The behavior of gases plays a central role in astronomy. Under "normal" conditions, gases obey approximately the Ideal Gas Law. For example, one property of ideal gases is that an increase in the temperature causes an increase in the pressure. Thus, a balloon expands if the air inside it is heated.

Degenerate Gases
Under conditions of very high pressure, gases can behave in a quite different way. These gases are called degenerate gases and their properties are governed by the principles of quantum mechanics.

One very important property of degenerate gases is that the pressure is independent of the temperature. Thus, contrary to our usual experience, if we could heat a balloon of degenerate gas it would not expand. This has very large consequences for thermonuclear reactions, such as those that occur in a nova outburst.

Thermonuclear Reactions in
Ideal Gases
If a thermonuclear reaction is ignited in matter it quickly raises the temperature to values approaching a billion degrees. For a normal ideal gas this would raise the pressure, causing the hot gas to expand rapidly. This would lower the density and cause the thermonuclear reactions to slow down.

Thus, normal explosions are "self-limiting", because the explosion tends to separate the fuel for the explosion, causing it to stop.

Thermonuclear Reactions in
Degenerate Gases
In degenerate matter the situation is completely different. Because the pressure does not increase with the temperature, as the temperature approaches a billion degrees the thermonuclear fuel is heated but not separated by the explosion. This causes the reactions to go even faster, because thermonuclear reaction rates increase rapidly with temperature. This is called a thermonuclear runaway, and can lead to gigantic explosions.

Thermonuclear ignition under degenerate conditions is thought to be a key component in nova explosions, X-ray bursters, type 1a supernovae, and in what is called the helium flash in red giant stars.