The Doppler effect produces different types of shift in respect to the waves and their frequency. The shorter the wavelength of light, the more blue shifted it is, and the longer the wavelength, the more red shifted it is. These shifts are used by modern-day cosmologists to determine whether an object in space is heading towards or away from us.
Many of the objects seen in the night sky, the average stars, nebulas and galaxies, emit light that becomes red shifted as it flies towards us. This was first seen by Edwin Hubble in the 1920s. At the time, the popular theory of the universe was that it was a static entity. However, this newfound fact, that many objects were heading away from us, seen in all directions, pointed to one theory, that the universe was expanding. It was also found that the farther one delves into the midst of space, the quicker the objects are receding from us.