Why do supernova spectra show blue-shifted emission lines?

Supernova spectra exhibit blue-shifted emission lines because these lines serve as evidence that hot gases are being ejected and blown outward from the exploding star toward the observer.1

This phenomenon is a manifestation of the Doppler effect, which describes how the observed frequency or wavelength of light changes based on the relative motion between a source and an observer. When a light source moves toward an observer, the light waves are compressed, resulting in a shift toward the blue (or violet) end of the spectrum, known as a blueshift. Conversely, if an object were moving away, the light would undergo a redshift. In the context of a supernova, the rapid expansion of gaseous material toward Earth causes this characteristic blue shift in the spectral lines.231