Chromosphere
The chromosphere is the second layer of a star’s atmosphere, sitting above the photosphere (the visible surface) and below the corona. The Sun’s chromosphere is a short, colorful cushion about 3,000 to 5,000 kilometers thick, making up just a small fraction of the Sun’s radius. It gets its reddish color from hydrogen emissions (the H-alpha line) and can usually be seen only with special equipment or during a total solar eclipse.
The layer contains features such as spicules—narrow jets of gas that shoot upward through the chromosphere and into the corona, reaching up to about 10,000 kilometers. Its light is dominated by emission in the H-alpha line, and it is also seen in the calcium II K line in the violet part of the spectrum.
In other stars, chromospheres exist too and can be very large. For example, in the giant star Antares, the chromosphere can be hundreds of millions of kilometers thick, about 2.5 times the star’s radius. The density of the solar chromosphere falls off very quickly with height, while the temperature changes from roughly 6,000 K at the bottom to a minimum around 3,800 K, then rises to well above 35,000 K at the outer edge as it transitions toward the corona.
The chromosphere is much thinner than the photosphere and is usually invisible unless viewed with special techniques. When seen in the Sun’s edge (the solar limb), it shows strong emission lines, especially H-alpha at 656.3 nm, which gives the red tint. The surface also features bright regions called plages and dynamic plasma loops near the disk’s edge; cool loops can rise and fall rapidly, while hotter, more stable loops exist farther toward the corona.
In stars beyond the Sun, chromospheric activity is often measured with the Mount Wilson S-index. Chromospheres are common in many luminous stars, especially cooler, magnetically active ones, as well as in certain brown dwarfs and giant stars.
This page was last edited on 3 February 2026, at 11:41 (CET).