Photosphere
The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately, or equivalently, a depth from which 50% of light will escape without being scattered.
A photosphere is the region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.
Stars, except neutron stars, have no solid or liquid surface. Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface.
Etymology
The term photosphere is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light.Temperature
The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Various stars have photospheres of various temperatures.Composition of the Sun
The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere. All heavier elements, colloquially called metals in stellar astronomy, account for less than 2% of the mass, with oxygen, carbon, neon, and iron being the most abundant.Sun's photosphere
Image:Sun Atmosphere Temperature and Density SkyLab.jpg|thumb|upright|Solar atmosphere: temperature and density. See here for meanings of extra lines in the graph.
The Sun's photosphere has a temperature between meaning human eyes perceive it as an overwhelmingly bright surface, and with a sufficiently strong neutral density filter, as a hueless, gray surface. It has a density of about 3 kg/m3; increasing with increasing depth. The Sun's photosphere is 100–400 kilometers thick.