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Sunspots are areas of the Sun's surface that are somewhat cooler and darker than the surrounding photosphere. The average temperature of a sunspot is about 4,000 degrees K, compared to the 6,000 K of the photosphere. Sunspots appear dark when photographed or viewed with a filter, but are actually have a surface brightness about 20 percent that of the photosphere. The central, darker portion of a sunspot is known as the umbra, while the outer, lighter portion is known as the penumbra. Sunspots form where solar magnetic field lines have broken through the surface. The number of sunspots increases and decreases in an eleven-year cycle. This large sunspot group was one of several visible on the Sun's disk at the time, typical of what is seen near solar maximum. The two largest spots are larger than the diameter of the Earth. Granulation is subtly noticeable in this image, seen as a mottling of the Sun's surface. Granulation results from the pattern of convective cells on the photosphere where columns of gas rise and fall. Very subtly visible near the Sun's limb at lower left are bright streaks known as faculae, which are often precursors to sunspots and may also be seen after the disappearance of sunspots. Instrument:
C-9.25, with 26mm Tele Vue Plossl
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