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Transcript
Outline
Introduction
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
90% hydrogen atoms
10% helium atoms
Less than 1%
everything else
M
Finding the Composition of a Star
Rainbows
from
starlight
The Solar Spectrum – In Detail!
Stellar Spectra
Abundances of the Elements
Log e (H=12)
14
11
H
He
CNO
8
Fe
5
Sr, Y, Zr
Sc
2
Li, Be, B
Ba
Eu
-1
10
20
30
40
50
Atomic Number
60
70
80
Solar Abundances EXPANDED
CNO
Log e (H=12)
8
Fe
5
Sr, Y, Zr
Sc
2
Ba
Li, Be, B
Eu
-1
10
20
30
40
50
Atomic Number
60
70
80
The Life Cycles of Stars
• Fundamental Properties
of Stars
–
–
–
–
Mass
Temperature
Luminosity or radius
Age
• Star Birth
• Middle Age
• Stellar Senior Citizens
But only
certain sizes
and colors
are allowed!
Stars come in many sizes and colors
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
The Creation of Elements…
“Just between you
and me, where does
it get enriched?”
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.
Hydrogen
Burning
Stars burn hydrogen in their
interiors to produce helium.
Hydrogen burning also rearranges
carbon, nitrogen, and oxygen.
Helium
Burning
Three helium atoms
combine to form carbon
Alpha-Process
Elements
CNO
Log e (H=12)
Carbon and oxygen
atomsFecombine
8
to form elements with even
5
numbers of protons.
Sr, Y, Zr
Sc
2
Ba
Li, Be, B
These elements come from
supernovae. -1
10
20
30
Eu
40
50
Atomic Number
60
70
80
The Iron
Peak Metals
In the cores of massive stars during
supernova explosions, atomic nuclei
exchange protons and neutrons to form
the iron peak metals.
CNO
Log e (H=12)
8
Fe
5
Sr, Y, Zr
Sc
2
Ba
Li, Be, B
Eu
-1
10
20
30
40
50
Atomic Number
60
70
80
Heavy Metals
All heavier elements are formed
when iron peak elements capture
neutrons
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
Eu
1
Br
Fraction of r-process
As
Se
Rh
Te
Sb
Ag
Ru
Rb
Pd
Ge
Kr
Zn
Sm
Xe
Pr
Cd
Mo
Y Nb
Tm
Lu
Yb Ta
Dy
Gd
Er
Os Au
Re Pt
Th
Bi
Ir
U
Hf
Hg
Sn
I
Sr
La Nd
Ce
W
Tl
Pb
Ba
By studying
Purple elements
the abundance
were formed
patterns
of
in the
supernovae
heavy metals,
(the r-process)
we can learn
and
whether
pink elements
the metals
were
were
formed
madeinin
supernovae
red giant stars
or in low
(themass
s-process)
stars.
Ga
Zn As Kr
Cs
In
Zr
0
Tb
Ho
Y
Mo Pd
In
Te
Cs Ce Sm Tb
Er
Lu
W
Ir
Hg Bi
A History of the Milky Way
What is a galaxy?
Structure of the
Milky Way
How did our galaxy
form?
You are
here.
The Milky Way….
Halo
Flattened Inner Halo
Disk
Bulge
Thick Disk
Dwarf Spheroidal Companions
Dark Matter Corona
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 Mystery
Metals
The abundances of the “light” heavy metals
in ancient stars are too high to explain with
our current theory of supernova r-process
production
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
La
Ba
138
139
p
s,r
130
132
134
135
136
137
138
p
p
s
s,r
s
s,r
s,r
133
Cs
Xe
s,r
128
129
130
131
132
134
136
s
s,r
s
s,r
s,r
r
r
s-process path
r-process path