An energy budget is a physical analysis of all of the energy coming into a system, all the energy going out, and any difference that might be internally transformed or stored. Almost all of the energy coming to Earth from outer space has been radiated by the closest star, the Sun. The Sun emits electromagnetic energy at a rate and spectral quality determined by its surface temperature—all bodies do this, as long as they have a temperature greater than absolute zero, or −459°F (−273°C). Fusion reactions occurring within the Sun maintain an extremely hot surface temperature, about 10,800°F (6,000°C). As a direct consequence of this surface temperature, about one-half of the Sun's emitted energy is so-called "visible" radiation with wavelengths between 0.4 and 0.7 µm (this is called visible radiation because it is the range of electromagnetic energy that the human eye can perceive), and about one-half is in the near-infrared wavelength range between about 0.7 and 2.0 µm. The Sun also emits radiation in other parts of the electromagnetic spectrum, such as ultraviolet and cosmic radiation. However, these are relatively insignificant amounts of energy (although even small doses can cause biological damage).
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