Photosphere

The photosphere is a star's outer shell from which light is radiated.

This article is about the astronomical object. For the image stitching feature, see VR photography.
Not to be confused with Photon sphere.
The structure of the Sun, a G-type star:
Not labelled: Solar wind
A miasma of plasma
The surface of the Sun

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.[note 1] 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%, respectively, of the mass of the Sun in the photosphere. 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.[5] See here for meanings of extra lines in the graph.

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] and a density of about 3×10−4 kg/m3;[8] increasing with depth into the sun.[5] Other stars may have hotter or cooler photospheres. The Sun's photosphere is around 100 kilometers thick.

Photospheric phenomena
Further information: Solar phenomena

In the Sun's photosphere, the most ubiquitous phenomena 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.[9] 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
  1. 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]

References
  1. Carroll, Bradley W. & Ostlie, Dale A. (1996). Modern Astrophysics. Addison-Wesley.
  2. Kanaan, A.; et al. (WET) (2005). "Whole Earth Telescope observations of BPM 37093: A seismological test of crystallization theory in white dwarfs". Astronomy & Astrophysics. 432 (1): 219–224. arXiv:astro-ph/0411199v1. Bibcode:2005A&A...432..219K. doi:10.1051/0004-6361:20041125. S2CID 7297628.
  3. Jones, P. B. (2003). "Nature of Fault Planes in Solid Neutron Star Matter". The Astrophysical Journal. 595 (1): 342–345. arXiv:astro-ph/0210207. Bibcode:2003ApJ...595..342J. doi:10.1086/377351. S2CID 119335130.
  4. Jones, P. B. (2004). "Heterogeneity of solid neutron-star matter: Transport coefficients and neutrino emissivity". Monthly Notices of the Royal Astronomical Society. 351 (3): 956–966. arXiv:astro-ph/0403400. Bibcode:2004MNRAS.351..956J. doi:10.1111/j.1365-2966.2004.07834.x. S2CID 11877513.
  5. John A. Eddy (1979). "SP-402 A New Sun: The Solar Results From Skylab". NASA. Archived from the original on 2004-11-18. Retrieved 2017-07-12.
  6. "Sun Fact Sheet". nssdc.gsfc.nasa.gov. Retrieved 2023-08-27.
  7. "Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties" (PDF). 2014. Archived (PDF) from the original on 2016-01-28.
  8. Stanford Solar Center (2008). "The Sun's Vital Statistics". Archived from the original on 2012-07-03. Retrieved 2018-02-20.
  9. "NASA/Marshall Solar Physics". NASA. Archived from the original on 2016-02-05. Retrieved 2008-02-19.
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