Download
1 / 47
480 likes | 744 Views
Nucleosynthesis. Remember that the Big-Bang 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
E N D
Nucleosynthesis • Remember that the Big-Bang 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 long-lived 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 with8Be
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
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.
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)
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
Constraints on Solar System Formation • All the planets orbit in the same direction. • Most, but not all, of them spin the same way they orbit, the same way the Sun spins? • All the planets orbit in nearly (but not exactly) the same plane. • 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?
Where did it go? • Planets: • 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
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?
