Blueshift of the semi-forbidden Al\,\textsc{ii}] $\lambda2669$ line
When they looked for clues from other sources – like how some light comes back slightly shifted as if it was affected by moving particles – they found a hint of shock-related motion!
Technical definition
The shift in the wavelength of certain light emitted or absorbed by atoms towards shorter (blue) wavelengths due to the effects of moving particles.