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The description of the word you requested from the astronomical dictionary is given below.
corona = [Latin] from koroonè = [Greek] crown
The corona is the outermost layer of the Sun, starting at about 1300 miles (2100 km) above the surface. The temperature in the corona is 500,000 K (900,000 degrees F, 500,000 degrees C) or more, up to a few million K. The corona is very dilute indeed (less than 1000 million atoms per cubic centimetre or 10,000 million atoms per cubic inch) and cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph.