The hotter the gas is, the faster the average speed of the atoms is. As the atoms move and change directions, they emit electromagnetic radiation. Sometimes you can see this radiation, somethimes you can't. There is an ideal case of this, called blackbody radiation . Many real cases of warm or hot bodies emitting radiation are pretty close to this ideal case.
Blackbody radiation is emitted over a range of wavelengths.
Very hot stars look a bit blue, and in fact radiate a lot of energy in the ultraviolet. We can use this information to learn about galaxies. Here is a picture of M100 .