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I recently got questioned on why stars are the color they are. I know the color of a star depends on its surface temperature where hotter stars produce more light towards the blue side of the spectrum and cooler stars towards the red.

My question, or I should say the question of the other person, is that why is this the case? If a cold and a hot star are both producing light/energy by nuclear fusion (hydrogen into helium) then why is one redder and one bluer?

My thought is that the energy of the star excites the surface material (the plasma) which produces light through the emission "line" of the material. Where the color depends on the electron and their orbital changes. Because the hotter star produces more energy overall, this means that more energy is available for the surface plasma to cause their electrons to jump to and fall from higher orbits.

Does this make sense? I don't want to pass on incorrect information.

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    $\begingroup$ It may be helpful to note that stars are essentially blackbodies and thus nearly follow the laws blackbodies and their radiation curves. See What is the RGB curve for blackbodies? $\endgroup$ Dec 27, 2021 at 2:20
  • $\begingroup$ this should give you some idea courses.lumenlearning.com/astronomy/chapter/colors-of-stars $\endgroup$ Dec 27, 2021 at 4:24
  • $\begingroup$ An interesting side note that’s somewhat related: when describing a star, there are 5 really important temperatures that are used to describe what best fits a given mechanism. In theory, if a part of a star was in thermodynamic equilibrium (never perfectly the case but sometimes an ok local approximation), 4 of the 5 temperatures would be the same. The 5th one that doesnt line up? The color temperature; the temperature that makes the black body curve that best fits the star. Like fasterthanlight mentions, stars are very close to black body curves; where they differ is where color temp comes in $\endgroup$
    – Justin T
    Jan 3 at 5:01

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The core of the star is the seat of nuclear fusion, yes, but by the time this energy reaches the surface (a few hundred thousand years at least in the case of our Sun), it has time to dissipate (from a [comparatively] small core to a huge outer surface). What’s left at the photosphere (the apparent surface of the star) is not nuclear fusion anymore, but radiation from “down below.” It’s a little like sitting next to an electric heater; while electricity makes it warm, it’s not electricity anymore that you feel on your skin.

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Okey in agree with @fasterthanlight a star can be treated as a black body. The derivation of the black body law is through thinking about a gas of photons wich obbeys the Bose-Einstein statistics distribution function. This function dependes on the temperature and frecuency, so that is why the radiation intensity varies in function of the two variables mentioned before.

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  • $\begingroup$ This is a good start to an answer, but it probably needs more improvement because it doesn't actually answer the question. Please provide more details as to why star color depends on temperature. $\endgroup$ Dec 31, 2021 at 14:09
  • $\begingroup$ Well it actually answers the question, you can see the typical graphics of the radiation intensity vs wavelength (inversely proportional to frecuency/energy) and see why one is redder or bluer in function of Temperature (en.wikipedia.org/wiki/Planck%27s_law#/media/File:Black_body.svg). If you want is like an phyiscial interpretation of what is happening, let me know and iĺl think about it. $\endgroup$ Jan 1 at 15:41

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