Being composed of gas, the Sun has no solid surface. What appears to an observer on Earth to be the visible surface of the Sun is actually a gaseous layer (known as the photoshpere) in which conditions promote the emission of electromagnetic radiation at visible wavelengths. Observing the Sun at other wavelengths - for example, X-rays, ultraviolet and so on - allows us to see other "surfaces" of the Sun, either above or below the visible surface, depending upon the wavelength being observed. The dark atomic absorption lines in the Sun's spectrum are imposed on the Sun's light by other atoms and ions in the cooler upper levels of the photosphere and in the lower part of the chromosphere, the region of gas just above it. These regions form the lowest layers of the Sun's atmosphere, above which is the more rarefied corona. Deep inside the solar core is the central region in which nuclear fusion takes place.

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