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HST
The
Interstellar
Medium
(ISM)
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Dust &
Gas
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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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
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Infrared Light
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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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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Dense Clouds
Floating in our Milky Way Galaxy
Gravity pulls atoms closer together
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Collapse of the Solar Nebula
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Artists Rendering
A Young Star Forming,
surrounded by a protplanetary disk
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As the nebula
contracts due to its
self-gravity, it heats
up, spins faster, and
flattens.
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Cloud Contracting
Due to Self-Gravity
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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Star and planet formation
Protoplanetary Disks…
Solar System size
Measured Sizes: 100-1000 AU
Masses: 10-3 – 10-1 Msun
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Formation of Planetary Systems
Observations

Models
of
Planet
Formation
Protoplanetary Disks
of Gas & Dust
Observations

Thermal Emission (Infrared)
fromTheory
Dust
of
Planet Formation:



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 Dust
sticks
Hubble
Spacecollides,
Telescope
Picturesand
of protoplanetary
grows  disks.
pebbles/rocks
 Gravity helps attract
 Mass of Disk = 10-100 MJUP
more rocks
DiskLifetime
~ 3attracts
Million years
Gravity
gas
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:
Safronov 1969
Greenberg et al 1978
Wetherill & Stewart 1993
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Lissauer 1987
Rafikov 2003
Chambers, Thommes 2002
Goldreich, Lithwick, Sari 2004
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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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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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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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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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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Origin of the Solar System
Our theory explains the properties
of the Solar System
1. 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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