Download Lecture (Powerpoint)

Announcements
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All students: email me this week ([email protected])
telling me whether you want to:
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Do a presentation Mar 12
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Do a presentation at end of term
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Do a research paper
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Do an art project
And a couple possible topics you are interested in covering.
List of some possible topics on course web page
(http://flash.uchicago.edu/~ljdursi/SETI)
under blog.
Marks – Reading Quizzes and Assignments
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Reading Quiz:
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5 NCRs, 9 CRs, 1 CR+
Assignments:
Review: The Distance Ladder
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Different `realms' of distance in Universe, each
requiring different units, techniques of
measurement:
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Solar System
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Nearby stars
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Galactic distances
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Extra-galactic distances
Measurement for each realm depends on
knowing distances from the nearer realms
`Rungs' on Distance Ladder
Review: The Distance Ladder
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Sun
Moon
Earth
Geometric
measurement of
distance to Sun
depends on knowing
distance to Moon
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Solar system
`rung' depends on
Earth-Moon
`rung'
Review: The Distance Ladder
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Parallax distance
measures of nearby
stars REQUIRES
knowing how big an
AU is
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`nearby star' rung
depends on `solar
system' rung
Summary of Last Class: Light
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Light is a form of electromagnetic radiation
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All EM radiation
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Dims with distance as the inverse square law
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Forms a broad spectrum
Dense, opaque material glows when hot as a
blackbody
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Hotter glows more, and at shorter (blue-er)
wavelengths
Other processes give rise to distinctive line spectrum
which can be used to determine
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Composition
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Speed (by Doppler shift)
Summary of Last Class: Galaxies
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Galaxies are `island universes' which contain
most of the matter, stars in the Universe
Can be spiral, elliptical, or irregular
Star formation continues in galaxies, particular in
spiral galaxies
Galaxies also contain gas clouds, dust
Galaxies are separating over time: expanding
universe
Feedback:
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Most unclear item from last week's readings?
What we're going to cover today
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The Stellar Cycle: Birth, Life, and Death of the Stars
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Birth: turbulent collapse of clouds of gas
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Life: ignition; burning; balance between
gravity and pressure
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Death: gravity begins to win; but burning has
one last hurrah.
Stars are crucial for life
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Stars are the main engines in the Universe
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Stars are where planets are found
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Stars produce energy that can power life
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Stars produce all the heavy elements (eg Carbon)
that build life
Stellar Cycle
The Birth of Stars
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At end of this, we'll know:
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Where stars are formed
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How they form
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What has to happen for a star to
`turn on'
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How planets form around stars
Turbulence
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Happens when flow velocities are too
large to be kept smooth by viscosity
Turbulence
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Gas clouds in the galaxy are
turbulent, too
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Very wispy, tenuous gas
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No viscosity to speak of
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`Stirred' by energetic
events in the galaxy
Gas Clouds
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Two broad types of clouds:
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Gas clouds
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Warm
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Very wispy
Molecular clouds
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Colder
Much denser
Gas has condensed
enough that complex
molecules have formed
Molecular Clouds
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Because molecular clouds are cooler and denser, atoms
collide more often
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Can form complex molecules
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Greatly helped by presence of grains
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Provides sites for atoms to latch onto
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Region of high atom density; atoms more easily find
other atoms to interact with
Gas Clouds
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All of these gas clouds are turbulent
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Random motions, eddies
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Where fluid comes together, dense
regions
Fluid is moving fast enough that
can compress very dense spots
Gas Clouds
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Gravity acts to try to pull
these dense spots together
However,
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Pressure in gas clouds
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Rotation
Gas Clouds
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If a large enough, dense
enough region is formed,
gravity can start to win and
core starts collapsing inward
Nearby material can also
start falling in
As collapses, `spins up' and
disk can form
Gas Clouds
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Collapse will usually
happen in many places
throughout the cloud at the
same time
This is why stars tend to
be clustered
Amount of stars depends
on size of gas cloud
producing stars
Gas Clouds
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As core collapses, gets hotter
and denser
Begins to glow
Begins to evaporate nearby
complex molecules
Any particularly dense
regions can (for a while)
protect columns in their
shadow
Gas Clouds
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Nearby gas evaporates, but
disk remains
Flattens out at spinning
increases with collapse
Can begin to coalesce as star
begins to form
Protoplanetary
Disks
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These protoplanetary
disks can be seen around
very young protostars
Protoplanetary Disks
Jets and Outflows
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As core collapses further,
heat increases and so gas
pressure increases
If core is small enough, this
ends the process
If core is large enough,
burning can `turn on' and
begins rather violently
Under some circumstances,
enormous jet can form
perpendicular to disk
Summary
The Life of Stars
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At end of this, we'll know:
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The structure of stars
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How stars burn
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How stars age
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Our Sun's life story
Failed Stars
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`Stars' that are too small (~8% of
the mass of the Sun, or ~80 Jupiter
masses) never ``turn on''
Central temperatures never get hot
enough for nuclear burning to
begin in earnest
Nuclear burning is what powers
the star through its life
Star sits around as a brown dwarf –
too big and hot to be a planet, too
small and cold to be a real star
Failed Stars
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Such brown dwarfs have been
observed
~100,000 times fainter than Sun
Stars, failed or otherwise, often
observed in binary systems
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(~30% of all stars in binary
systems?)
Turbulent collapse makes it very
likely that two cores form nearby
or large core splits into two.
