Photosphere

A miasma of plasma

Sun's surface in false color

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 Template:Frac,<ref>Template:Cite book</ref> 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.Template:NoteTag 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.Template: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

File:Sun Atmosphere Temperature and Density SkyLab.jpg

Solar atmosphere: temperature and density.<ref name=Eddy /> See here for meanings of extra lines in the graph.

The Sun's photosphere has a temperature between Template:Convert (with an effective temperature of Template:Convert)<ref>Template:Cite web</ref><ref>Template:Cite web</ref> 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 3Template:E kg/m³;<ref name="Stanford University">Template:Cite web</ref> increasing with increasing depth.<ref name=Eddy>Template:Cite web</ref> The Sun's photosphere is 100–400 kilometers thick.<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref>

Photospheric phenomena

In the Sun's photosphere, the most ubiquitous phenomenon are granules—convection cells of plasma each approximately Template:Cvt in diameter with hot rising plasma in the center and cooler plasma falling in the spaces between them, flowing at velocities of Template:Cvt. 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 Template:Cvt in diameter with lifespans of up to 24 hours and flow speeds of about Template:Cvt, 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.<ref>Template:Cite web</ref> 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.

External links

• Template:Commons category-inline
• Animated explanation of the Photosphere (University of South Wales).
• Animated explanation of the temperature of the Photosphere (University of South Wales).
• Solar Lower Atmosphere and Magnetism (MPS)