“We Are All Stardust” Origin of the Chemical Elements

1. “We Are All Stardust”Origin of the Chemical Elements Second NRAO/AUI Image Contest Awards John M. D’Auria Professor Emeritus Simon Fraser University A Majestic Gas Bubble Shell in the Milky Way J. English of Univ. of Manitoba First place award Radio Pulsar / White Dwarf J. VanLeeuwen et al, UBC Cassiopeia A at Dusk superimposed) M. Bietenholz of York Univ. BCAPT/TRIUMF Professional Development Day Oct. 20, 2006 http://www.nrao.edu/

2. SOME FUNDAMENTAL QUESTIONS • What are the building blocks of matter ? THE CHEMICAL ELEMENTS (e.g. Carbon, Oxygen, …) • Where were these elements produced ? STARS and the BIG BANG • How were these elements produced ? NUCLEAR REACTIONS How do we know? Examining the clues -processes on earth simply recycle elements -unusual abundance distribution of elements (follow nuclear properties) -high power intensities available from the sun (cannot be chemical rxs.) SUN(~1033 g) gives  ~4 X 1026 Joules/sec [energy/sec] Chemical reactions produce 106 Joules/gram per reaction Then Sun gone in (1033 x 106 )/4 x 1026= 2.6 x 1012 sec ~ 80000 years [mass x energy per mass ÷ energy per time] [lifetime of the sun if chem reactions are source of energy]

3. Talk Outline • Some basic concepts about the nucleus • Take a look at elemental abundances • Primordial nucleosynthesis • What are stars? • Stellar nucleosynthesis • Nucleosynthesis in Exploding Stars • (Cosmic ray nucleosynthesis) • Summary Lecture available on http://trshare.triumf.ca/~isol/ BCAPT/TRIUMF Professional Development Day Oct. 20, 2006

4. Atomic Nuclei and Elements • Atoms made up of electrons and nuclei • Nuclei made up of protons and neutrons • Number of protons, Z, (atomic number) determines element • Number of neutrons, N, determines isotope • Sum of the Z + N = A, the so-called atomic mass number • Relative numbers of protons and neutrons determine stability Z  AX 1H – proton 2H – proton and neutron 3H – 1 p and 2 n N 

5. Forces of Nature • Gravity: affects everything, even light, but particularly important in planets, stars, galaxies, clusters of galaxies, the universe • Weak Nuclear Force: responsible for radioactive beta decay & some nuclear reactions • Electromagnetic Force: electricity, magnetism, felt by all charged particles; gamma decay • Strong Nuclear Force: felt by protons and neutrons, holds nucleus together so involved in all nuclear reactions; alpha decay Cat’s eye nebula - HST All play a role directly or indirectly in origin of the elements

6. Radioactive decay of unstable isotopes • alpha decay – release of a 4He++particle() (ΔA = 4; ΔZ = 2) • 235U(uranium)231Th(thorium) + 4He(helium) • beta decay – (ΔA = 0; ΔZ = ±1) • emission of an electron (e-) or positron (e+)(anti-matter) • electron capture (capture of atomic electron by nucleus) • 14C(carbon) 14N(nitrogen) + beta particle + neutrino • gamma decay – (ΔA = 0; ΔZ = 0) • internal conversion (release of atomic electron) • pair production(creation of e+ and e) Lifetime of Decay depends upon type of interaction/type of decay and other factors

7. Nuclear Masses and Binding Energy • Einstein: E0 = mass x c2, where E0 is the rest energy • The mass of a stable nucleus is less than sum of component masses • Mass = [Z·mass (1H) + N·mass (1n)] – Binding Energy [B.E.is like Heat of formation] • Binding Energy Calculation • - Consider the nucleus 16O (8 p + 8 n), what is its binding energy? • e.g. Mass of 16O = 15.9959 atomic mass units (u) (1 amu = 1.66 x 10-24 g) • Mass of 8 1H + 8 1n = 8.0624 + 8.0696 = 16.1524 u (note: p = 1H+) • Difference = 0.1371 u (mass) = 127.71 MeV (1 MeV=1.6 x 10-13J) • or 127.71/16 = 7.94 MeV/u = Binding energy for 16O (one nucleus) • - Given 1 mole (16g) of 16O, it would take about 2.04 x 1013Joules to break it up • (or if released, with all mass converted to energy). • In nuclear reactions, difference between the rest energies of reacting nuclei and product nuclei is released as kinetic energy, i.e., DE = Dmc2 • Fusion of light nuclei & Fission of heavy nuclei release lots of energy

