Photosphere

The photosphere of an astronomical object is the region at which the optical depth becomes one for a photon of wavelength equal to 5000 angstroms. (Photo means light, hence the term photosphere.) In other words, the photosphere is the region 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 5800 kelvins; other stars may have hotter or cooler photospheres. The Sun's photosphere is composed of convection cells called granulesâ€"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 eight 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.

The Sun's visible atmosphere has other layers above the photosphere: the 10,000 kilometer-deep chromosphere (typically observed by filtered light, for example H-alpha) lies just between the photosphere and the much hotter but more tenuous corona. Other "surface features" on the photosphere are solar flares and sunspots.