Plural of chromosphere; the layer of the sun's atmosphere located just above the visible photosphere, visible as a red rim during solar eclipses.
From Greek 'chroma' (color) and 'sphaira' (sphere). The layer appears reddish because it's colored by hydrogen gas, which is why 'chromo-' (color) is appropriate for this part of the sun.
The chromosphere was named for its gorgeous color—that brilliant red ring you see during a total solar eclipse is hydrogen gas at about 6,000 degrees, thousands of times hotter than the sun's visible surface!
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