Transcript document

Nucleosynthesis
• Remember that the BigBang created the H and
He in the universe, but
not much more.
• H fusion in stars makes
more He
• But where do we get the
stuff we are made of?
Z
N
Star-stuff…..stellar fusion
creates the elements up to Iron
Iron is dead end for fusion
because fusion reactions
involving iron do not release
energy
(Iron has the lowest mass per
nuclear particle -- that’s just the
way it turned out.)
Table of
Nuclides and
their half-lives
if unstable
• Gray is stable
• White is unstable
• Hatched is longlived unstable
Fusion typically
creates elements in
jumps of 2 atomic
numbers (4 atomic
mass units) He + n
Except the first
jump…. there is
a problem early
on with 8Be
Nucleosynthesis
• What happens when you fuse He
• 4He + 4He → 8Be
• BUT 8Be is unstable (half-life of 10-17
seconds)
• To make stable 12C would require
three He to fuse within 10-17
seconds….it happens in normal
stars, but not often enough to create
this universe.
Nucleosynthesis II
• To raise the probability of the three
4He reaction, we have to raise the
temperature and the density
• This happens during core collapse at
the Red Giant stage of massive stars
– Gravitational collapse provides extra
energy and heating (we will calculate
that energy later)
• Once you get 12C we have a stable
base to start several element
formation cycles.
Once we are past
the Be barrier we
can make heavier
elements by fusing
He to a stable atom.
Note we take jumps
of 4 atomic
numbers
Nucleosynthesis III
• This sort of burning
of He combined with
C, O, Ne, Mg, Si and
so on, takes place in
massive stars with
higher temperatures
• Another element
creation process is
the CNO bi-cycle
– It starts with stable
Carbon and fuses H
• This produces lots of
energy and fills in the
periodic table
Advanced nuclear burning occurs in multiple
shells --
Evidence for
helium
capture:
Higher
abundances
of elements
with even
numbers of
protons
• This adding and filling process
liberates lots of energy, but the
amount liberated per reaction
generally drops to 56Fe.
• 56Fe is bottom of the energy
valley. To make heavier
elements will require energy
input.
• To make the elements beyond
Fe requires two processes….
S-Process
• Some reactions liberate neutrons.
• The larger nuclei tend to have a larger capture cross-section
for neutrons and can absorb lots of neutrons.
• This puts energy into the atom, pumps up its atomic
number, and pushes it into really unstable isotopes (look at
66Zn going to 73Zn.
• Then beta decay turns a neutron into a proton, pushing the
atom up the periodic chart.
• This makes about 75% of isotopes heavier than Fe
R-Process
• The S-process goes only “slowly”, so there are some
isotopes that it cannot make.
• Look at 144Sm.
– Remember that the S-process zig-zags. You push stable
isotopes to the right by adding neutrons, then zag up and to
the left by beta decay.
– But Pm (anybody know what Pm is????) doesn’t have any
stable atoms
– Nd has lots of stable atoms, so they will not beta decay to form
Pm, and therefore cannot form 144Sm
R-Process
• The way we get around this problem is the R-process
• It occurs during a supernova. While the supernova is in the
process of ripping apart a star, the stellar material is flooded with
neutrons.
• During the few seconds (or milliseconds) of the explosion
neutrons are absorbed much faster then the atoms can decay, so
the isotopes are pushed far to the right on the chart.
R-Process
• THEN decay begins and continues moving up and to
the left until stable configurations are reached.
• BUT so many neutrons are absorbed during this
RAPID process that it allows nuclides to decay to a
wide range of stable configurations.
R-Process
Supernovas
Products of Supernovas
• Explosive nucleosynthesis.
– This creates and recycles heavy elements.
– Seeds the elements into a new star/solar system
formation cycle
• Neutrino burst.
– Neutrinos are little bits of pure energy produced by
various nuclear reactions….
– During the core collapse, protons and electrons are
forced to merge, creating a sea of neutrons and a
huge number of neutrinos
– During a supernova the mass-energy equivalent of
50 Earths are created in just neutrinos
• Expanding shock wave and shell
Cosmic Abundances
• So we have made a bunch of heavy
elements via Nova and Supernova
processes
• But, light elements are still far more
abundant.
• We can look at meteorites and stellar
spectra, and samples of the solar wind
to determine the relative abundances of
elements in our solar system.
