See my copyright notice for fair use practices. A rainbow is an example of a continuous spectrum .
Most continuous spectra are from hot, dense objects like stars, planets, or moons.
The continuous spectrum from these kinds of objects is also called a thermal spectrum , because hot, dense objects will emit electromagnetic radiation at all wavelengths or colors. A thermal spectrum is the simplest type of spectrum because its shape depends on only the temperature. A discrete spectrum is more complex because it depends on temperature and other things like the chemical composition of the object, the gas density, surface gravity, speed, etc. For now, let's look at a thermal spectrum. Some thermal spectra for objects of different temperatures are illustrated in the figure below.