The chromosphere (literally, "sphere of colour") is the second of the three main layers in the Sun's atmosphere and is roughly 2,000 kilometers deep. It sits just above the photosphere and just below the solar transition region.

The density of the chromosphere is very small, it being only 10^–4 times that of the photosphere, the layer just below it, and 10^–8 times that of the atmosphere of Earth. This makes the chromosphere normally invisible and it can only be seen during a total eclipse, where its reddish colour is revealed. The color hues are anywhere between pink and red. However, without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.

The density of the chromosphere decreases with distance from the center of the sun. This decreases logarithmically from 10¹⁷ particles per cubic centimeter, or approximately 2 × 10^–4 kg/m³ to under 1.6 × 10^–11 kg/m³ at the outer boundary. The temperature begins to decrease from the inner boundary of about 6,000 K to a minimum of approximately 3,800 K, before increasing to upwards of 35,000 K at the outer boundary with the transition layer of the corona. Figure 1 shows the trends which density and temperature follow through the chromosphere.