Particles in gas move faster with temperature in a linear motion (root mean velocity equation?) right? It explains increase in pressure and effusion proportional to temperature.
Solids emit radiation if their temperature is high enough because heat makes their atoms vibrate thus producing electromagnetic waves.
Gas, though, emits radiation due to temperature too.
How is this possible for a gas with its particles moving in a linear motion?
The key to emitting radiation is acceleration. Acceleration can arise due to a change in speed or due to a change in direction.
We could take bremsstrahlung as an example of radiation emitted by a gas of ionised atoms and electrons. The radiation here occurs because the electrons are accelerated by the electric fields of the ions in the gas. The acceleration can take the form of a change of speed or change of direction (usually both).
In an atomic gas the radiation comes from atomic processes. This might involve the excitation of atoms, perhaps by collisions, followed by the dexcitation by the emission of radiation. Or one could imagine photo-recombination, where a free electron is "captured" by an atom to form an ion, which obviously leads to an acceleration and the emission of light.
