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Transcript of © 2005 Pearson Education Inc., publishing as Addison-Wesley The Planets Lecture 9 Geoff Marcy Light...
© 2005 Pearson Education Inc., publishing as Addison-Wesley
The Planets
Lecture 9
Geoff Marcy
Light&
Origin of the Solar System
© 2005 Pearson Education Inc., publishing as Addison-Wesley
• Read Chapter 5 : “Light”
• Homework: MasteringAstronomy
Assignment Chapter 5
Due Friday
• Midterm: • - Next Tuesday, Sept. 30 - Covers Chapters 1-5 - 30 Multiple Choice
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Light as a Wave
• f is frequency• is wavelength
• For light: f = c
c = speed of light
= 300,000 km/s• Our eyes recognize
f (or ) as color .
Last Time:
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Light as photons
Light as a particle . . . .
E = hf
Each Photon has an Energy:
Last Time:
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Energy Levels of Atoms• Electron can only have specific
energies in an atom.• Electrons can absorb energy,
from collisions or light.• The energy “excites” them
to higher energy levels.• Then they emit light when they
lose energy (drop to lower energy level).
• Only photons whose energies (colors) equal the difference in electron energy levels can be emitted or absorbed.
HydrogenAtom
Absorption of Light by Atoms & Molecules
• Atoms absorb photons whose energies (i.e. wavelengths) match
the energy difference
between the current energy level
and a higher energy level.
• The resulting spectrum has
all wavelengths (all colors),
but is missing wavelengths
that were absorbed.
• You can determine what types of atoms are in an object
by the emission & absorption lines in the spectrum.
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Warm, Opaque Objects Glow by
Thermal Emission of Light
Cool Warmer Hot HotterRed & Faint White & Bright
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Warm, Solid Objects Glow by
Thermal Emission of Light
Cool Warmer Hot HotterRed & Faint White & Bright
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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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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),
proportional to Temperature 4:
“Thermal Emission” from Warm, Opaque Objects
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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).
3. Hotter objects emit bluer photons (having shorter wavelength, and higher average energy.)
average increases as 1/ T (using kelvin Temp scale)
“Thermal Emission” from Warm, Opaque Objects
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Warm, Solid Objects Emit Light:“Thermal Emission”
Examples:
• Electric Stove Filaments
• Fireplace Coals
• Light bulb filament
• Warm human body
© 2005 Pearson Education Inc., publishing as Addison-Wesley
1. Hotter objects emit more light energy per unit surface area per second. Energy emitted = 6x10-8 T4
(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)
Summary
“Thermal Emission” from Opaque Objects
© 2005 Pearson Education Inc., publishing as Addison-Wesley
“Spectrum” of Light carries information
about the Planets and Stars
By studying the spectrum of an object, we can learn its:1 Chemical Composition2 Temperature (from thermal emission intensity and wavelength.3 Velocity (from Doppler effect)
Separate light into its different wavelengths (spectrum).
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Spectrum from a Typical Planet, Comet, or Asteroid
Spectrum reveals:1 Chemical Composition – from abs. of wavelengths2 Temperature – from peak wavelength of IR light 3 Velocity – from Doppler effect
Reflected visible light from Sun Thermal Emission (IR)
Absorption by moleculesin gases in atmosphere
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. Formation of the Solar System
Clouds of Gas and Dust in the Milky Way Galaxy
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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
Overall Properties of our Solar System
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
Dust and GasIn
InterstellarClouds !
Light absorbedfrom distant starsalong mid-plane.
our Milky Way Galaxy
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The Dark Clouds in the Milky Way
Centaurus AHST
Milky Way
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The Interstellar Medium(ISM)
Dust & Gas
98% is H and He
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Dark Clouds in our GalaxyDense gas and dust. 1% (by mass) is “rocky/icy” dust particles that could eventually make terrestrial planets.
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Absorptionof Light by DustDust 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 containhydrogen, helium, carbon,nitrogen, oxygen
and complex molecules
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Small Dust particle:Only a few thousand atoms
Dust is Made of Atoms
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Interstellar Dust Grain:
C, O, Si, H20 ice, Si-O.
Large Dust Particle: 10,000’s of Atoms!
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Stars are continuously forming in the galaxy.
Basic Observation
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Dense CloudsFloating in our Milky Way Galaxy
Gravity pulls atoms closer together
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A Young Star Forming,surrounded by a protplanetary disk
Artists Rendering
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As the nebula contracts due to its self-gravity, it heats up, spins faster, and
flattens.
Cloud ContractingDue to Self-Gravity
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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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Protoplanetary Disks…
Solar System size
Star and planet formation
Measured Sizes: 100-1000 AU
Masses: 10-3 – 10-1 Msun
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Protoplanetary Disks of Gas & Dust
Theory ofPlanet Formation:
Dust collides, sticks
and grows pebbles/rocks Gravity helps attract
more rocks Gravity attracts gas
Formation of Planetary Systems
Observations Thermal Emission (Infrared)
from Dust
Hubble Space Telescope
Pictures of protoplanetary disks.
Mass of Disk = 10-100 MJUP
Disk Lifetime ~ 3 Million years
Observations Models of Planet Formation
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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
Safronov 1969Greenberg et al 1978Wetherill & Stewart 1993Kokubo & Ida 2000
Lissauer 1987Rafikov 2003Chambers, Thommes 2002Goldreich, Lithwick, Sari 2004
Analytical and N-body:
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Building the Planets Inward of 3 AU
At 3 AU is “Snow Line” : Hotter than 0COnly 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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Building the Planets
accretion -- small grains stick to one another via electromagnetic force until they are massive enough to attract via gravity to form...
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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)
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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