Transcript The Origin of the Elements

H205
Cosmic Origins
APOD
Tom Lehrer's "The Elements".
A Flash animation by Mike
Stanfill, Private Hand
Our last class!
Origin of the Elements
Hand in EP7
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
Primordial
Nucleosynthesis
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.
The Creation of Elements…
“Just between
you and me,
where does it
get enriched?”
STARS!
The Composition of Stars
90% hydrogen atoms
10% helium atoms
Less than 1%
everything else
(and everything
else is made in stars!)
everything
else
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 + energy
Where does the energy come from ?
Mass of four 1H > Mass of one 4He
E=
2
mc
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 counteracts 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
Helium
Burning
Three helium atoms
combine to form carbon
The end for small stars
After helium exhausted, outer layers of star expelled
Planetary Nebulae
Heavy Elements from
Massive Stars
Large stars also fuse hydrogen into
helium, and helium into carbon
But their larger
masses lead to higher
temperatures, which
allow fusion of carbon
into magnesium, etc.
“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
Calcium
Silicon
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
Making
Elements Up to
Iron
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 b-decay
Makes Eu, Gd, Dy
Plus some Sr, Y, Zr,
Ba, La…
Red Giants
Low neutron flux
Time for b-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
Cosmic ray
Light nucleus
Interstellar matter
(~1 hydrogen atom per cm3)
Light nucleus
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)
Cosmic Elements
White - Big Bang
Pink - Cosmic Rays
Yellow - Small Stars
Green - Large Stars
Blue - Supernovae
Nucleosynthesis in Stars
p3
Betelgeuse
steadily making He.
Future C, N
Red Giant making Ca
and beyond. Future
supernova.
Orion Nebula
New stars getting
heavy elements.
Future Earths?
Rigel -
Blue Supergiant
making, He, C, N. Future
heavy elements.
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
 Primordial
mechanisms of
nucleosynthesis
nucleosynthesis
 Hydrogen burning
 The variables are:
 Star formation rate
 Initial mass function
 Yields
 Stellar evolution time
 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!