Electromagnetic radiation emitted by a moving object exhibits the Doppler effect. The radiation emitted by an object moving toward an observer is compressed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted. As with the pitch changes for a moving ambulance, blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.