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Announcements
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Learn about SAIC's own Graduating Fellowship
Program from Romi Crawford, Director, Visiting
Artists Program and Denenge Akpem,
Fellowship & VAP Coordinator Thursday,
February 12 at 12:15 in 112 S. Michigan
Building, Room 816
Class website
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If there are any questions about the course
material or assignment, check the web page:
http://flash.uchicago.edu/~ljdursi/SETI/
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Class notes, assignments, blog, answers to
quizes/assignments there.
Can also email me:
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[email protected]
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Comments on assignment, quiz,
marking:
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Marks:
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Very `coarse' marking system; some NCRs were
very nearly CRs.
Reading Quiz 1:
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4 CR+, 11 CR, 4 NCR
Assignment 1
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Full credit (CR+) full points
Credit (CR) half points (passing grade)
No Credit (NCR) no points
5 CR+, 7 CR, 2 NCR
Answers are on the web page
Comments on assignment, quiz,
marking:
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Quiz: Lots of problems with the lifetime question
(how does lifetime of civilization play a role in
the Drake equation)
If civilizations are very short-lived on average,
there will be fewer that are still alive today, so N
drops.
If civilizations are very long-lived on average,
most will still be alive, so N is longer.
L / LMW is the fraction of civilizations born that
are still alive if civilizations are born throughout
the lifetime of the Milky Way
Comments on assignment, quiz,
marking:
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Assignment: Lots of problems with `scientificsounding claim problem'.
Need to provide either tests of a theory, or
theories to explain an observation.
Summary of Last Class: Distance Ladder
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Four `realms', with their own scales and methods
of measurement
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Solar system
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Nearby stars
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Galactic distances
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Inter-galactic distances
When saying something is far away, important to
ask – compared to what
Summary of Last Class: Drake Equation
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Can use to estimate number of civilizations in
galaxy today
Process of going through the estimate shows
what we know fairly well (number of stars) and
what needs more work (everything else, but gets
worse as you go on)
Will use to structure rest of the course.
Feedback:
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Most unclear item from last week's readings?
Feedback:
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How much time was spent on readings?
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How much time was spent on homework?
What we're going to cover today
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Observing the Universe: The Electromagnetic
Spectrum (Ch 2)
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The EM spectrum
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Inverse Square Law
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Processes by which light is emitted/absorbed
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Spectra
Galaxies and The Expanding Universe (Ch 6)
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Types of Galaxies
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Expanding Universe
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Dark Matter
Observing the Universe:
Electromagnetic waves
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At end of this lecture/reading, should be able to:
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Explain the inverse square law and use it to
solve (non-algebraic) problems
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Define the electromagnetic spectrum
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Explain what's meant by blackbody
radiation and line spectrum
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Explain the wave nature of light
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Describe how spectra are used to determine
the composition (and speed) of astrophysical
object
Electromagnetic Radiation
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There are only two long range forces
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Electromagnetism
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Gravity
This is how we must observe the distant Universe
Only now beginning to be able to observe
gravitational waves
Most of our observations come from
electromagnetic radiation.
Inverse Square Law
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Electromagnetic (and most other
kinds) of radiation obey the InverseSquare Law
Intensity of radiation (brightness)
falls off with the square of the
distance
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Doubling the distance to
something makes it appear
four times as dim (¼ as bright)
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Tripling the distance makes it
appear nine times as dim (1/9
as bright)
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etc.
Electromagnetic Radiation
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Electromagnetic radiation
from a source is in the form
of waves
Both Electric and Magnetic
components
Wave travels at speed of
light
Waves
Speed
wavelength
frequency
Waves
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(speed) = (wavelength) x (frequency)
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Higher frequency – more energy
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Higher amplitude – more energy
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Have to look on scales ~ wavelength to see
the wave
Electromagnetic Waves
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(speed) = (wavelength) x (frequency)
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But speed is fixed (all EM waves travel
at the speed of light)
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So given frequency, you can know the
wavelength and vice versa.
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Higher frequency – more energy
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Higher amplitude – more energy
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But EM waves come in bundles
(`photons') with fixed amplitude
Wave nature usually only noticeable on scales
~ wavelength
Electromagnetic Waves
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Wave nature of light usually not noticeable
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Wavelength of light ~ 1/40000th of an inch
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In common experience light behaves as if it
were made up of particles or rays which
emanate from source
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Newton's `corpuscular' (particle) theory
of light
Electromagnetic Waves
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Light is one facet of the entire electromagnetic spectrum
Our eyes have dedicated cells which are sensitive to
electromagnetic radiation in this range
Eyes most sensitive to yellow light – this is where the sun
emits the peak amount of energy
Electromagnetic Waves
15”
~9'
TV Antenna
VHF: ~200 MHz; wavelength~60”
UHF: ~575 MHz; wavelength~20”
~4.5”
Satellite TV dish
~12 GHz; wavelength ~9”
CB Radio Antenna
~27 MHz; wavelength~ 36 ft
What Generates Electromagnetic Waves?
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Thermal radiation: Hot things glow.
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Heat causes atoms to rattle about in an object
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Atoms contain charged particles (electrons, protons)
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Accelerating charged particles emit electromagnetic
radiation.
Thermal Radiation
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If material is dense enough to be opaque, hot body emits
radiation in a characteristic `blackbody' spectrum
High frequency
Short wavelength
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Low frequency
Long wavelength
Hot objects emit more and at shorter wavelengths
(higher frequencies)
Temperature and Spectrum
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Can put temperatures on
frequency chart
Our suns temp ~6000 K
(~10,000 F) means that the
peak of its radiation is in
yellow part of visible light
Room temperature: infrared
X-rays, gamma-rays: so
high energy that would
thermal emission is modest
up there
Line Spectra
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For non-opaque materials,
spectra can look quite
different.
