blue shift

The term blue shift (aka blueshift) refers to the shortening of wavelengths due to the Doppler effects of motion of the radiation-source toward from the observer. It is analogous to redshift, the lengthening of the wavelength if the source is receding. When viewing a spectral line of a star, if the star is coming toward us, the wavelength we observe will be shorter than the line's rest wavelength. The phenomenon is also termed a negative redshift, and can be quantified by the redshift parameter (z), which is negative when the source is blueshifted. Like redshift, if the rate of the radial motion is relativistic, special relativity is relevant, and calculating z is inaccurate unless the Lorentz factor is used.