Main sequence stars

Main sequence stars
Haris Ðapo
1 Akdeniz
University, Antalya
Antalya
Haris Ðapo (Akdeniz University)
• Lecture 3.
Main sequence stars
Main sequence stars
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Outline
1
Introduction
2
Hydrogen burning
pp chain
CNO cycle
3
Energy transport
4
Surface
5
Summary
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Main sequence stars
Main sequence stars
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Outline
1
Introduction
2
Hydrogen burning
pp chain
CNO cycle
3
Energy transport
4
Surface
5
Summary
Haris Ðapo (Akdeniz University)
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Main sequence stars
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Main sequence star
a main sequence star burns hydrogen
Proxima Centauri and Sun
Procyon Celestia and Archenar
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Hertzsprung - Russell diagram
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Basics
After the collapsing phase to a main-sequence star, along the path
on the H-R diagram, a star starts to burn hydrogen
the longest phase in the stars life
M (M⊙ )
30
15
9
5
3
1.5
1.0
0.5
0.1
Haris Ðapo (Akdeniz University)
MAIN SEQUENCE (106 yrs)
4.9
10
22
68
240
2,000
10,000
30,000
107
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Outline
1
Introduction
2
Hydrogen burning
pp chain
CNO cycle
3
Energy transport
4
Surface
5
Summary
Haris Ðapo (Akdeniz University)
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pp chain - Idea
proton - proton chain reaction (pp chain) is how stars converts
hydrogen to helium
pp chain dominates in stars the size of the Sun or smaller and was
the only way Pop. III stars could burn hydrogen
pp fusion if kinetic energy of the protons is high enough to
overcome Coulomb barrier
the reaction proceeds via tunneling and would not be practical
otherwise
however helium-2 is unstable!; we need beta decay to change one
proton into a neutron immediately
In the Sun, deuterium-producing events are rare enough that a
complete conversion of its hydrogen would take more than 1010
years
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pp chain - Network
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pp chain
e− + e+ → 2γ(1.02MeV )
ppI chain releases a net energy of 26.7 MeV.
the pp I branch is dominant at temperatures of 10 to 14 MK
the pp II branch is dominant at temperatures of 14 to 23 MK
90% of the neutrinos in the reaction 7 Be(e− , νe )7 Li ∗ have energy
of 0.861 MeV, while the rest 10% carry 0.383 MeV
the pp III chain is dominant if the temperature exceeds 23 MK
pp III chain generates high energy neutrinos (up to 14.06 MeV)
hep has never been observed due to its great rarity
the total energy yield of one whole chain is 26.73 MeV
only energy released as gamma rays supports a star against
gravity, neutrinos do not
the neutrinos in the ppI, ppII and ppIII chains carry away 2.0%,
4.0% and 28.3% of the energy in those reactions
in more massive stars this ratios will change in favor of neutrinos
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Sun’s neutrino spectrum
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CNO cycle I (the other hydrogen burning)
CNO for carbon-nitrogen-oxygen
CNO cycle dominates in stars > 1.3M⊙
pp-chain starts at 4 × 106 , CNO chain starts at 13 × 106 K
because of stronger temperature dependence CNO starts to
dominate at 17 × 106 K (Sun: 15.7 × 106 K)
carbon, nitrogen and oxygen are catalyst for conversion of
hydrogen into helium
there is also and extended CNO cycle (with fluorine), but it is rare
while in CNO no “new” C or N or O nuclei are produced in run up
to equilibrium, relative proportions of the nuclei are altered
star with CNO are limited to max mass of about 150 M⊙ while
primordial starts with only hydrogen and helium could go up-to
250 M⊙
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CNO cycle II
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Helium core
As it converts hydrogen to helium star
forms a helium core.
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Outline
1
Introduction
2
Hydrogen burning
pp chain
CNO cycle
3
Energy transport
4
Surface
5
Summary
Haris Ðapo (Akdeniz University)
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Radiation and Convection zones
convection zone of a star is the range of radii in which energy is
transported primarily by convection
in the radiation zone, energy is transported by radiation
stellar convection consists of mass movement of plasma
convection only occurs if the temperature changes rapidly (i.e.
there is a large difference between their bottom and top
temperatures
because of CNO cycle strong dependence on temperature stars
slightly more massive than the sun will have a convective core
in sun and similar stars the core will be in radiative zone
however in very low mass stars and red giants the entire star will
be in convection zone
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Convection
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Outline
1
Introduction
2
Hydrogen burning
pp chain
CNO cycle
3
Energy transport
4
Surface
5
Summary
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Photosphere
the photosphere is the region from which externally received light
originates
a stars spectral type and temperature originate in the photosphere
of course the atmosphere of a star is partially transparent so the
last scattering point for photons coming out of the star lies in a
region and is not a sharp surface
it also depends on the energy of the photon
this leads to various definitions of where you define the surface
and thus the temperature
we can only investigate the photosphere of sun in detail
the sun: 4500 - 6000 K; effective temperature 5800 K
material constantly rises to the surface and falls down (boiling)
above photosphere are chromosome and corona
features: solar flares and sunspots
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Outline
1
Introduction
2
Hydrogen burning
pp chain
CNO cycle
3
Energy transport
4
Surface
5
Summary
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Summary
main sequence stars burn hydrogen
via pp chain reaction of CNO cycle
stars spend most of there life as a main sequence star
convective and radiative zones
what astronomers observe is the photosphere of a star
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Îlginiz için teşekkür ederim!
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Main sequence stars
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