Redshift or blueshift that help us see which stars/galaxies are approaching or receding are determined by what effect named for an Austrian physicist?
Doppler effect at which stars and galaxies are approaching or receding from us; that is, their radial velocities. This may be used to detect if an apparently single star is, in reality, a close binary, to measure the rotational speed of stars and galaxies, or to detect exoplanets. This redshift and blueshift happens on a very small scale, if an object is moving toward earth, there would not be a noticeable difference in visible light Note that redshift is also used to measure the expansion of space, but that this is not truly a Doppler effect. Rather, redshifting due to the expansion
Blueshift evolves while a photon is passing, the amount of blueshift on approach will differ from the amount of gravitational redshift as it leaves the region. There are faraway active galaxies that show a blueshift in their [O III] emission lines. One of the largest blueshifts is found in the narrow-line quasar, PG 1543+489, which has a relative velocity of -1150 km/s. These types of galaxies are called "blue outliers". In a hypothetical universe undergoing a runaway big crunch contraction, a cosmological blueshift would be observed, with galaxies further away being increasingly blueshifted—the exact opposite of the actually observed cosmological redshift