STELLAR STRUCTURE

Star structure is based upon four laws:

To visualize stellar structure, astronomers think of a star like an onion, with concentric spherical and very thin layers. We call these layers "shells". In computer models, astronomers calculate the mass, density, temperature, pressure, and energy transport mode of each shell. To do so, all layers must be solved simultansously, because what happens in one shell is closely related to the properties in its neighboring shells (since they share boundaries).

The computer codes apply the four laws of stellar structure as follows:

Conservation of Mass

Mass can be transported from one shell to the other (like in the convection zone), but the whatever mass comes into a shell, it pushes out an equal amount. Thus, the mass in each shell is constant with time (it is a conserved amount). Another way to state this law is to say that stars are not clumpy inside. The matter is distributed smoothly from the core to the surface.

Conservation of Energy

Energy can also be transported from one shell to the other (see below), but the whatever energy is transported into a shell, an equal amount must be transported out of the shell. Thus, the energy in each shell is constant with time (it is a conserved amount). Another way to state this law is to say that there are no hot or cold spots inside stars. The energy is distributed smoothly from the core to the surface. Also, note, this also implies that the total energy generated in the core is equal to the energy that escapes the photosphere (the luminosity!).

Hydrostatic Equilibirum

Hydrostatic equilibrium is the balancing between outward pressure in response to inward gravitational weight. For example, water has weight. As you dive deeper under the water, you feel the cummulative weight (gravitation) of all the water above you and your body is compressed a little bit, which you feel as pressure on your body/head. The same principle applies for each shell in a star. A given shell feels the cummulative weight of all the shells above it. It responds by being compressed and its resistance to being completely compressed is its own outward pressure.

The deeper a shell is in a star, the greater its pressure because the greater is the cummulative weight of all the shells above it. The core has the highest pressure and the surface has the lowest pressure.

Energy Transport

Temperature is the measure of the energy content of a gas. The high the temperature of a gas, the faster the particles in the gas are moving. It is these fast motions of particles crashing into one another which creates the pressure in the gas. Thus, higher temperature gas has higher pressure content, and low temperature gas has lower pressure. Clearly, then, the temperature and prssure in a star increase toward its core (this is a consequence of hydrostatic equilibrium). But how is this energy trnasported from shell to shell.

Energy transport in stars is either as a convective transport or a radiative transport. Normal stars never use conduction as transport mechanism.

Convective Transport: When energy is transported from one place to another by the bulk motion of hot gas into regions of cold gas (and visa versa). Boiling water is an example of convective transport.

Radiative Transport; When the energy is transported in the form of light (electromagentic energy). The warmth of sun light on your face is radiative transport.

How the energy transport occurs in the star depends upon stellar mass.