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OutlineIntroduction
The Life Cycles of Stars
The Creation of Elements
A History of the Milky Way
Nucleosynthesis since the Beginning of Time
INTRODUCTION
Abundances of the elements Composition of stars How do we learn what stars
are made of?
The Composition of Stars
M90% hydrogen atoms
10% helium atoms
Less than 1% everything else
Finding the Composition of a Star
Rainbows from
starlight
The Solar Spectrum – In Detail!
Stellar Spectra
Abundances of the Elements
HeH
Eu
BaSr, Y, ZrSc
Li, Be, B
CNO
Fe
-1
2
5
8
11
14
10 20 30 40 50 60 70 80
Atomic Number
Lo
g e
(H
=12)
Solar Abundances EXPANDED
Eu
BaSr, Y, ZrSc
Li, Be, B
CNO
Fe
-1
2
5
8
10 20 30 40 50 60 70 80
Atomic Number
Lo
g e
(H
=12
)
The Life Cycles of Stars• Fundamental
Properties of Stars– Mass– Temperature– Luminosity or radius– Age
• Star Birth• Middle Age• Stellar Senior Citizens
Stars come in many sizes and colors
But only certain
sizes and colors are allowed!
The Main Sequence
The sun is an
ordinary, yellow main
sequence star
Giants are cool and very large
Most stars occur in these main groups in the luminosity-temperature diagram
Main Sequence
Giants Supergiants White
Dwarfs
Quiz: Which star is the biggest?
A B C D
Quiz: Which star is the smallest?
A B C D
The Evolution of Stars
The Ages of Stars
How old is the Jewelbox?
The Jewelbox Cluster is about 12 million years old
Star Birth
• New stars are born continuously• Stars form in dense regions of gas and
dust
Stellar Middle Age
Stars produce energy by converting hydrogen into helium through nuclear reactions in their interiors.
The Proton-Proton Cycle
Stars produce energy by fusing hydrogen nuclei into helium, and helium into heavier nuclei
Stellar Senior Citizens
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
“Just between you and me, where does it get enriched?”
The Creation of Elements…
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.
Primordial Nucleosynthesis
Hydrogen Burning
Stars burn hydrogen in their interiors to produce helium.
Hydrogen burning also rearrangescarbon, nitrogen, and oxygen.
HeliumBurning
Three helium atoms combine to form carbon
Alpha-ProcessElements
Carbon and oxygen atoms combineto form elements with even numbers of protons.
These elements come fromsupernovae.
Eu
BaSr, Y, ZrSc
Li, Be, B
CNO
Fe
-1
2
5
8
10 20 30 40 50 60 70 80
Atomic Number
Lo
g e
(H
=12
)
The Iron Peak Metals
In the cores of massive stars duringsupernova explosions, atomic nuclei exchange protons and neutrons to form the iron peak metals.
Eu
BaSr, Y, ZrSc
Li, Be, B
CNO
Fe
-1
2
5
8
10 20 30 40 50 60 70 80
Atomic Number
Lo
g e
(H
=12
)
Heavy MetalsAll heavier elements are formed
when iron peak elements captureneutrons
Heavy Metals from Ancient Superstars
Some heavy metals are made in supernovae when massive stars explode
The explosion produces many free neutrons, which combine with iron to make heavier metals
Making Heavy Metals in Low Mass Stars
• In low mass stars like the sun, neutrons are created when protons mix into the hot, helium-burning region
• With only a few neutrons available, a different set of heavy metals are created
Neutron Capture Elements in the Solar System
Zn
Ga
Ge
AsSe
Br
Kr
Rb
Sr
Y
Zr
NbMo
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
YbLu
Hf
Ta
W
ReOs
Ir
Pt
Au
Hg
TlPb
Bi
Th
U
0
1
Fra
ctio
n o
f r-
pro
cess
Zn As Kr Y Mo Pd In Te Cs Ce Sm Tb Er Lu W Ir Hg Bi
Purple elements were formedin supernovae (the r-process) and
pink elements were formed inred giant stars (the s-process)
By studying the abundance patterns of the heavy metals, we can learnwhether the metals were made insupernovae or in low mass stars.
A History of the Milky Way
What is a galaxy?Structure of the Milky WayHow did our galaxy form?
You are
here.
The Milky Way….
Bulge
Flattened Inner Halo
Thick Disk
Dwarf Spheroidal Companions
Dark Matter Corona
Halo
Disk
Formation of the Milky Way
Galaxy Collisions and Mergers
Chemical Evolution
The creation of elements in stars leads to chemical enrichment of the Galaxy
The composition of the Galaxy depends on How fast stars form What kinds of stars
form How long it’s been
since stars started to form
The Chemistry of Stars
The chemical compositions of stars reflect the star formation histories of stellar populations
The complexity of the Milky Way’s history is reflected in the compositions of its stars
Heavy Metals in the Early Galaxy
In the early galaxy, elements were forming very quickly from new star formation.
The chemical mixture we see is different from the Solar System
We find a much smaller amount of “metals” mixed in with the hydrogen and helium
Heavy metals come only from supernovae – not from low mass stars
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
Low Mass Stars Begin to Contribute Heavy Metals
Low mass stars begin to contribute heavy metals when the Galaxy reaches an age of a few hundred million years.
The abundances of the “light” heavy metalsin ancient stars are too high to explain with our current theory of supernova r-process production
The Mystery Metals
Where to the first metals come from?
Ancient Superstars!
Formation of stars as “pre-galactic” objects from small density fluctuations
Masses from a few tens to a few hundred solar masses
Low mass star formation is suppressed because the first, massive stars reheat the gas
These stars form the first metals
Metals in Very Distant Galaxies Studies of the most metal-poor stars in the
Galaxy give us access to the state of the Universe at very early times
The most metal-poor stars in our Galaxy contain only 1/10,000 of the amount of metal that the sun has.
The most distant galaxies we can study contain 1/100 (1%) of the amount of metal that the sun has
The Epochs of Galactic Chemical Evolution
Primordial Epoch -The Big Bang (hydrogen, helium, lithium)
Epoch of Massive Stars – the first few million years Ca, O, and the “mystery metals”
Supernova Epoch - r-process elements from 8-10 MSun SNII The first few 10’s of millions of years
The Red Giant Epoch yields s-process elements The first few hundreds of millions of years
The Iron Epoch – most of the iron comes from low mass stars The first billion years
The Lithium Epoch – Where does lithium come from??
Websites of Interest
Jewels of the night http://www.noao.edu/education/jewels/home.html
National Optical Astronomy Observatory Image Gallery http://www.noao.edu/image_gallery
Space Telescope Science Institute http://www.stsci.edu
Amazing Space http://amazing-space.stsci.edu
NASA’s Astronomy Picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html
Astronomical Society of the Pacific http://www.astrosociety.org
The Stonebelt Stargazers http://www.mainbyte.com/stargazers/
n-capture Synthesis Paths
Ba
La
Cs
Xe
139
132131130129128
130 132
133
134 136
134 135 136 137 138
138
pp s,rs,r s,r
s,r
s,r
s,r
s
rs,r r
p
s
s,r ss
r-process paths-process path
