To understand redshift, think of a passing ambulance. The sound of the siren changes as the ambulance goes by. This is because sound waves are compressed as the ambulance moves toward you, and they are stretched as it moves away. Scientists call this Doppler shift.

Stars, galaxies and quasars all emit energy that travels through space in the form of waves, both visible light waves and higher-energy light, such as X-rays, that are not visible.

As an object moves away from the Earth, the waves are stretched, since they have farther to travel as the object moves. When the energy reaches Earth it will appear to be at a lower level than it is at the source of emission.

Across each object's spectrum of emission are black lines, places where energy is absorbed. This is due to gases in the composition of the object that absorb only specific levels of energy.