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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 2⁄3,^[1] 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.^{[note 1]} 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.^{[citation needed]}

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 (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.

Sun's photosphere

The Sun's photosphere has a temperature between 4,400 and 6,600 K (4,130 and 6,330 °C) (with an effective temperature of 5,772 K (5,499 °C))^[6]^[7] meaning human eyes perceive it as an overwhelmingly bright surface, and with sufficiently strong neutral density filter, as a hueless, gray surface. It has a density of about 3×10⁻⁴ kg/m³;^[8] increasing with increasing depth.^[5] The Sun's photosphere is 100–400 kilometers thick.^[9]^[10]^[11]

Photospheric phenomena

In the Sun's photosphere, the most ubiquitous phenomenon are granules—convection cells of plasma each approximately 1,000 km (620 mi) in diameter with hot rising plasma in the center and cooler plasma falling in the spaces between them, flowing at velocities of 7 km/s (4.3 mi/s). Each granule has a lifespan of only about twenty minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are supergranules up to 30,000 km (19,000 mi) in diameter with lifespans of up to 24 hours and flow speeds of about 500 m/s (1,600 ft/s), carrying magnetic field bundles to the edges of the cells. Other magnetically related phenomena in the Sun's photosphere include sunspots and solar faculae dispersed between granules.^[12] These features are too fine to be directly observed on other stars; however, sunspots have been indirectly observed, in which case they are referred to as starspots.

Notes

^ As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet;^[2] and only neutron stars are believed to have a solid, albeit unstable,^[3] crust ^[4]