The photosphere is a star's outer shell from which
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
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
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s.
Stars, except
neutron star
A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
s, have no solid or liquid surface. Therefore, the photosphere is typically used to describe the
Sun's or another
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
's visual surface.
Etymology
The term ''photosphere'' is derived from
Ancient Greek
Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), 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
The Stefan–Boltzmann law, also known as ''Stefan's law'', describes the intensity of the thermal radiation emitted by matter in terms of that matter's temperature. It is named for Josef Stefan, who empirically derived the relationship, and Lu ...
. Various stars have photospheres of various temperatures.
Composition of the Sun
The
Sun is composed primarily of the chemical elements
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
; 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
Astronomy is a natural science that studies astronomical object, celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall chronology of the U ...
, 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
Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
of about 3
kg/
m3;
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—
convection cell
In fluid dynamics, a convection cell is the phenomenon that occurs when density differences exist within a body of liquid or gas. These density differences result in rising and/or falling convection currents, which are the key characteristics o ...
s 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
supergranules up to in diameter with lifespans of up to 24 hours and flow speeds of about , carrying
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
bundles to the edges of the cells. Other magnetically related phenomena in the Sun's photosphere include
sunspot
Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...
s and solar
faculae 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 ''
starspots''.
Notes
References
External links
*
Animated explanation of the Photosphere (University of South Wales).
(University of South Wales).
Solar Lower Atmosphere and Magnetism(
MPS)
{{Portal bar, Astronomy, Outer space, Solar System, Science
Stellar astronomy
Sun
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