Astronomers today use the concept proposed by Christian Doppler and Edwin Hubble to determine the motion of stars and galaxies. Write a brief essay describing how the Doppler effect explains why some stars are "blue shifted" and others are "red shifted."

So we want to know how does the Doppler effect describe why some stars are "blue shifted" and some stars are "red shifted". Light is an electromagnetic wave and therefore acts like a wave, it has its frequency and wavelength. If the star is moving away from the observer, the light that it emits reaches us with a longer wavelength and we say that it is "red shifted" or it has moved to the red part of the EM spectrum. If the star is coming towards the observer, its wavelength will be shortened or "blue shifted" since its wavelength has moved towards the blue part of the EM spectrum. This shortening and stretching of the wavelength is called the Doppler effect.