A complex structure of warm gas above the visible surface, or photosphere, of the Sun and most stars. The term “chromosphere” was first applied to the red ring and large prominences seen at the edge of the eclipsed Sun with the unaided eye. Emission in the Balmer-alpha line of hydrogen at 653-nanometer wavelength accounts for the red color. The chromosphere is transparent in visible light, but is opaque and bright in the ultraviolet continuum and in strong lines of abundant elements, including hydrogen, helium (first observed during the 1868 solar eclipse), oxygen, calcium, and magnesium. Gas temperatures range from 3500 to 30,000 K (5800 to 54,000°F) and densities are between 10⁹ and 10¹² particles per cubic centimeter.