Stellar Structure

Temperature, density and pressure decreasing

Energy generation via nuclear fusion

Energy transport via radiation

Energy transport via convection

Flo

w o

f en

erg

y

Basically the same structure for all stars with approx. 1 solar

mass or less.

Sun

What would happen if the sun suddenly stopped producing

energy in its center?

1. It would explode.

2. It would rapidly collapse.

3. It would gradually cool off and shrink.

4. It would gradually cool off and expand.

5. It would maintain its current structure.

Outward pressure of hot gas in the center

balances the

inward force due to gravity.

Hydrostatic Equilibrium

Hydrostatic Equilibrium

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.

How does the sun produce energy in its

center?

1. Through chemical burning of hydrogen (2 H + O → H2O).

2. Through thermonuclear fusion of helium into Carbon.

3. Through thermonuclear fusion of hydrogen into helium.

4. Through thermonuclear fission of helium into hydrogen.

5. Through thermonuclear fission of Uranium into lighter nuclei.

The CNO Cycle

In stars slightly more massive than the

sun, a more powerful energy generation

mechanism than the PP chain takes over:

The CNO Cycle.

In the sun, energy production is

dominated by direct fusion of H into He

(PP chain).

Energy Transport Structure

Inner radiative, outer convective

zone

Inner convective, outer radiative

zone

CNO cycle dominant PP chain dominant

MassSun

Radiative Core, convective envelope;

Energy generation through PP Cycle

Convective Core, radiative envelope;

Energy generation through CNO Cycle

Summary:

Stellar Structure