The corona is the very hot outer part of the solar atmosphere which consists of highly ionised gas superheated to temperatures in exces of 1 million degrees Celsius. It can be seen visually during a total solar eclipse as a halo or cloud of pearly white light. Phenomena such as solar flares and coronal mas ejections (CME) which occur in the corona emit masive amounts of radiation acros the whole electromagnetic spectrum. This can cause radio blackouts on Earth, and can be dangerous to astronauts beyond the protection of the Earth's atmosphere. Skylab, America's first space station launched in 1973, carried the Apollo Telescope Mount which contained eight instruments to study the Sun at various wavelengths.
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