Transcript 6. Light: The Cosmic Messenger

Absorption Spectra
• Light shines through a gas,
atoms will absorb those
photons of specific
wavelengths that match the
atom’s electron energy
levels.
• Spectrum is missing those
wavelengths that were
absorbed.
• We can determine which elements are present in an
object by the emission & absorption lines in the
spectrum.
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Light
&
Origin of the Solar System
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• Homework #3 Due Tomorrow at noon .
• New Homework posted tomorrow:
Read: Chapters 5: “Light” Reading.
Chapter 8: First 10 pages only.
• Midterm #1: 12 days - Feb 26.
–
Covers Chapters 1-5 & 8 (skip 6,7) .
– 6 problems, similar to homework
Equations provided;
no calculator
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Telescope Observations Project
Two Parts
Good Night to Use Telescope & Detect Mars . . .
1. Make Telescope Observations of two objects
Suggestions: Saturn, Mars, Orion Nebula
Telescope Hours: Tue & Thu 7-8 pm, 7th floor of Campbell Hall
Sketch both objects on 1/2 sheet of paper. Note Date and Time.
2.
Mark the position of Mars with a dot,
at three times during the Semester,
early, middle, late. (Use either map.)
Note date of each observation.
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Wed,
Totality: 7:00 - 7:50 pm
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5:43
10:50
9:09
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7:00
Light
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Last Time:
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Spectra Tell us:
What a Planet is made of
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• Chemical Composition of
Surface and Atmosphere ?
• Temperature
• Ices, liquids, gases?
Artist’s rendering: Recently discovered “dwarf planet”, Eris.
One of many icy objects larger than Pluto in the
Kuiper Belt of the outer Solar System.
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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:
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E = hf
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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Emission Spectra
• Each type of atom or
molecule has a unique set
of electron energy levels.
• Each emits its own set of
wavelengths of light.
• Unique Emission line
spectrum for each atom
or molecule.
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Absorption Spectra
• Light shines through a gas,
atoms will absorb those
photons of specific
wavelengths that match the
atom’s electron energy
levels.
• Spectrum is missing those
wavelengths that were
absorbed.
• We can determine which elements are present in an
object by the emission & absorption lines in the
spectrum.
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Warm, Solid Objects Glow by
Thermal Emission of Light
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Cool
Red & Faint
Warmer
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Hot
Hotter
White & Bright
“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 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)
Thermal Radiation
Hot: more Blue
Cold: more Red
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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 = 2.9 x 106 / T (nm)
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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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The Doppler
DopplerEffect
Effect
The
Waves emitted from an object moving towards
you will have its wavelength shortened.
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The Doppler Effect
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The Doppler Effect
with Light
1. Light from an object moving towards you
will have its wavelength shortened.
BLUESHIFT
2. Light from an object moving away from
you will have its wavelength lengthened.
REDSHIFT
3. Light emitted from an object moving
perpendicular to your line-of-sight will not
change its wavelength.
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The Doppler Effect
Change in Wavelength
 = velocity

c
Wavelength
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Measuring the Doppler Effect
• Measure the Doppler shift
of emission or absorption
lines in the spectrum of a
planet.
• Calculate the velocity of
the object in the direction
either towards or away
from Earth.


=
velocity
c
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Chapter 8
.
Formation of the Solar System
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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 orbits & 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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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
Associated with dense gas is about 1%
(by mass) of “rocky/icy” grains that
could eventually make terrestrial
planets.
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Absorption
of 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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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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The Orion Nebula
Infrared View
Stars: Only 1 Million years old.
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80% of young stars
have protoplanetary
disks.
Planet - building
material is common.
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Disk masses measured
from
Millimeter-wavelength
thermal emission
of dust.
Star and planet formation
Protoplanetary Disks…
Solar System size
Measured Sizes: 100-1000 AU
Masses: 10-3 – 10-1 Msun
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Disk of dust
around AU Microscopii
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AU Microscopii
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Artists Rendering
of Young Star Forming,
and protplanetary disk
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Formation of Planetary Systems
Observations  Models of
Protoplanetary Disks
of Gas & Dust
Observations


Thermal Emission (Infrared)
from Dust
Theory
of ( = 1 mm)
Thermal
Emission
from Planet
cold dustFormation:
far from star.
Dust collides, sticks
and grows

Hubble Space
Telescope
pebbles/rocks
Pictures of
protoplanetary disks.
 Gravity helps attract
more rocks
 MDISK = 10-100 MJUP
 Gravity attracts gas



