The photosphere is the thin layer of a star from which the light we see escapes, effectively its visible surface even though the star has no solid boundary. On the Sun it is a few hundred kilometers deep and sits near 5,500 degrees Celsius, cool enough to appear dark against the million-degree corona above it in certain wavelengths. Sunspots are photospheric features where intense magnetic fields suppress convection and the gas cools. Nearly everything known about stellar composition comes from absorption lines imprinted on light leaving this layer.