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 it being 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,[1] or equivalently, a depth from which 50% of light will escape without being scattered.
A photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.
Temperature
The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Stars, except neutron stars, have no solid or liquid surface.[2] Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface.
Composition of the Sun
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.
Sun
Solar atmosphere: temperature and density.[3] See here for meanings of extra lines in the graph.
The Sun's photosphere has a temperature between 4,500 and 6,000 K (4,230 and 5,730 °C)[4] (with an effective temperature of 5,777 K (5,504 °C))[5] and a density of about 3×10−4 kg/m3;[6] increasing with depth into the sun.[3] Other stars may have hotter or cooler photospheres. The Sun's photosphere is around 100 kilometers thick, and 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, flowing at velocities of 7 kilometer per second. Each granule has a lifespan of only about twenty 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 and flow speeds of about 500 meter per second, carrying magnetic field bundles to the edges of the cells. Other magnetically-related phenomena include sunspots and solar faculae dispersed between the granules.[7] These details are too fine to be seen when observing other stars from earth.
Other layers of the Sun
The Sun's visible atmosphere has other layers above the photosphere: the 2,000 kilometer-deep chromosphere (typically observed by filtered light, for example H-alpha) lies just between the photosphere and the much hotter but more tenuous corona. Other "surface features" on the photosphere are solar flares and sunspots.
References
Carroll, Bradley W. & Ostlie, Dale A. (1996). Modern Astrophysics. Addison-Wesley.
As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet [1]; and only neutron stars are believed to have a solid, albeit unstable [2], crust [3]
John A. Eddy (1979). "SP-402 A New Sun: The Solar Results From Skylab". NASA.
The Sun – Introduction
World Book at NASA – Sun
Stanford Solar Center (2008). "The Sun's Vital Statistics".
"NASA/Marshall Solar Physics". NASA.
External links
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)
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The Sun
Internal structure
Core Radiation zone Tachocline Convection zone
The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819.jpg
Atmosphere
Photosphere
Supergranulation Granule Faculae Sunspot
Chromosphere
Plage Spicule Moreton wave
Corona
Transition region Coronal hole Coronal loop Coronal mass ejection Prominence Helmet streamer Supra-arcade downflows
Variation
Solar cycle
List of solar cycles Solar maximum Solar minimum Wolf number Solar wind Flare Helioseismology
Heliosphere
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Related
Eclipse Heliophysics In culture
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Spectral class: G-type main-sequence star
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Shell B[e]
White dwarf
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Burster
Hypothetical
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Symbiotic Remnant Luminous red nova
Structure
Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism
Properties
Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram
Star systems
Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system
Earth-centric
observations
Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard
Lists
Proper names
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Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy
Related articles
Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event
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