Because the light from it travels to you about 874 thousand times
as fast as the sound does, so the hearing part falls behind the seeing
part.
As the gas cools down, particle movement slows down with it, and so does the temperature of the gas. This is because the more heat or thermal energy a gas has, the faster the particles move. This is why absolute zero, or 0 degrees Kelvin is defined as zero particle movement, because it has zero energy.
Hope this helps
