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( Sunspot)
A sunspot is a region on the Sun's surface (photosphere) that is marked by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. They can be visible from Earth without the aid of a telescope. Although they are at temperatures of roughly 4,000–4,500&_160;K, the contrast with the surrounding material at about 5,800&_160;K leaves them clearly visible as dark spots, as the intensity of a heated black body (closely approximated by the photosphere) is a function of T&_160;(temperature) to the fourth power. If a sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. A minimum in the eleven-year sunspot cycle happened during 2008.[1] While the reverse polarity sunspot[2] observed on 4 January 2008 may represent the start of Cycle&_160;24, no additional sunspots have yet been seen in this cycle. The definition of a new sunspot cycle is when the average number of sunspots of the new cycle's magnetic polarity outnumbers that of the old cycle's polarity[citation needed]. Forecasts in 2006 predicted Cycle&_160;24 to start between late 2007 and early 2008, but new estimates suggest a delay until 2009. Sunspots, being the manifestation of intense magnetic activity, host secondary phenomena such as coronal loops and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.[3] Sunspot populations quickly rise and more slowly fall on an irregular cycle about every 11&_160;years. Significant variations of the 11-year period are known over longer spans of time. For example, from 1900 to the 1960s the solar maxima trend of sunspot count has been upward; from the 1960s to the present, it has diminished somewhat.[4] The Sun is presently at a markedly heightened level of sunspot activity and was last similarly active over 8,000 years ago.[5]
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