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 NucleosynthesisWithin 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 surewhere 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

everythingelse

90% hydrogen atoms

10% helium atoms

Less than 1% everything else(and everythingelse 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 (1H) => 4He + 2 e+ + 2 neutrinos + energyWhere does the energy come from ?Mass of four 1H > Mass of one 4He

E = mc2

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 (4He) => 12C + energy

After helium exhausted, a small star is not large enough to reach temperatures necessary to fuse carbon to heavier elements

HeliumBurning

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

SupernovaFusion of iron takes energy, rather

than releases energySo 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 beforesupernova 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 elementsDisperses 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 byneutron 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 processesSupernovae

High neutron fluxType II Supernovae

(massive stars)No time for-

decayMakes Eu, Gd, DyPlus some Sr, Y, Zr,

Ba, La…

Red GiantsLow neutron fluxTime for -decay

before next neutron capture

Makes Sr, Y, Zr, Ba, La, Ce

But no Eu, Gd, Dy

Heavy MetalsAll heavier elements are formed

when iron peak elements captureneutrons

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

Light nucleus

Interstellar matter (~1 hydrogen atom per cm3)

Cosmic ray

White - Big Bang Pink - Cosmic RaysYellow - Small Stars Green - Large Stars

Blue - Supernovae

Cosmic Elements

Nucleosynthesis in Stars

BetelgeuseRed Giant making Ca and beyond. Future supernova.

Rigel - Blue Supergiantmaking, He, C, N. Futureheavy elements.

Orion NebulaNew stars gettingheavy 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!