Everything emits radiation. The peak wavelength emitted by an object is inversely proportional to the temperature of the object emitting it. This is Wien's Displacement Law. Also, the amount of energy output by an object is proportional to its temperature to the fourth power.

So hotter objects emit more intense radiation with shorter peak wavelengths than cooler objects.

In the picture below, the "extreme UV" radiation has been emitted by a much hotter source than the "red light" radiation. Therefore, extreme UV radiation has shorter wavelengths than red light.