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The Sun and the Stars
The Sun and the Stars
Dr Matt Burleigh
The Sun and the Stars
A star like our Sun never becomes hot enough in core
to burn heavier elements!!
Evolutionary tracks for stars of differing mass
II evolution of 5 solar mass star (pop I)
For a M> 1Msun core hot enough (>10 million K)
for CNO cycle to operate
12
C 1H 13N  
13
N 13C  e   
C 1H 14 N  
13
14
15
15
CNO cycle
N H O  
1
15
O15N  e   
N 1H 12 C  4He
Net effect, 4 H 1He (CNO just catalysts)
Enuc  Tc
18
c.f.
Enuc  Tc
4
for PP chain
(Enuc = energy released per unit mass)
O stars only spend 106 years on main sequence!!
Dr Matt Burleigh
The Sun and the Stars
Stellar structure :
Lower Main Sequence
(Eg Solar masses)
Upper Main Sequence
(Eg 5 Msun)
radiative core,
convective outer layers
well-mixed convective core,
radiative envelope
energy via PP chain on MS
energy mostly by CNO cycle
on main sequence
CNO dominates above 20million K
Dr Matt Burleigh
The Sun and the Stars
NB UMS stars are convective in the core, materials are well mixed
Supernova?
Log(L/Lsun)
Once H in core virtually exhausted,
core contracts due to drop in pressure,
H burns in shell around He core. Burnt
material (He) added to core, density increases.
Eventually core becomes so dense
contracts and heats up energy generation
in shell accelerates, outer envelope expands,
surface temperature drops (moves to right
on HR diagram).
Lower temperature increases opacity, convection
in envelope increases transport of energy to
surface, luminosity rises RGB
~1.34Msun
PN ejected
to WD
4
AGB
He
H
Thermal Pulse begins
C+O
He ignition
Helium
Flash
He
RGB
2
0
WD
cooling
4.2
3.8
3.4
Log Teff (K)
Core continues to contract (T> 100million K)
 Triple-alpha process in core. He ignites, but no
helium flash)
Dr Matt Burleigh
The Sun and the Stars
NB UMS stars are convective in the core, materials are well mixed
Supernova?
Log(L/Lsun)
Rapid expansion of core leads to reduction in
luminosity (much less dramatic in this case). He
burning core H burning shell.
PN ejected
to WD
4
He
H
Eventually He burning dominates, surface temp
rises (moves to left on H-R diagram)
2
Finally He in core exhausted, C core contracts,
now He burning in shell, and H burning in shell,
heat from contraction, accelerates process, outer
envelope expands  AGB
0
12
Thermal Pulse begins
C+O
He ignition
Helium
Flash
He
RGB
WD
cooling
After this evolution uncertain, thermal pulses drive
strong wind leaving PN and exposed core, or star
may become Supernova
5-8Msun stars can burn Carbon to Oxygen via
(2nd loop on H-R diagram)
AGB
4.2
3.8
3.4
Log Teff (K)
C  4He 16O  
Serious mass loss via strong superwinds
Dr Matt Burleigh
The Sun and the Stars
III Evolution of >8 solar mass star
Apart from an initial rise on main sequence, evolution is almost horizontal.
i.e. evolution occurs at almost constant luminosity (blue giant  red-giant  blue giant etc)
Large stellar winds/mass loss even on main sequence (~10-6 to 10-7 Msun/yr).
Usual reactions - PP-chain+CNO 4 H  4 He
3 4 He 12 C
12
But also
T~107 K
T~108 K
C  4He16O  
16
O 4He 20Ne  
12
C  12C  23Na  H
 23Mg  n
T~108 K
T~6x108 K
 24 Mg  
16
O  16O 24 Mg  2 4 He
T~6x108 K
 28Si  4 He
 31P  H
 31S  n
 32 S  
Dr Matt Burleigh
The Sun and the Stars
and, -processes, e.g.
28
Si  4He  32 S
32
S  4He  36 Ar

56
Fe
56
Ni
-process must end at Fe because Fe is at peak of BE curve
All reactions beyond Fe endothermic i.e. require more energy than they produce
Dr Matt Burleigh
The Sun and the Stars
Ignition of each new burning process is
preceded by contraction and heating up of core.
