Just above the Sun's surface is a layer called the chromosphere. Looking like a field of fiery grass, this layer can be seen only by using a special filter that lets through only one wavelength of light, a reddish colour given off by ionised hydrogen gas (hydrogen alpha). This artificially-coloured image also shows an optical effect known as limb brightening, where the edge of the Sun appears brighter than the middle.
Credit: Image made in red light by Robert Gendler.
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