Chromosphere of Stars:

The chromosphere is a pinkish atmosphere above a stars surface (the photosphere). It emits an emission spectrum, which indicates it chromosphere's are composed of a very hot gas (20,000 K or more). The most complex and transient stellar phenomena occur in the chromosphere including:

  1. stellar flares
  2. stellar arcs
  3. prominences
Flares, arcs and prominences are linked to stellar activity, very small changes in stellar luminosity . Gas is trapped in the flux lines created by the starspots and lifted off the photosphere into the chromosphere. Over a period of a few hours, the magnetic fields collapse hurling the hot gas outward (much like a breaking rubber band).

Stellar Corona:

The corona of a star is a large, white halo of glowing gas. The corona gas is extremely hot (temperatures on order of a million degrees) and is the source of the stellar winds.

A stellar wind is a constant stream of sub-atomic particles moving at faster than the escape velocity of the star's gravitational field. They escape through windows in the corona, called coronal holes, regions where the magnetic fields are weak and the charged stellar wind particles are not trapped in magnetic bottles.

X-ray and UV pictures of the corona show that the hot gas is connected to the magnetic features in the photosphere. Those low level structures extending into long streamers in the outer corona and heat the corona to its million degree temperatures.

Degeneracy:

Gases normally obey the Ideal Gas Law. However, when the atoms become very densely packed, special rules of quantum mechanics control the behavior of the atoms. In particular, the Pauli exclusion principle limits the number of particles of a given kind that can be put into a cubic centimeter of space. When the density of the gas reaches this limit it begins to behave like a liquid.

For normal gases, when the density or temperature go up, the pressure goes up. For extremely high density gases, the pressure becomes almost independent of temperature and the gas is said to be degenerate. This happens for normal matter when the density reaches 1015 gm/cc (for example, if you squeeze the whole Earth into a football stadium).

The cores of normal stars are an ideal gas, and follow the ideal gas law. However, as hydrogen fuel is burned, helium is produced as a byproduct and begins to build up in the center of the star. Helium cannot burn at these lower temperatures, so the helium ash becomes denser and denser to reach degeneracy. The core can cool, but it doesn't contract because its internal pressure remains high even as its temperature falls. Thus, stars with low mass cores become white dwarfs, slowly cooling and radiating their heat into space.