Across each object's spectrum 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. However, these absorption lines often do not match what an observer would expect to see. This is due to what is called a Doppler shift.

As an object moves away from the earth, the waves this energy is travelling towards us in are being stretched, since they have farther to travel as the object moves. Because these waves are stretched, when the energy reaches earth it will appear to be at a lower level than it is at the source of emission.

In the visible light range, red light is less energetic than blue light. When a visible object is redshifted, its movement away from us causes absorption lines in the spectrum to appear closer to the lower-energy red end of the visible range than would be expected if the object were not moving. Conversely, if a visible object were moving towards us, it would appear more energetic than it really was. The light waves would be pushed together, and the absorption lines would be shifted towards the blue side of the visible light range.

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