RANDOM THERMAL MOTION
Within a gas particles dart about rapidly, colliding millions of times each second and changing their direction of motion just as frequently. Each gas particle has a kinetic energy proportional to the product of its mass and the square of its speed. After a collision the speed can be either greater or smaller than it was before the collision; the kinetic energy of each particle changes in its repeated collisions. Collectively, however, the gas particles will have some average kinetic energy, which changes only when energy is added to the gas or removed from it. Another way of saying this is that the average kinetic energy changes when the gas is heated or cooled.
Temperature is a measure of the average kinetic energy of gas particles. The motion of the particles composing a body, like ice or water or water vapor, is called random thermal motion for a gas. It increases as the temperature goes up, and it decreases as the temperature goes down. Absolute zero is reached when the average kinetic energy is zero. Seen in terms of the motion of the particles in the gas, temperature is a measure of that motion: The greater the temperature of the gas, the greater the random thermal motion.
Temperatures in astronomy are usually measured on the absolute, Kelvin (K), scale. In this system there are 100 divisions (degrees) between the freezing point (273 K) and the boiling point (373 K) of water.
