Sunspots. Swedish One-metre Solar Telescope image of sunspots on the surface of the Sun. Sunspots are cooler regions of the Sun's surface that appear dark against their brighter, hotter surroundings. They comprise a dark umbra surrounded by a lighter, filamentous penumbra. The individual filaments comprising the penumbra are around 150-180 kilometres in diameter. It is thought sunspots are formed by powerful magnetic fields intersecting the Sun's visible surface (photosphere). The amount of sunspots on the Sun varies over an eleven year cycle, for unknown reasons. Image taken on 15th July 2002.