Photosphere

The photosphere of an astronomical object is the region at which the optical depth becomes one. In other words, the photosphere is the place where an object stops being transparent. It is typically used to describe the Sun or another star. Because stars are large balls of gas, they have no solid surface. However, there is a depth at which the gas stops being transparent to photons, and this depth provides a visual surface to the star.

The Sun's photosphere has a temperature of about 6000 Kelvins; other stars may have hotter or cooler photospheres. The Sun's photosphere is composed of convection cells called granules, firestorms each approximately 1000 kilometers in diameter with hot rising gas in the center and cooler gases falling in the narrow spaces between them. Each granule has a lifespan of only about 8 minutes, resulting in a continually shifting "boiling" pattern. Amid the typical granules are supergranules up to 30,000 kilometers in diameter with lifespans of up to 24 hours. It is unknown whether these features are typical of other stars.

Other "surface features" on the photosphere are solar flares and sunspots.