Photosphere

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 2⁄3,^[1] or equivalently, a depth from which 50% of light will escape without being scattered.

Stars, except perhaps 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.

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.^{[citation needed]}

Sun's photosphere

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] meaning human eyes perceive it as an overwhelmingly bright surface, and with a sufficiently strong neutral density filter, as a hueless, gray surface. It has a density of about 3×10⁻⁴ kg/m³;^[8] increasing with increasing depth.^[5] The Sun's photosphere is 100–400 kilometers thick.^[9]^[10]

Photospheric phenomena

In the Sun's photosphere, the most ubiquitous phenomenon 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.^[11] These features are too fine to be directly observed on other stars; however, stellar spatial structures have been indirectly observed, and can behave like sunspots. We can refer to these as starspots, although clearly non-analogous features also may be present as well.

Notes

^ 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

^ Carroll, Bradley W. & Ostlie, Dale A. (1996). Modern Astrophysics. Addison-Wesley.
^ 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.
^ 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.
^ 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.
^ ^a ^b 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.
^ "Sun Fact Sheet". nssdc.gsfc.nasa.gov. Retrieved 2023-08-27.
^ "Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties" (PDF). 2014. Archived (PDF) from the original on 2016-01-28.
^ Stanford Solar Center (2008). "The Sun's Vital Statistics". Archived from the original on 2012-07-03. Retrieved 2018-02-20.
^ "The Photosphere". NASA. Retrieved 5 November 2023.
^ "Layers of the Sun". NASA. Retrieved 5 November 2023.
^ "NASA/Marshall Solar Physics". NASA. Archived from the original on 2016-02-05. Retrieved 2008-02-19.

External links

Media related to Photosphere at Wikimedia Commons
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)

[5] 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]

[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.

[Eddy-6] 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.

[7] "Sun Fact Sheet". nssdc.gsfc.nasa.gov. Retrieved 2023-08-27.

[8] "Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties" (PDF). 2014. Archived (PDF) from the original on 2016-01-28.

[Stanford_University-9] Stanford Solar Center (2008). "The Sun's Vital Statistics". Archived from the original on 2012-07-03. Retrieved 2018-02-20.

[10] "The Photosphere". NASA. Retrieved 5 November 2023.

[11] "Layers of the Sun". NASA. Retrieved 5 November 2023.

[12] "NASA/Marshall Solar Physics". NASA. Archived from the original on 2016-02-05. Retrieved 2008-02-19.

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