Different objects in the universe emit different forms of electromagnetic radiation in waves. X-rays are among the most energetic emissions. Visible light is also radiation, and is less energetic than x-rays, but more so than radio waves.

Visible objects in the sky are releasing energy across the visible part of the electromagnetic spectrum. Since each object is composed of different gases, different objects release different levels of radio, visible, or x-ray energy towards the earth. Each object's range of energy emission is referred to as its spectrum.

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