Atoms/molecules can emit
or absorb photons only of
particular energies.
If dense enough, these lines
get blended out into
blackbody spectrum
If not (like gas in flame) the
spectrum is composed of
lines
Line Spectra
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If electron is an excited state
(e.g., from thermal jostling
around) can fall back down and
lose a specific amount of
energy
Corresponds to specific
frequency/colour
All possible combinations of
going down levels makes for
the emission spectra of that
material
Line Spectra
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If photon is coming towards
electron of right energy, can be
absorbed by electron
Electron jumps up a level
After a while, electron will
`fall' back down, emitting
photon, but maybe:
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in another direction
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in stages, emitting at
other frequencies
Net effect: absorption spectra
Atmospheric Absorption
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Our own atmosphere absorbs a lot of radiation
Absorbs UV (ozone): keeps energetic radiation from
destroying life
Absorbs infrared (greenhouse gasses): good to keep
things warm, but too much can cause troubles
Solar Spectrum
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Central region of sun fairly
dense
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hot core
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Outer layers progressively less
dense
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Wispier outer layers
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Emits as blackbody
Line effects start
becoming noticeable
We see continuum blackbody
spectrum from the inner star
with absorption features from
the outer layers
Solar Spectrum
Calcium
Sodium
Hydrogen
Oxygen Molecules
The Sun throughout the spectrum
Other Physical Processes
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Other, non-thermal processes can generate
electromagnetic radiation:
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Hot electrons in magnetic fields can
generate X-rays
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Nuclear decays can produce X-rays
or gamma rays
Thus, spectra can tell us composition of an
object and what physical processes are
occurring.
The Galaxy throughout the spectrum
Break!
Doppler Shift in Light
Doppler Shift in Light
Doppler Shift in Light
Doppler Shift in Light
Doppler Shift in Light
But what if it moves?
Doppler Shift in Light
Doppler Shift in Light
Doppler Shift in Light
Doppler Shift in Light
Doppler Shift in Light
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Sound or light from a source moving towards
you is shifted to higher frequencies (light is
bluer)
From a source moving away from you, shifted to
lower frequencies (redder)
Doppler Shift in Light
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Effect is fairly modest, but spectra can be
measured very accurately
Astronomers can measure velocities
towards/away very precisely
Galaxies
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At end of this lecture/reading, should be able to:
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Describe the differences between spiral,
elliptical and irregular galaxies
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Sketch the structure of a spiral galaxy
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Explain what evidence there is for galaxies
moving apart (expansion of the universe)
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Describe the evidence for galactic dark
matter
Galaxies
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Amongst the first things born
in Universe
Hubble Deep Field, looks back
up to 12 billion years in past
(Universe ~ 15 billion years)
Already galaxies like todays
existed, although not as much
structure
Galaxies
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Island universes
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Building blocks of the universe
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Contain millions, billions, or trillions of stars
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Also contain gas clouds (from which new stars
can be born), dust, star clusters,…
Typically several tens of thousands of parsecs
across
Vary in type and structure
Spiral Galaxies
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Flat, disk-shaped
galaxies with spiral
arms
Rotate (our part of our
galaxy rotates around
the center every ~200
million years)
Gas clouds, dust, stars
Spiral Galaxies
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Forms very early in
universe from huge
cloud of gas
Some gentle initial
rotation around
center
As collapses,
rotation increases,
flattens disk
Star clusters form
before collapse is
complete, orbit the
newly-forming disk
Elliptical Galaxies
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Simpler
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No initial rotation on collapse
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Forms a spherical or elliptical sea
of stars
Uses up most of gas in star
formation at original collapse --mostly old stars
Also contains globular clusters
Irregular Galaxies
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Neither spiral or elliptical
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Shapes vary
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Probably disrupted in the process
of forming, or afterwards by
neighboring galaxies
Clusters and superclusters
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Under gravitational attraction,
galaxies begin clustering
together:
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First in clusters (like our
`local group’
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These then cluster into
superclusters (like our
`Virgo supercluster’)
Galaxies moving away from us!
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Once `spiral nebulae’
were established as
galaxies, Hubble
examined their redshifts,
and distances
Found that galaxies were
all moving away from us;
faster
Expanding Universe
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Either we are very special
and everything is moving
away from us, or Universe
as a whole is expanding
But if universe is steadily
increasing in size, implies
that at some time in the
past, Universe was a
single point.
`Start of the Universe’
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Big Bang
Evidence for the Big Bang:
Microwave Background
At very beginning, Universe would have been very
hot
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Now, looking back, it is greatly redshifted
(cooler)
Can calculate what temperature it would be
now: ~ 3 degrees Kelvin
Microwave temperatures
The Microwave background
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Accidentally discovered
by radio astronomers
(thought it was noise)
1980s, COBE satellite
went up to take careful
measurements
Blackbody temperature
agrees with predictions
Slight fluctuations; hot
spots which eventually
gave rise to galaxies!
`Big Bang’ Nucleosynthesis
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Can also predict what nuclei are formed at such temperatures
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Too cold: can’t form nuclei
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Too hot: large nuclei are torn apart
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Prediction: Universe should be mostly Hydrogen, Helium, some
Lithium: Prediction agrees with observation
Reading for next class
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Birth, life, and death of stars
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Chapters 3, 4, 5 (~61 pages)