8. Thermonuclear Fusion Reactions • Positively charged nuclei repel one another due to electromagnetic force – Coulomb force • If they get close enough together, the dominant strong nuclear force can be felt • Probability of a reaction depends very strongly on temperature • Only at very high temperatures of a few million K or higher can fusion occur, for typical pressures

9. Nuclear Reactions energetics and cross sections projectile, a + target, A  recoil, b + product, B A(a,b)B If mass of reactants < mass of products, reaction requires energy: Q value(endo) If mass of reactants > mass of products, reaction goes (energy released) BUT For charged particles, there is an electrostatic repulsion inhibiting reaction (Coulomb barrier – positives repel ..like activation energy) With neutrons there is no barrier….. + + cross section () is the probability of the reaction occurring - depends primarily upon incident energy/velocity - increases with energy (velocity) of projectile - as energy/ velocity increases, more exit channels open up - various reactions can proceed simultaneously depending upon 

10. Shell Model of the Nucleus (simple version) In the atom there are stable closed shells of electrons around the nucleus. e.g. noble gases have closed shells and are unreactive Similarly in the nucleus, there are magic stable configuration of nucleons (protons or neutrons) forming closed shells called magic numbers. These occurs at # of nucleons = 2, 8, 20, 28, 50, 82, 126 Chart of the Nuclides

11. Abundances of Atomic Nuclei • What nuclei are found in the universe, and in what amounts? • 2 Sources of information: Sunlight(provides elemental abundances) Primitive Meteorites (provides isotopic abundances) (cannot use earth or moon info) • Once we know the abundances, we can use physical laws to determine the conditions needed to produce them M51, the Whirlpool Nebula, NGC5194/5195.

12. Absorption Lines • A thermally radiating light bulb viewed from behind a relatively cool gas leads to the continuous spectrum as before, but with dark, lines present at certain colours. • These absorption lines are characteristic of the elements in the gas

13. The Solar Spectrum • Relatively cool, diffuse gas near the Sun’s surface absorbs continuous, thermal radiation from the opaque, hot solar interior • Temperature of surface is 6000 K, and temperature of centre is 16 million K • Comparing solar absorption lines with lab spectra allows determination of elemental abundances in solar atmosphere • Keep in mind that they are red shifted (wavelength longer than in the lab) since stars are moving away from the earth

14. Solar & Meteoritic Abundances • Solar absorption lines in general yield only chemical information (which element) • Meteorites can be examined in the lab much more thoroughly and precisely, allowing determination of isotopic abundances too • Agreement between two sources of abundance information is excellent Log Log

15. Solar System Abundances Iron Log Platinum Lead Uranium • 1H by far most abundant • By mass, 4He second, then 16O, 12C, 20Ne, and 56Fe • Sun (and therefore, solar system) is 71% hydrogen & 27% helium; all other elements make up only 2% of the mass of the solar system, even though they’re all heavier than H and He • Other stars: 70-75% H, 25-30% He • This gives a clue to the origin of the lightest elements, that they are produced differently from all the rest

16. Origin of the Chemical Elements Primordial Nucleosynthesis Stellar Nucleosynthesis Cosmic Ray Nucleosynthesis

17. The Big Bang • Expansion of the universe: distant galaxies are flying away from us at speeds proportional to their distance (the Hubble law) • There is a cosmic microwave background radiation that has nearly the same intensity in every direction; it is the thermal radiation of the universe, the radiant heat left over from the Big Bang, and corresponds to a temperature of 2.7 K

18. Big Bang Nucleosynthesis Log • The abundances of the lightest elements agree with calculations of a period of thermonuclear fusion reactions in the first few minutes after the Big Bang • Big Bang nucleosynthesis produced H, He, & trace quantities of Li & Be • How ? Log

