• The diagram below gives an example of an absorption spectrum and shows how the black lines shift to the red end of the spectrum if the star is moving away from an observer, or to the blue end if it is moving towards the observer.

un-shifted

red-shifted

blue-shifted

• When we view stars in a nearby rotating spiral galaxy, some stars will be moving towards us and others will be moving away from us as the galaxy rotates. We observe both blue- and red-shift.

star in rotating galaxy