The Internal Structure of the Sun
The Sun: 4.5 billion years old
• The Sun has been a main-sequence star for 4.5
billion years, and at the core
1. Hydrogen depleted by about 35%
2. Helium amount increased
3. Become 40% more luminous
4. Has grown in radius by 6%
• The Sun should remain on the main sequence for
another 7 billion years
• The Sun or 1 M star has a main sequence
lifetime of 12 billion years
The Sun: 4.5 billion years old
The Evolution of the Sun
Asplund et al., 2009, Annual Review of
Astronomy & Astrophysics, 47, 481
Change of chemical composition
Next Phases
Subgiant Phase
• Core hydrogen fusion ceases when the
hydrogen has been exhausted in the core of a
main-sequence star
• This leaves a core of nearly pure helium
• The core shrinks under self-gravitation due to
the loss of hydrostatic equilibrium
• The core becomes hotter
• Shell hydrogen fusion occurs just outside the
core
• No fusion in the helium core
• Subgiant Phase (SGB)
Red Giant Phase
• Shell hydrogen fusion works its way outward in
the star and adds more helium into the core
• Core becomes hotter
• Shell hydrogen fusion occurs at a greater rate
• Outer layers expands because of the increased
energy flow
• At some point the core reaches the SchönbergChandrasekhar
limit
Schönberg-Chandrasekhar limit
• The mass limit for the core which is capable of
supporting the gravitational pressure of core
and envelope
• Core too massive => rapid contraction
• Release of a lot of gravitational potential energy
• Dumped into the envelope => heating up
• The Subgiant is now becoming a Red Giant (Red
Giant Branch, RGB)
First dredge-up
As the outer atmosphere
expands it becomes
convective => Mixing
First dredge-up
Composition depending on mass
initial metallicity, and used
theoretical model
1 M Initial After FDU
Y 0.280 0.3025
C/Z 0.1733 0.1530
N/Z 0.0531 0.0771
O/Z 0.4823 0.4822
3
He 8.4E-5 0.001368
12
C/13
C 90.0 29.8
16
O/17
O 2660 2610
16
O/18
O 500.1 526.9
1995astro.ph.12121B
Red Giant Phase
Red Giant Phase
• With time, more helium “ash” adds into the
core
• Core contracts more and becomes even hotter
• When the central temperature reaches 108
K,
helium fusion ignites (Helium Flash) inside the
core
• Helium fusion process (triple alpha process),
converts helium to carbon
Helium Flash
Helium Flash takes
only seconds
Helium Flash
• In low-mass star, the compressed core is in an
electron-degeneracy state
• Electron-degeneracy: the electrons are so closely
packed that they can not be further compressed,
due to the Pauli exclusion principle
• Pauli exclusion principle: two particles can not
occupy the same quantum state
• The core becomes supported by degenerateelectron
pressure, i.e independent of temperature
• As helium fusion ignites in the core, temperature
rises exponentially, but pressure does not rise
Helium Flash
• Helium fusion rate rises exponentially => Helium
Flash
• During the time of Helium Flash, the core is
extremely bright (1012
L)
• At certain core temperature no longer electrondegeneracy
state => core becomes an ideal gas
• The core expands, and cools, terminating the
Helium Flash
• The core becomes a steady state of helium fusion
• A red giant enters into the stage of “Horizontal
Branch” in the HRD
Horizontal Branch
• Red Giant becomes less luminous and smaller,
but hotter
• Steady State of helium fusion in core: no
contraction and no temperature rising
• The core acts like an ideal gas:
1. Temperature increases, core expands
2. Core expands, temperature decreases
• Shell hydrogen fusion rate drops => lower
luminosity
• Red Giant shrinks because of less energy output
• It becomes hotter at surface as it compresses