19. Primordial Nucleosynthesis First there was the Big Bang - red Doppler shift of spectral line measurements (Hubble) - universal isotropic 2.8 K background photon radiation observed initially using radio astronomy At t = 1 second (after BB) - T = ~1010 K (10000000000 K); (very high temperature) - combination of neutron and proton to form 2H (deuteron) is not possible when exposed to such temperatures as it is above the binding energy; p + n  d +  THE UNIVERSE COOLS AS IT EXPANDS At t = ~ 3 minutes; universe cooled to ~109 K, formation of deuterium works p + n  d +  reaction goes to the right - at this point in time/temperature, % neutrons is 13% - additional reactions occur using d, leading to 4He p + d 3He +  ; 3He + 3He 4He + 2p - The % by weight of 4He is 2 ~ (2) (13 %) = 26 % since 4He = 2p + 2n. 4He (and H) are dominant products from the early universe nucleosynthesis

20. Why is 4He a dominant product in early universe nucleosynthesis ??? Because: a. It has the highest binding energy of the light nuclei. b. The lower/lighter mass nuclei (d, 3H, 3He) have lower binding energies and will mainly provide fuel in the chain of reactions. c. There are no stable nuclei at A = 5 & 8 (synthesis stops/hindered). d. Some higher mass Li’s and Be’s may be produced by reactions but the higher Coulomb electrostatic barrier will hinder their production. Calculated primordial abundances (mass fraction) 1H 0.75 ; 2H 2.5 x 10-5 ; 3He 4.2 x 10-5 4He 0.23 ; 6Li 300 x 10-12 ; 7Li 4600 x 10-12 These are reflected in abundances observed today

21. Stellar Nucleosynthesis The next phase requires STARS Most elements made in stellar factories Cosmological dust embryonic stars (Mainly H and He) gravity What is a star?Gigantic ball of burning gas (plasmas) Very high temperature in the core Initial early stars ‘clean’ (H, He) and very large THE SUN Mass: ~ 2 x 1030 kg; Radius: ~ 7 x 105 km; Surface Temp: ~ 5700 K Core Temp: 16 x 106 K Core Density ~150 x Density of Water

22. Stars • Stars are hot spheres of gas powered by internal energy sources • The pressure at the centre must support the weight of the overlying layers: gravity tends to collapse a star under its own weight; as it shrinks, the pressure, temperature, and density all increase until the pressure balances gravity, and the star assumes a stable configuration • For gas spheres at least 1/10 the mass of the Sun, the central temperature becomes hot enough to initiate thermonuclear fusion reactions • Nuclear reactions in the hot, dense core are the power source of the Sun and all other stars

23. How Do We Know All That? • Helioseismology (study of surface vibrations of the Sun) yields temperature, density, composition, sound speed, from 5% of solar radius to the surface • Sudbury Neutrino Observatory and other detectors measure solar neutrinos, weakly interacting particles created in nuclear reactions and radioactive decays in the solar core, which then fly right through the Sun and are occasionally detected on Earth

24. Nuclear Reactions in Stars • Stellar temperatures are usually below Coulomb Barrier, the minimum energy required for nuclear reaction • Reactions penetrating the barrier are low probability but can happen, e.g. alpha emission occurs through barrier • Overlap of velocities of species in stars with probability of the reaction occurring – GAMOW Peak Probability of a reaction Distribution of Velocities Energy or velocity

25. How Does the Sun Generate Energy? strong weak First step of process • 4 hydrogen nuclei (protons) are fused into a single 4He nucleus, with the emission of 2 positrons and 2 neutrinos • This is very efficient, releasing about 1% of the binding energy, an enormous amount of energy • But very slow reaction (~109 years) since involves weak interaction (see above) and strong nuclear force. • The vast majority of stars generate energy the same way, fusing hydrogen into helium (Hydrogen Burning)

26. How Long Can This Go On? • Sun started as 71% H by mass, 27% He • Gradually, H in the core is fused into He • Presently, ~ 4.6 Gyr after it started shining, the Sun’s core H is about 1/2 gone • Will continue fusing H into He in the core for another 5 Gyro • Why? Low probability reaction • Then what?

