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Chapter 5: Cosmic foundations
for origins of life - stars
Stellar evolution: forming the elements
for biolmolecules and planets….
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Stars are fusion reactors that convert lighter elements into
heavier ones, liberating energy (from E=mc^2)
They therefore continuously evolve as their fuels are used
up. H burns to He, He burns to C, etc…
Stellar end-states: white dwarfs, neutron stars, or black
holes.
In all of these cases, significant fraction of stellar mass,
ejected into interstellar medium
Planets, and biomolecules made out of these materials
Energy liberated when light atomic nuclei
undergo fusion! (eg. Proton-proton reaction)
• Two protons colliding at
high enough speed,
undergo fusion.
• Products: a deuteron
(heavy water), a positron
(positively charged
electron), and a neutrino
(very weakly interacting
particle) + energy release:
• Special Relativity: Energy
release per fusion is
proportional to mass
difference between products
and reactants
E fusion  (m)c
2
Energy production in the Sun’s core –
the proton-proton chain
Net result of p-p burn:
For each Helium-4 nucleus produced:
- Consume 4 protons
- Liberate energy and 2 neutrinos
- Neutrinos arise from weak interaction.
eg. they arise during conversion of a proton to
a neutron in building a deuteron
4(1H )4He  2  energy
Nuclear reactions yield predictable neutrino fluxes
from the Sun that directly reflect reaction rates
Stellar temperatures:
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Stars have
different colours
Corresponds to
different
temperatures
from black body
curves
Note *huge*
(and harmful)
UV fluxes
produced by
massive stars
(life possible on
planets around
them?)
Spectral Classification of Stars – Consequence of
stellar temperatures:
- Stellar spectra can be
divided into “spectral
classes” of stars –
O,B,A,F,G,K,M
- Atomic theory: this
represents a sequence
of decreasing
temperature - hot stars
are more completely
ionized than cool stars
so see fewer absorption
features.
- The Sun is a G2 star.
Hertzsprung-Russell Diagram: Plotting L vs. T
Luminosity L and temperature T of a star are
independent physical properties of a star.
-Temperature correlates with colour of a star (hot is
blue, cool is red). L varies by factor of 100 million!
-Plot L of a star vs. its colour on a diagram: find that
these are correlated with one another. Known as
“colour-magnitude diagram”.
- Most stars occur along “main-sequence”, where they
burn hydrogen.
H-R Diagrams (L vs. T) of
Nearest, and brightest stars
Stars within 5pc of Sun
100 brightest stars in the sky
STELLAR RADII:
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Range from
0.08 of the Sun,
to 630 times the
Sun’s radius
(Betelgeuse)
Giants: radii of
10 – 100 solar
radius
(Mira is Red
Giant)
Supergiants: up
to 1000 solar
radii
Main Sequence: Stars confined to well defined band from
top left (high T, high L), to bottom right (low T, low L).
•Temperature range over main sequence: 3,000K (M
type) – 30,000 K (O type); 1 decade in temperature
• Range in luminosities over 8 decades!
- partly explained by black body relation;
L T R
4
2
• At top end – stars are hot and large: blue supergiants
• At bottom end – stars are cool and small: red dwarfs
• O and B stars extremely rare: one in 10,000
Stars spend most of their life on main-sequence burning
hydrogen
Off the main sequence:
 Red giants (upper right of H-R diagram:
high L, low T); and white dwarfs (lower
left: low L, high T).
 Red giants burn hydrogen in a shell
 White dwarfs hard to detect – very faint
 Sun will go through red-giant phase and
end up as a cooling white dwarf.
 Red giant will swell to orbit beyond
Earth… consequences for life!
Main sequence is a
mass sequence: ie
stellar mass determines
stellar properties
LM
3
Structure of Red Giant star – furious
hydrogen burning occurs in a shell
gradually moving out through unburned
material. Non-burning He ash
accumulates in core.
After 10 billion years, core of solar
mass star uses up H, and consists
of He. Fusion ceases at centre of
core, and it begins to contract.
Star leaves main sequence.
Red Giant Branch:
Subgiant Branch: Stage
7 – Stage 8;
-H burns in a shell, He
ash accumulates in core.
Red Giant Branch: Stage
8 – stage 9:
- Outer layers of star so
cool that convection
throughout star occurs –
so ascend a vertical
track
Tip of Giant Branch:
-Radius is 100 solar radii
(size of Mercury’s orbit)
- He core is 1/1000 size of
star - few times larger
than the Earth. 25% of
stellar mass locked up in
core
- 10,000 times the
luminosity of the Sun.