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
Disk Lifetime ~ 3 Myr
Origin of the Solar System
Our theory must explain the data
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
There are exceptions to these patterns.
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Origin of the Solar System
Theory – our Solar System formed from a giant,
swirling cloud of gas & dust.
Depends on two principles of Physics:
• Law of Gravity:
gravitational attraction of gas and
• Conservation of angular momentum
and on
•Basic chemistry
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Gravitational Collapse
of Original Cloud
• The solar nebula was initially somewhat spherical and a
few light years in diameter.
– very cold
– rotating slightly
• Gravity pulled the atoms and molecules together.
• As the nebula shrank, gravity increased, causing collapse.
• As the nebula “falls” inward, gravitational potential
energy is converted to heat.
– Conservation of Energy
• As the nebula’s radius decreases, it rotates faster
– Conservation of Angular Momentum
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As the nebula
collapses, it heats
up, spins faster, and
flattens.
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Collapse of the Solar Nebula
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Building the Planets
So only rocks & metals condensed within 3.5 AU
of the Sun… the snow line.
Hydrogen compounds (ices) condensed beyond the
frost line.
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Building the Planets
• Each gas (Jovian) planet formed its own “miniature”
solar nebula.
• Moons formed out of the disk.
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Lecture8
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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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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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Orderly Motions in the Solar System
• The Sun formed in the very center of the nebula.
– temperature & density were high enough for nuclear fusion reactions to
begin
• The planets formed in the rest of the disk.
• This would explain the following:
–
–
–
–
–
–
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 (collisions in the disk)
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Observe Radio Waves
to Search for Water
H2O
Production:
Earth’s
Ocean
every
24 min
H 20
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All planetary systems
are like our
Solar System . . .
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Rotating Molecules Detected
by
Emission of Radio Waves
CO
Neutral Carbon
Water in the
Interstellar
Medium.
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The Solar Nebula
• The nebular theory holds that our
Solar System formed out of a
nebula which collapsed under its
own gravity.
• observational evidence
– We observe stars in the process of
forming today.
– The are always found within
interstellar clouds of gas.
newly born stars in the Orion Nebula
solar nebula – name given to the cloud of gas from
which our own Solar System formed
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Seeing Through the Dark
Optical HST View
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Ring of Dust
around Fomalhaut
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More Support for the Nebular Theory
• We have observed disks around other stars.
• These could be new planetary systems in formation.
 Pictoris
AB Aurigae
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9.3 Creating Two Types of Planets
Our goals for learning:
• What key fact explains why there are two types
of planet?
• Describe the basic steps by which the terrestrial
planets formed.
• Describe the basic steps by which the Jovian
planets formed.
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Building the Planets
Condensation – elements & compounds began
to condense (i.e. solidify) out of the nebula….
depending on temperature!
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Building the Planets
…and temperature in the Solar nebula
depended on distance from the Sun!
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Building the Planets
…planetesimals which will:
• combine near the Sun to form rocky planets
• combine beyond the frostline to form icy planetesimals
which…
• capture H/He far from Sun to form gas planets
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Building the Planets
solar wind --- charged particles streaming out
from the Sun cleared away the leftover gas
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9.4 Explaining Leftovers and Exceptions to
the Rules
Our goals for learning:
•
•
•
•
What is the origin of asteroids and comets?
What was the heavy bombardment?
How do we explain the exceptions to the rules?
How do we think that our Moon formed?
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Origin of the Asteroids
• The Solar wind cleared the leftover gas, but not the leftover
planetesimals.
• Those leftover rocky planetesimals which did not accrete onto a
planet are the present-day asteroids.
• Most inhabit the asteroid belt between Mars & Jupiter.
– Jupiter’s gravity prevented a planet from forming there.
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Origin of the Comets
• The leftover icy
planetesimals are the
present-day comets.
• Those which were
located between the
Jovian planets, if not
captured, were
gravitationally flung in
all directions into the
Oort cloud.
• Those beyond
Neptune’s orbit
remained in the ecliptic
The nebular theory predicted the existence plane in what we call
of the Kuiper belt 40 years before it was
the Kuiper belt.
discovered!
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Exceptions to the Rules
So how does the nebular theory deal with exceptions,
i.e. data which do not fit the model’s predictions?
• There were many more leftover planetesimals than we
see today.
• Most of them collided with the newly-formed planets
& moons during the first few 108 years of the Solar
System.
• We call this the heavy bombardment period.
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Exceptions to the Rules
Close encounters with and impacts by planetesimals could explain:
• Why some moons orbit opposite their planet’s rotation
– captured moons (e.g. Triton)
• Why rotation axes of some planets are tilted
– impacts “knock them over” (extreme example: Uranus)
• Why some planets rotate more quickly than others
– impacts “spin them up”
• Why Earth is the only terrestrial planet with a large Moon
– giant impact
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Formation of the Moon
(Giant Impact Theory)
• The Earth was struck by a
Mars-sized planetesimal
• A part of Earth’s mantle was
ejected
• This coalesced in the Moon.
– it orbits in same direction as
Earth rotates
– lower density than Earth
– Earth was “spun up”
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