Cosmic Abundances
• What would be the abundances for a
system near the galactic center?
• In a Population II star cluster?
Abundance is key…
• What we get in planets
is largely determined
by what is available.
• Strontium has about
the same chemistry
and size as
calcium….why aren’t
our bones made of
strontium?
• Why don’t we breathe
fluorine?
• What about europium,
or hafnium or
praseodymium?
• (Baron Carl Auer von
Welsbach...didymium)
Star-forming region: Young
stars and a molecular cloud
Stages in Star Formation
• Shock wave from a nearby supernova compresses a
molecular cloud
• A slowly rotating core forms
• The core collapses into a protostar and disk
• Central pressures increase and initiate fusion
• The solar wind starts to push “left-overs” out of the
system.
• Newly ignited stars can be very unstable T-Tauri Stars
Early stages of protoplanetary
disk formation
Constraints on Solar
System Formation
1) All the planets orbit in the same direction.
2) Most, but not all, of them spin the same way
they orbit, the same way the Sun spins?
3) All the planets orbit in nearly (but not exactly)
the same plane.
4) All the planets orbit in nearly (but not exactly)
circular orbits.
Why does the cloud
collapse to form a disk?
• All molecular clouds spin a bit (we will discus
that a bit later)
• The centrifugal force within the disk should
tend to support gas around the equator of the
collapsing sphere.
• Gas at the poles will continue to collapse in
towards the center without being held out by
centrifugal force.
• Because of centrifugal force a collapsing gas
sphere becomes a fattened disk.
Constraints
• Forming from a spinning cloud answers constraints 1
and 2 (orbits and planetary spins)
• The flattened disk forces everything down into the
same plane (constraint 3)
• Rotating gas disks are likely unstable and tend to
breakup into protoplanetary lumps.
• The lumps form from self gravity, grains become
pebbles, pebbles become boulders….and so on
• Planetesimals form.
– The ones without circular orbits tend to collide with those that
have circular orbits.
– The collisions may change orbits and spin states
– This may account for constraint 4
The Angular Momentum Problem
• Remember we are dealing with a
rotating cloud
• The definition of angular momentum
L = mrv
– m is mass, r is the distance of the
spinning body from the center, v is
velocity
• How much angular momentum does
the solar system start with?
Angular Momentum
• Assume….
– The protoplanetary cloud has a radius of about
1/3 light years (3.6 x1015 m)
– One solar mass is about 2 x 1030 kg
– The rotational velocity of the cloud from just
rotating the galactic center is about 3.6 m/s
• L = (3.6 x1015 m) * (2 x 1030 kg) * 3.6 m/s
• L ≈ 1046 kg m2/s
• How fast would the Sun be spinning once
everything accreted to one solar radius?
Angular Momentum
• Remember that angular momentum is
conserved.
• v = L/mr
• If r drops to 7 x 108 m, then v has to go to
about 2 x 106 m/s (about 1% of the speed
of light)
• But the Sun spins about 105 slower than
that. Its angular momentum is about 1041
kg m2/s, a factor of 100,000 less!
• What has happened to all that angular
momentum?
• Planets:
Where did it go?
– The orbits of the planets contain about 1043 kg
m2/s
– But even with the planets we are really missing
about 99.9% of the original molecular cloud
angular momentum.
• Randomness:
– The spin of the original cloud may not have
oriented with the rotation of the galaxy.
– The cloud would have not have started with as
much momentum
• Ejection:
– The solar system certainly lost mass during all
phases of its formation (T-tauri, Oort cloud).
– Ejection of this material would have drained off
angular momentum
In the Beginning
The Hadean
• The first geologic eon of Earth and lies before the
Archean. It began with the formation of the Earth about
4.5 billion years ago and ended about 4,000 million
years ago
• What is the age of the solar system?
• What are the oldest rocks on Earth?
The Hadean
• What does 182Hf–182W tell us about Earth’s
accretion and core formation?
• What is the evidence from Mars?
• What was the primary atmosphere of
Earth?
• Where did the secondary atmosphere and
water come from?
• What happened to the oceans during the
giant impact?
• What is the role of radioactive heating vs
tidal heating vs impact heating?
The Earth in the Hadean
The Earth in the
Hadean
The “Crust” in the Hadean
The End of Hadean (the LHB)
The End of Hadean (the LHB)
The End of Hadean (the LHB)