chroma + sphaira

Greek chrōma (χρῶμα) meant color, and sphaira (σφαῖρα) meant sphere or ball. The chromosphere is the 'color sphere' — the thin layer of the sun's atmosphere between the photosphere (the visible surface) and the corona (the outer atmosphere). It appears as a bright red ring during the seconds when a total solar eclipse first covers the sun's disc. The red comes from hydrogen-alpha emission at 656 nanometers.

The term was coined in the 1860s, when astronomers began using spectroscopy to study the sun's composition. During the total solar eclipse of August 18, 1868, observed from India, Jules Janssen and Norman Lockyer independently discovered helium in the chromosphere — an element unknown on Earth at the time. They found it by analyzing the light from the color sphere. Helium was named for Helios, the sun.

The chromosphere is violent. Spicules — jets of gas — shoot upward from its surface at speeds of 20 kilometers per second, reaching heights of 10,000 kilometers before falling back. Solar prominences, arcs of plasma held aloft by magnetic fields, can extend hundreds of thousands of kilometers into space. The thin red line is anything but calm.

Photographing the chromosphere requires specialized filters. In hydrogen-alpha light, the sun transforms from a featureless yellow disc to a roiling surface of dark filaments, bright plages, and erupting prominences. The color sphere reveals what the eye cannot see: the sun is not a lamp. It is a storm.