H205 Cosmic Origins  Our last class!  Origin of the Elements  Hand in EP7 APOD Tom Lehrer's "The Elements". A Flash animation by Mike Stanfill, Private Hand

Abundance of Elements in the Galaxy  How are the chemical elements created  in the Early Universe?  in Stars?  in Supernovae?  How is the Galaxy enriched in chemical elements?

Big Bang Nucleosynthesis  Within first three minutes, hydrogen & helium formed  At t =1 s, T=10,000,000,000 K: soup of particles: photons, electrons, positrons, protons, neutrons. Particles created & destroyed  At t =3 min, T=1,000,000,000 K: p+n => D  D + D => He

Hydrogen and helium were created during the Big Bang while the Universe was cooling from its initial hot, dense state. About 10% of the lithium in the Universe today was also created in the Big Bang. We’re still not sure where the rest comes from. The first stars formed from this material. Primordial Nucleosynthesis

“Just between you and me, where does it get enriched?” The Creation of Elements… STARS!

The Composition of Stars everything else 90% hydrogen atoms 10% helium atoms Less than 1% everything else (and everything else is made in stars!)

The Evolution of Stars

Making Elements in Stars Some elements are created in the cores of low mass stars like the Sun

Small Stars: Fusion of light elements Fusion: (at 15 million degrees !) 4 ( 1 H) => 4 He + 2 e neutrinos + energy Where does the energy come from ? Mass of four 1 H > Mass of one 4 He E = mc 2

Hydrogen Burning Stars burn hydrogen in their interiors to produce helium. Hydrogen burning also rearranges carbon, nitrogen, and oxygen.

Small Stars to Red Giants After Hydrogen is exhausted in core, energy released from nuclear fusion no longer counter- acts inward force of gravity. Core collapses, Kinetic energy of collapse converted into heat. This heat expands the outer layers. Meanwhile, as core collapses, Increasing temperature and pressure...

A Red Giant You Know

Beginning of Heavier Elements  At 100 million degrees Celsius, helium fuses:  3 ( 4 He) => 12 C + energy  After helium exhausted, a small star is not large enough to reach temperatures necessary to fuse carbon to heavier elements

Helium Burning Three helium atoms combine to form carbon

The end for small stars Planetary Nebulae After helium exhausted, outer layers of star expelled

Heavy Elements from Massive Stars  But their larger masses lead to higher temperatures, which allow fusion of carbon into magnesium, etc.  Large stars also fuse hydrogen into helium, and helium into carbon

“Alpha” Elements

The “Odd-Even” Effect  Carbon, oxygen, neon, magnesium, silicon, sulfur, arcon, calcium are **high**  Nitrogen, fluorine, sodium, aluminum, phosphorus, chlorine, potassium are **low**

Stellar Senior Citizens Many elements are made in supernovae when massive stars explode When stars finally deplete their nuclear fuel, they become white dwarfs, neutron stars, or black holes. In the process, much of the stellar material is returned to interstellar space

Supernova  Fusion of iron takes energy, rather than releases energy  So fusion stops at iron No nuclear fusion energy to balance inward force of gravity  Nothing to stop gravity  Massive star ends its life in supernova explosion

The Iron Peak Metals In the cores of massive stars just before supernova explosions, atomic nuclei exchange protons and neutrons to form the iron peak metals.

The “Iron Peak”  The “transition metals” vanadium through zinc, including copper, iron, and nickel are produced in supernove explosions  Most of the SN production is through Type IA supernovae (exploding white dwarfs)

Supernovae  Explosive power of supernovae:  Creates new elements  Disperses elements created in large stars All X-ray Energies Silicon Calcium Iron

Making Elements Up to Iron Hydrogen – from big bang nucleosynthesis Helium – from big bang and from hydrogen burning via the p-p chain and CNO cycle Nitrogen – from CNO cycle Carbon, Oxygen – from helium burning Light elements (Neon, Magnesium, Calcium – from carbon and oxygen burning Iron metals – from the final burning

Elements Heavier than Iron … Once iron is formed, it is no longer possible to create energy via fusion.  Elements heavier than iron require a different process (Iron is atomic number 26.) The heaviest naturally occurring nucleus is uranium (atomic number 92). How do we get to uranium then? Elements heavier than iron are created by neutron capture The neutron is converted into a proton and added to the nucleus, increasing the atomic number to make the next element in the periodic table.

Making Heavy Metals in Stars In low mass stars like the Sun, heavy metals are created when the star is a giant Massive stars make heavy metals when they become supernovae

Isotopes built by n-capture syntheses The valley of beta-stability Rolfs & Rodney (1988)

How to Make Heavy Metals: neutron-capture processes  Supernovae  High neutron flux  Type II Supernovae (massive stars)  No time for  -decay  Makes Eu, Gd, Dy  Plus some Sr, Y, Zr, Ba, La…  Red Giants  Low neutron flux  Time for  -decay before next neutron capture  Makes Sr, Y, Zr, Ba, La, Ce  But no Eu, Gd, Dy

Heavy Metals All heavier elements are formed when iron peak elements capture neutrons

Nucleosynthesis from Cosmic Rays  Lithium, beryllium, and boron are difficult to produce in stars Li, Be, and B are formed in the fusion chains, but they are unstable at high temperatures, and tend to break up into residues of He, which are very stable  So what is the origin of these rare elements?  Collisions of Cosmic Rays with hydrogen & helium in interstellar space

Cosmic Rays Collisions with ISM Lithium, beryllium, and boron and sub-iron enhancements attributed to nuclear fragmentation of carbon, nitrogen, oxygen, and iron with interstellar matter (primarily hydrogen and helium). (CNO or Fe) + (H & He) ISM  (LiBeB or sub-Fe) Light nucleus Interstellar matter (~1 hydrogen atom per cm 3 ) Cosmic ray

White - Big Bang Pink - Cosmic Rays Yellow - Small Stars Green - Large Stars Blue - Supernovae Cosmic Elements

Nucleosynthesis in Stars Betelgeuse Red Giant making Ca and beyond. Future supernova. Rigel - Blue Supergiant making, He, C, N. Future heavy elements. Orion Nebula New stars getting heavy elements. Future Earths?  3 steadily making He. Future C, N

Matching Our Universe  What would the composition of our universe be if only very large stars formed?  Only very small stars?

Composition of the Universe (Actually, this is just the solar system) Composition varies from place to place in universe, and between different objects

Chemical Evolution  Elements are created in stars and mixed back out into the Galaxy  The Galaxy’s composition changes as stars form, evolve, and die

Chemical Evolution  Nucleosynthesis in stars leads to chemical enrichment of the Galaxy  Rate of enrichment depends on sites and mechanisms of nucleosynthesis  The variables are:  Star formation rate  Initial mass function  Yields  Stellar evolution time  Primordial nucleosynthesis  Hydrogen burning  Proton-capture chains  Helium Burning  Alpha Process  Equilibrium process  Neutron-capture processes  Odd-ball stuff

Nucleosynthesis since the beginning of time By studying stars of different ages, formed at different times in the Galaxy’s history, we can trace the history of the Milky Way

The Galaxy (and the universe) is gradually enriched in heavy elements Despite all the nucleosynthesis that has occurred since the creation of the universe, only 2% of the ordinary matter in the universe is now in the form of heavy elements. Most is still hydrogen and helium

Your Cosmic Origin

We’re Done! Final Reflection due by 4:45 PM on Friday, May 8 Earlier submission is welcome!