27. Stage 2. Helium burning Red Giant Phase • Inert He core contracts, outer layers expand, Sun becomes red giant • H fuses into He in a shell surrounding the He core • Size and mass of inert He core continue to expand • More massive the core becomes, more pressure is required to support it and the overlying layers, so temperature rises • Eventually, becomes hot enough for He to fuse into carbon via 3 alpha reaction [triple alpha rx.] • Some carbon fused with helium to form oxygen 4He + 4He  8Be 4He + 8Be 12C (carbon) 4He + 12C 16O (oxygen)

28. After Core Helium Exhaustionstars < 10 Solar Masses • Inert carbon core (the ashes of helium burning) • He-burning shell • He layer (the ashes of hydrogen burning) • H-burning shell • Hydrogen-rich envelope • Core never gets hot enough to burn carbon

29. Planetary Nebula Phase • Star ejects outer layers, enriching interstellar medium with elements produced during the star’s life • The core, composed predominantly of carbon, oxygen, and helium, becomes a white dwarf, a stellar cinder that gradually cools, no longer able to generate energy by nuclear reactions • This is for stars < 10 Solar Masses The Helix Nebula in the constellation Aquarius

30. How About More Massive Stars(>10 Solar masses)? • After core helium fusion comes carbon fusion, oxygen fusion, & silicon fusion • Creates an onion-like structure, with an inert iron core surrounded by progressively cooler burning shells and a hydrogen-rich envelope

31. Stage 3: Pre-explosive burning stages (summary) - As massive star uses up fuel, it is pulled tighter by gravity and the core gets hotter. - More reactions open up/ several phases are initiated. a. Carbon burning 12C + 12C  many particle emission reactions b. Oxygen burning 16O + 16O  “ “ “ Leads to a core of mainly 28Si Then: Equilibrium process is achieved Photodisintegration (, nucleon) and alpha capture occur • Leads to core of even isotopes in the IRON region • which is thermodynamically the most stable region • of the chart of nuclides (highest binding energy region) • - Lifetime of the star getting shorter as it gets hotter.

32. Major Stellar Fusion Processes(non-explosive scenario)

33. Stage 4. Heavy Element Production: Elements above iron region probably made by two different but similar processes, namely; s – process (slow neutron capture) r – process (rapid neutron capture) Basically competition between fusion of a neutron with the nucleus and beta decay. Remember there is no electrostatic (Coulomb) barrier for neutron capture. National Research Council– This is considered one of greatest unanswered questions

34. s-process (Slow neutron capture process) - nucleus captures a neutron forming an unstable isotope, which then decays by beta emission n + AX  A+1X  Z+1X ; - slow process allowing beta decay to occur. - this process works with heavy star (not a first generation star), needs neutrons, and takes a long time to complete. - produces neutron-rich isotopes close to stability. 117In beta decay S-process 116Cd 117Cd stable

35. EXPLODING STARS Nucleosynthesis in Cataclysmic Scenario

36. r-process At the end of the e-process, with high concentration of iron region in heavy mass stars, lack of nuclear reactions leads to core collapse, leading to a catastrophic implosion in a time scale of seconds…..SUPERNOVA CRAB NEBULA (remanant of SN observed in 1054) SN 1987A ( in Magellanic Cloud: occurred 169 kly ago; observed in 1987) Elements dispersed into the cosmos and many new elements created….. How???

37. Supernova Remnant • Core-Collapse Supernova • When nuclear fuel becomes exhausted • in a massive star, the core collapses • under tremendous force of gravity under • the weight of the produced iron. • Major implosion, and then explosion • (gravitational bounce), in seconds, • releasing much material, and forming • neutron star or a black hole. • SN process still not well understood • (sound could play a role) • Elements heavier than iron produced. CRAB NEBULA (remnant of SN observed in 1054)

38. Type Ia Supernovae • Another type of supernova takes place in binary star systems • If a white dwarf accretes sufficient matter from its stellar companion, it detonates in a cataclysmic thermonuclear explosion • Standard Candles – Used to study explanding universe