- Core density, about 100
million kg/ cubic metre.
- Envelope density, about
1/1000 kg/ cubic metre
Helium fusion: the Triple-Alpha Process
fine tuning!
At stage 9 – tip of Giant Branch – central temperatures are
100 million K, at densities of 108 kg/cubic metre, conditions
allow ignition of helium “ash” accumulating in stellar core:
4
He  4He8Be  energy
8
Be  4He12C  energy
• Beryllium – 8 highly unstable. Decays very quickly into 2
16
alpha particles again - about 10 sec ! SLIGHT CHANGE
IN STRENGTH OF NUCLEAR FORCE AND THIS
REACTION IS IMPOSSIBLE!
• Resonant interaction between Be and alpha particle allow
second reaction above to occur - carbon is the “ash”
Horizontal Branch – Helium “Main-Sequence”
Helium Flash: Explosive
onset of He burning at tip
of Red Giant Branch
(RGB). (stage 9)
• He burning core (stage
10) known as Horizontal
Branch.
Ascending the Asymptotic Giant Branch – the
Accumulation of Carbon
When He in core of star
on Horizontal Branch is
used up – He shell burn
commences – star moves
off Horizontal Branch.
• Now have 2 burning
shells, H, and deeper in,
He – with Carbon “ash”
accumulating in core
• Star moves up
“asymptotic giant branch”
increasing in size and
luminosity. Carbon core
continues to contract
• Horizontal Branch (stage
10): He core burn – and H
shell burn. The “MainSequence” for He burning.
• Asymptotic Giant Branch
(AGB) stage 10 – stage
11: Shell burning for both
He and H. Carbon ash
accumulates in core.
•Produces much larger red
star – Red Supergiant
[500 solar radii – swallows
Mars!, surface
temperature 4000 K,
central T 250 million K.
5 billion years into the future – the fate of the Sun
Planetary nebula – NGC
3132.
- ejection of envelope of
star leaving a degenerate
stellar core (white dwarf).
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White dwarf
- Outer edge of envelope
Evolution of Massive Stars
Stars more massive than 8
solar masses lead to
supernova explosions
• High mass stars move almost
horizontally (rather than
vertically) in post mainsequence evolution:
- luminosity of star stays fairly
constant but radius
increases,reducing surface
temperature
• High mass stars fuse carbon,
oxygen, and other elements
For massive stars
(more than 8 solar
masses) – series of
burning shells – ash
of burn above it
igniting producing ash
beneath it.
• Creates an “onionlike” series of burning
layers…. at bottom of
which is iron ash.
• Carbon burns for
1000 yr, oxygen for a
yr, silicon for a week.
• Iron core grows for
less than a day!
Iron is nature’s most stable element
• Small nuclei
liberate energy
by fusion
• Elements
more massive
than iron
liberate energy
by fission into
smaller nuclei.
• IRON DOES
NOT BURN!
• Degenerate
iron core is
end-state of
nuclear fusion
Carbon burning: (a) occurs at T= 600
million K, while (b) occurs at 200 million K
The route to iron:
Oxygen Fusion:
O O S  energy
16
16
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O He Ne  energy
16
4
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Silicon Burning: Building up
to Nickel (T = 3 billion K !)
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Si  7( 4He)56Ni  energy
Nickel-56 quickly
decays via cobalt56 into stable
iron-56
Synthesizing Elements Beyond
Iron
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Occurs by “neutron capture” to iron (which just changes the
isotope), followed by radioactive decay into stable element:
eg.
56
Fe  n57Fe
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Fe  n58Fe
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Fe  n59Fe
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Fe59Co  e   
Neutron capture occurs during supernova explosion (high
density and temperature) – either by “rapid” (r) or “slow” (s)
process
Elements produced during explosion much rarer because
time available to produce them is so short
Nucleosynthesis in stars: explains
abundances of the elements
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Sharp drop in
abundance as go
to higher atomic
number – reflects
increasing
Coulomb barrier
to fusion
Peaks and
troughs in
distribution –
reflect stable
“closed shell”
nuclei, etc.
Supernova remnant: the Crab nebula (supernova
seen by Chinese astronomers in 1054 A.D.)
Canadian Galactic Plane Survey (CGPS): the interstellar
medium… stirred by supernova explosions and stellar
winds….Map of atomic hydrogen.
[Midplane of Milky Way - near constellation Perseus]