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photosphere
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 the fact that it is 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.
In other words, 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, excepting neutron stars, have no solid surface.〔As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet (); and only neutron stars are believed to have a solid, albeit unstable (), crust ()〕 Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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