Stellar evolutionary track oscillates on HR
diagram as each new energy source
becomes available
H+He envelope
H burning
He burning
C burning
O burning
Si burning
Fe core
NB burning timescales very short at end
(e.g. HeC 106 years, CO 103 yrs,
SiFe in just a few days)
If at the end of -process,core mass exceeds 1.4Msun (Chandrasekhar limit),
electron degeneracy pressure can no longer support star and core collapses (1~sec).
When T> 6x109K, photodisintegration occurs
56
Fe    134 He  4n
4
Endothermic!!
requires 100Mevcollapse accelerates
He  2 p  2n
p  e   n  e
Energy removed rapidly from core, core contraction accelerates
Dr Matt Burleigh
The Sun and the Stars
Core: R~ 0.01Rsun, ~1015 g/cm3, M~1.4Msun
Eventually neutron degeneracy pressure opposes collapse (can be exceeded by ~50%),
core-bounce – contraction of core heats outer layers which burn explosively, star literally
explodes as a Type II Supernova leaving dense core of neutrons (R~5km)
– a neutron-star
Simulations suggest initial shock wave may stall, neutrino trapping may help
drive off outer layers. r-processes (rapid) manufacture heavy elements
L~109 Lsun (Mv= -20)
If M>25 Msun , neutron degeneracy pressure cannot halt collapse
– star becomes a black-hole
Dr Matt Burleigh
Type II Supernovae
Dr Matt Burleigh
Evolutionary phases of a massive star
Stage
Physical processes
Protostar
Dust and gas cloud collapses rapidly,
accompanied by heating of the interior and
ionisation of atoms
PMS
Semihydrostatic equilibrium; contraction and
heating continue
ZAMS
Hydrogen burning commences
Initial evolution on the main
sequence
Hydrogen consumed in the core; some
contraction occurs
Evolution off the main sequence
Hydrogen depleted in the core, isothermal helium
core and hydrogen-burning established
Evolution to the right in the H-R
diagram
Core rapidly contracts, envelope expands,
hydrogen-burning shell narrows
Red giant
Energy output increases, convective envelope
forms, helium burning begins
Cepheid
Convective shell contracts, core helium burning
becomes the major energy source
Supergiant
Helium-burning shell forms
Dr Matt Burleigh
Type II Supernovae
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Explosion of stellar core stellar core to form neutron star
Absolute magnitudes from –16 to –20 (energy ~1044J)
– e.g. China, SN of 1054 reached mV=-6 (remnant is Crab Nebula)
Ejects a large fraction of original mass with v~5000-10000 km s-1
Seen in spiral galaxies only, especially in spiral arms… Population I stars
SN 1987A in LMC
Dr Matt Burleigh
Dr Matt Burleigh
Type Ia Supernovae
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Seen in both elliptical and spiral galaxies… Population I & II stars
Progenitors are H-deficient, highly evolved stars
Mechanism not well understood
– Single degenerate: Accretion onto a white dwarf from a companion star increases MWD >
Chandrasekhar limit
– Double degenerate: Merger of two WDs to give M > 1.4M
– Both mechanisms may operate
Dr Matt Burleigh
SN2014J in M82: closest SNIa for 42 years
Discovered by Dr Steve Fossey & students at University College London’s Mill Hill
Observatory (0.35m telescope) on 21st January 2014
Dr Matt Burleigh
Supernovae: Key Points
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SN responsible for nucleosynthesis of element above 56Fe
Remnant neutron stars… sometimes revealed as pulsars
Shockwave heating of interstellar medium… Supernova Remnants
Dr Matt Burleigh
Vela
Supernova Remnants
Crab Nebula
Dr Matt Burleigh
Supernova Remnants
Cassiopiea A
Dr Matt Burleigh
Supernova
expansion
Dr Matt Burleigh
The Sun and the Stars
Example SN light-curves, type I and type II
Crab nebula 1054 SN
Dr Matt Burleigh
The Sun and the Stars
IV Evolution of very low mass stars
Stars < 0.08Msun never make it onto the main sequence. Gravitational contraction
does not heat the gas efficiently, and their cores are degenerate long before they
are hot enough to start thermonuclear reactions.
These “failed stars” simply cool as Brown dwarfs
Dr Matt Burleigh