Hydrostatic Equilibrium
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Once collapse has halted in a star,
force inward (gravity) must be
balanced by force outward (gas
pressure)
(Much of the rotation has been
taken away by the planetary disk
by this point)
Central region is hottest because
pressure from the entire star is
pushing down on it
Star as a whole is hot enough that
no molecules are left; everything is
broken into components
Nuclear Reactions
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Nuclei of atoms themselves
interact
Change the elements: alchemy
The star, like the cloud it came
from, is mostly hydrogen
So hot the electrons are stripped
off; left with bare protons
(hydrogen nuclei)
Under extreme heat, protons can
fuse together to produce helium:
and more heat!
Higher temperatures – faster
reactions
Question
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What happens if an external force
`squishes' the star a little bit?
Built In Thermostat
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If star is squished in,
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Central region gets hotter
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Reactions speed up
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Star gets hotter
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Gas pressure increases
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Star fluffs out
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Central temperature returns
to normal
Built In Thermostat
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If star is pulled out a little,
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Central region gets cooler
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Reactions slow down
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Star gets cooler
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Gas pressure decreases
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Star falls back
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Central temperature returns
to normal
Star is STABLE
Given that burning is stable,
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What effects how hot a star is?
Given that burning is stable,
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What effects how hot a star is?
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MASS
The bigger the star that forms from the
collapse
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More pressure on the central region
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More burning
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Hotter
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Brighter
What color are more massive stars?
HR diagram and Main Sequence
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From previous, expect that
hotter stars should be brighter
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Blackbody
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More massive -> bigger
When temperature vs brightness
is plotted, see `Main Sequence'
Other populated regions show
later stages in stellar evolution
Stellar Evolution
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Nuclear reactions are very
sensitive to temperature
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Massive stars burn MUCH
faster than smaller stars
Even though massive stars have
more fuel (hydrogen) to begin
with, it is exhausted more
quickly
Everything happens faster with
more massive stars because
pressure is higher
Stellar Evolution
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As burning in core progresses,
Hydrogen in center becomes
depleted (Sun: ~10 billion
years)
Core of Helium `ash' left behind
Shell of Hydrogen burning
slowly moves outwards
As heat source moves further
out, star `puffs out'
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Outer regions cool, redden
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Red Giant (Sun: 1 billion years)
Stellar Evolution
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Eventually Helium core gets so
hot that even it can burn, to
Carbon
New energy source: star gets
hotter and bluer, and shrinks
back to more normal size
Burning happens faster with
heavier elements; soon Helium
becomes exhausted, a Carbon
core forms; becomes giant
again
Low Mass stars: envelope ejection
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Helium burning can be very
unstable
Outer layers begin pulsing;
blows most of the envelope off
of the star
(so called) `Planetary nebula'
forms
Only the core is left behind, still
glowing (because hot) but inert
White dwarf
High Mass Stars: Continue Burning
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Slightly more massive stars (4
to 8 solar masses):
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Everything happens faster
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Carbon can burn, as well;
one more stage of burning
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Then again leave (larger)
white dwarf and planetary
nebula behind
Very High Mass Stars: Continue Burning
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Very massive stars burn VERY fast
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Main sequence stage – 10 million years
Burning happens so quickly that outer layer
can't go unstable
Burning progresses faster and faster through
higher and higher elements until Iron
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No further burning is possible
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Left with a large envelope and very heavy core
Life Story of Our Sun
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Formed in ~50 million years
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Began life about 5 billion years ago
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A little dimmer (¾ current brightness)
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A little cooler, smaller
Slowly getting bigger and hotter:
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1 billion yrs from now: 10% brighter
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Greenhouse effect
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5 billion years from now: 40% brighter
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Earth like Venus today
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Still main sequence
Life Story of Our Sun
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Red giant branch begins
Next 700 million years; sun doubles in energy
output
Doubles in size, gets little redder
Next 600 million years; very strong wind; planets
pushed somewhat outwards
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At biggest, sun almost out to Venus' orbit
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Helium Flash!!
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Helium begins burning, process repeats itself but
10x faster
Ends with ½ of suns mass blown away; white
dwarf remains
The Old Age and Death of Stars
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At end of this, we'll know:
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The final stages of stellar life
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How stars of different mass die
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How they feed back material into
the interstellar environment, to be
made into new stars
The Old Age and Death of Stars
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Small stars end their life quietly
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White dwarf remnants
Massive stars continue burning in outer layers even
when they have burned all the way to iron in the
core.
New ash from burning continues to pile onto iron
core until pressure cannot support it any more
Type II Supernova
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The result is a collapse to a different
form of matter – a neutron star, or a
black hole -- and a release of energy
Energy release can be equal to the
entire energy of the host galaxy
Entire envelope is blown apart
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Heavy elements from burning
blown into surrounding gas
Type Ia Supernova
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Almost as much energy can come
from another kind of supernova
If a star which ended up as a white
dwarf has a companion, matter can
`rain in' on the inert white dwarf until
it gets hot enough to burn
Can burn catastrophically, exploding
and releasing heat, heavy elements
into surrounding gas
Supernova Feedback
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Originally, gas was all hydrogen and helium
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No planets, life
Generations of stars produced all the heavy elements
which make up planets and living things
Supernova explosions release these heavy elements into
the galaxy
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New stars are formed
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Can make planets, life
Supernova energy contributes to the turbulence in the
gas clouds, and can compress gas to start new cycle of
star formation
Stellar Cycle Revisited
Reading for Next Week
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Chapter 7, 8 – origins of life on earth