Stellar Structure

1
Stellar Structure
Energy transport via convection
Sun
Energy transport via radiation
Flow of energy
Energy generation via nuclear fusion
Basically the same structure for all stars with
approx. 1 solar mass or less.
Temperature, density and pressure decreasing
2
What would happen if the sun suddenly stopped
producing energy in its center?

• It would explode.
• It would rapidly collapse.
• It would gradually cool off and shrink.
• It would gradually cool off and expand.
• It would maintain its current structure.

3
Hydrostatic Equilibrium
0
Outward pressure of hot gas in the center
balances the
inward force due to gravity.
4
Hydrostatic Equilibrium (II)
0
Outward pressure force must exactly balance the
weight of all layers above any given point in the
star.
This condition uniquely determines the interior
structure of the star.
This is why we find stable stars on such a narrow
strip (Main Sequence) in the Hertzsprung-Russell
diagram.
5
0
6
How does the sun produce energy in its center?

• Through chemical burning of hydrogen (2 H O ?
  H2O).
• Through thermonuclear fusion of helium into
  Carbon.
• Through thermonuclear fusion of hydrogen into
  helium.
• Through thermonuclear fission of helium into
  hydrogen.
• Through thermonuclear fission of Uranium into
  lighter nuclei.

7
The CNO Cycle
0
In the sun, energy production is dominated by
direct fusion of H into He (PP chain).
In stars slightly more massive than the sun, a
more powerful energy generation mechanism than
the PP chain takes over The CNO Cycle.
8
Energy Transport Structure
Inner convective, outer radiative zone
Inner radiative, outer convective zone
CNO cycle dominant
PP chain dominant
9
Summary Stellar Structure
Convective Core, radiative envelope Energy
generation through CNO Cycle
Sun
Mass
Radiative Core, convective envelope Energy
generation through PP Cycle