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The appearance to us of the Sun is not uniform over the whole surface. The most obvious feature are the sunspots - slightly cooler magnetically active regions of the Sun, which appear darker than the surrounding area. The centre of the Sun appears brighter than the edges. This is called 'limb darkening' and occurs because when we look at the edges of the Sun we are looking through a thin layer of the Sun's atmosphere, on the other side of which is empty space, but when we look at the centre of the Sun we are looking into regions of dense hot gas.