Wien's Law is a very useful in cosmology. If we assume that we are observing light from a black body source, then we can use Wien's Law to estimate the temperature of the body, typically a could of gas or a star. In fact, no object, star or gas is a perfect black body, but Wiem's Law provides a useful approximation so the temperature can be estimated. The Law states that the temperature of the body is inversely proportional to the wavelength of light that gives a peak of intensity.
The Law can be stated in the form of an equation:
Hotter objects emit most of their radiation at shorter wavelengths; hence they will appear to be bluer .
Cooler objects emit most of their radiation at longer wavelengths; hence they will appear to be redder.
Furthermore, if an object is heated, so that the radiation curve moves from the low temperature curve on the diagram above to any of the higher temperature curve, it's intensity at any wavelength increases, so the hot object radiates more at any temperature than a cold one.
Suppose then that light from a star has a peak in intensity at a wavelength ofm. We estimate it's temperature as