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 ,
A photosphere is the region of a luminous object, usually a star, that is transparent to photons of certain .
Stars, except , 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
Metallicity 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 (with an effective temperature of )
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
kilogram/
cubic meter;
increasing with increasing depth.
The Sun's photosphere is 100–400 kilometers thick.
Photospheric phenomena
In the Sun's photosphere, the most ubiquitous phenomenon are granules—
of plasma each approximately in diameter with hot rising plasma in the center and cooler plasma falling in the spaces between them, flowing at velocities of . Each granule has a lifespan of only about twenty minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are
Supergranulation up to in diameter with lifespans of up to 24 hours and flow speeds of about , carrying
magnetic field bundles to the edges of the cells. Other magnetically related phenomena in the Sun's photosphere include
and solar
Solar facula dispersed between granules.
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
.
Notes
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