Scientists can determine whether a star is approaching the earth by looking at it’s blue shift. What is blue shift?

Ablueshiftis any decrease inwavelength(increase in frequency); the opposite effect is referred to asredshift. Invisible light, this shifts the color from the red end of the spectrum to the blue end. The term also applies when photons outside the visible spectrum (e.g.x-rays and radio waves) are shifted toward shorter wavelengths, as well as to shifts in thede Broglie wavelengthof particles. Blueshift is most commonly caused by relative motion toward the observer, described by theDoppler effect. An observer in a gravity well will also see infalling radiation gravitationally blueshifted, described byGeneral Relativityin the same way asgravitational redshift. In acontracting universe, cosmological blueshift would be observed; the expanding universe gives acosmological redshift(increase in wavelength), and the expansion is observed to beaccelerating

Dopplerblueshift is caused by movement of a source towards the observer. The term applies to any decrease in wavelength (increase in frequency) caused by relative motion, even outside thevisible spectrum. Only objects moving at near-relativistic speeds toward the observer are noticeably bluer to thenaked eye, but the wavelength of any reflected or emitted photon or other particle is shortened in the direction of travel.

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