How Stars Work

What happens in a star’s chromosphere?

The chromosphere, the thin and usually transparent layer of the Sun’s atmosphere between the photosphere and the corona, is a highly energetic plasma that is punctuated with flares—bright, hot jets of gas—and faculae consisting of bright hydrogen clouds called plages. The chromosphere is generally not visible except with ultraviolet or X-ray telescopes.

The chromosphere is around one thousand to two thousand miles thick and has some unexpected physical properties. For example, while the density of the gas decreases from the inner edge of the chromosphere to the outer edge, the temperature of the gas increases dramatically—from about 7,250 to 180,000 degrees Fahrenheit (4,000 to 100,000 degrees Celsius)—even though the distance to the Sun is actually increasing. At its outer limit, the chromosphere breaks up into narrow gas jets called spicules and merges into the Sun’s corona. I


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