A cluster of sunspots. Sunspots appear to be black because their temperature, about 4000 Kelvin, is much lower than the surrounding Sun's photosphere temperature (about 6000 Kelvin). The lower temperature inside sunspots arises from the presence of a strong magnetic field which blocks the new energy supplied by the Sun's interior. The black inner region of a sunspot is termed umbra whereas the more luminous surrounding fringes penumbra. The average size of a spot is roughly 10,000 km but some as big as 150,000 km have been recorded. The image was taken on May 17th 1951.