Photosphere
The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3. In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.
The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Stars, excepting neutron stars, have no solid surface. Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface.
The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.
The Sun's photosphere has a temperature between 4,500 and 6,000 K (4,230 and 5,730 °C) (with an effective temperature of 5,777 K (5,504 °C)) and a density of about 2×10−4kg/m3; other stars may have hotter or cooler photospheres. The Sun's photosphere is composed of convection cells called granules—cells of plasma each approximately 1000 kilometers in diameter with hot rising plasma in the center and cooler plasma falling in the narrow spaces between them. Each granule has a lifespan of only about eight minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are super granules up to 30,000 kilometers in diameter with lifespans of up to 24 hours. These details are too fine to see on other stars.
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