Stellar Structure:

Stars form from clouds of gas and collapse under self-gravity. The collapse is stopped by internal pressure in the core of the star. During the collapse, the potential energy of infalling hydrogen atoms is converted to kinetic energy, heating the core. As the temperature goes up, the pressure goes up to stop the collapse.

The heat from the collapse is sufficient for a star to shine, but only for a timescale of 15 million years (called the Kelvin-Helmholtz time). Since most stars are over 10 billion years old, then they must be producing its own energy rather than shining on leftover energy from formation.

The structure of stars is determined by 5 relations or physical concepts:

  1. hydrostatic equilibrium - Most stars, like the Sun, are not expanding nor contracting. They are stable in size. Therefore, this fact means that the internal pressure must balance the weight of the material above it (self-gravity)

  2. thermal equilibrium - the amount of energy generated in the core of a star by thermonuclear fusion must equal the amount radiated away (the only place for the energy to go is outward)

  3. opacity - how fast energy is radiated is determined by the resistance of the stellar envelope to the flow of photons. If a star has low opacity, it can radiate its energy fast and its temperature and pressure will be lower = smaller radii

    At a star's surface the energy is released to form the spectrum of the star.

  4. energy transport - how energy is transported from the core to the stellar surface determines the surface temperature of a star (its color)

    There are three ways to transfer energy; conduction, convection and radiation. Conduction, the collisional transfer of energy between atoms, only occurs between solids (such as a hot pan and your hand), so is not found in stars. Only convection and radiation transfer are important in stars and the opacity determines which method is used. When the temperature is high and all the atoms are stripped of their electrons, the opacity is low and radiation transfer is dominant.

    When the temperature drops, such as in the outer layers of a stars interior, the protons and electrons recombine to form atoms and the opacity goes up. High opacity slows the transfer of energy by radiation, so bubbles form. These bubbles are hot and low in density, thus starting a convective flow.

    Whether convection or radiation transport is used depends on the temperature make-up of the stellar interior. When the changes in temperature are sharp, convection is used. Think of the photons as grains of sand on a pile. If the pile is low, radiation is used. If the pile is high, the sand tumbles down, convection is used.

    At a star's surface the convective cells release energy, as shown in this supercomputer movie.

  5. energy production - in the case of stars, energy is produced by thermonuclear fusion. This can be either the proton-proton chain or the CNO cycle.

These 5 relationships, stated as mathematical formula, show how energy is generated, how that energy effects the structure of stars and how that energy is transported to the surface to make a star shine.

Stellar Interior:

A star is divided into six regions based on the physical characteristics of these regions. The boundaries are not sharp, and the regions vary in size from star to star. For example, hot stars have larger radiative zones and smaller convective zones. The reverse is true of cool stars.