39. r-process (Rapid neutron capture process) - High heating leads to extensive nuclear reaction network involving the high number of neutrons produced in explosion. - Multiple, rapid (seconds) neutron capture produces very neutron-rich nuclides 5-10 neutrons - Capture only limited by hitting the neutron drip line or if the probability of capturing a neutron is small. This occurs at the magic numbers/closed neutron shells. These are waiting points …… - At these points, beta decay can occur leading to stable nuclei and the full process leads to elements up to uranium and thorium. (n,) vs. (,n)

40. Chart of the Nuclei

41. Solar System Abundances, Revisited  nuclei Highest BE/A r-process s-process

42. How do we know this? For over 60 years scientists have collected a great deal of data on abundances and on nuclear reactions. These data have been put into involved nuclear reaction network codes. In general these codes can calculate the abundance that should be observed and compare them to what is observed. Circles – data Line - theory While the agreement is ok, it is not perfect and we still need more experimental data. Such studies are now in progress using exotic, radioactive beams. Is there any other process that we have not considered? Yes….

43. Cosmic Ray Nucleosynthesis - Have discussed all except three elements, namely B,Be and Li - Concentrations observed in cosmic rays at earth’s surface very high - Cannot explain their production in stellar environments or BB Cosmic ray abundances Cosmic rays are very energetic, fast moving nuclei, primarily H and He. Interaction of such species with gaseous materials found in universe such as hydrogen, helium, carbon, oxygen, etc. can induce spallation reactions and this could be the production source of the Li, B, Be. Solar

44. SummaryWe are all stardust: nuclear debris • Lightest elements, H, He, some Li: Big Bang • Li, Be, B: cosmic rays • C-Ni: made in stars, ejected in in a supernova • All elements heavier than Fe, Ni: neutron capture • Slow neutron capture: formed in giant stars • Rapid neutron capture process: occurs in supernovae, neutron star mergers? • Many, many tons of material !! Lecture available on http://trshare.triumf.ca/~isol/

45. We Are All Stardust: Origin of the Chemical Elements Questions 1. Indicate the number of nucleons in each of the following, namely, 12C, 18C, 56Fe, 235U, _____ protons, ______neutrons; _____ protons, ______neutrons; _____ protons, ______neutrons; _____ protons, ______neutrons. 2. Calculate the number of atoms in 10 g of 12C and in natural carbon. (stable isotopes of carbon are 12C: 98.89% and 13C: 1.11%) ___________. 3. The atomic mass of a hydrogen atom (1H) is 1.0078252 u and of the neutron is 1.0086649 u. If the atomic mass of 12C is 12.0000 u, calculate the energy (J) needed to split 1 atom of 12C (1 u = 1.66 x 10-27 kg; c = 3.00 x 108 m/s). Remember A. Einstein’s equation with Energy = mass x c2. 4. If the Sun releases energy at the rate of 3.8 x 1026J/s and the Sun has a mass of ~1033g, how long would the Sun last if a nuclear reaction releases about 1 MeV per atom? (1 MeV = 1.6 x 10-13 J; Avogadro’s number is 6.02 x 1023) ____________. 5. Name three of the most abundant elements in the Universe. _________ __________ __________. 6. What elements were created in the first three minutes of the Big Bang? _________ __________ __________. 7. Calculate the energy released in the fusion reaction of two 3He nuclei to form the 4He and two 1H particles. (use mass excesses as if they are mass values; 3He: 14.931 MeV, He: 2.425 MeV; 1H: 7.289 MeV) ______________. 8. Give a concise reason why the p + p reaction in stars leading to the production of deuterium is very, very slow. _________________________________________________________. 9. What elements, which are not produced in large numbers in normal stars, are produced by and with cosmic rays? ______________ ____________ ___________. 10. Name the process by which heavy elements such as lead and uranium are produced in exploding stars. _________________________________. 11. Name the process by which the element Carbon is produced in stars. 12. What is the name of the scientific technique by which the abundance of elements are measured in stellar environments such as the Sun? 13. In your own words describe briefly what a star is.