photosphere

Photosphere is the visible surface of a star, the quasi-spherical shell from which most of the visible light escapes. It is the layer where the optical depth at visible wavelengths is approximately 2/3, marking the transition between optically thick deeper layers and optically thin outer layers. Because it is a gaseous layer rather than a solid surface, a star does not have a sharp edge.

In the Sun, the photosphere has an effective temperature of about 5,500 to 6,000 Kelvin, giving it a yellow-white color. The temperature varies among stars, with hotter stars having bluer photospheres and cooler ones redder. The photosphere is characterized by a granulated pattern produced by convective cells; limb darkening, where the edge appears dimmer than the center due to looking through cooler, shallower layers at the limb. The spectrum shows a continuum with many absorption lines formed within the photosphere, imprinted on light from deeper layers.

Geographically, the photosphere is the lowest portion of the Sun's atmosphere; just above lies the chromosphere and corona, while below it lie the convection zone layers. Its thickness is small compared with the stellar radius, on the order of a few hundred kilometers for the Sun, though the precise extent depends on wavelength and the stellar atmosphere model. The term photosphere derives from Greek words for light and sphere and is used to describe the radiating surface of stars in general, with properties inferred from spectroscopy rather than direct imaging for distant stars.