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The primary driver of a star's colour is its temperature, by way of black-body radiation. enter image description here

Black body radiation restricts stellar colour to a narrow slice of the full RGB spectrum.

However, all stars have emission and absorption spectra that result in a small deviation from idealised black-body radiation, as can be seen below in the Sun's spectrum compared to an idealised 5777 K blackbody spectrum. Sun's spectrum compared to an idealised blackbody spectrum at 5777 K (All images from wikimedia commons)

There are some mentions in Wikipedia articles of unusual coloured stars. For instance, Mu Cephei is otherwise known as Herschel's Garnet Star, and Carbon stars are described as being 'ruby red'.

It is unclear to me if this is poetic license or that these stars do indeed have a redness to them that lies outside the bounds of the normal blackbody radiation line.

It stands to reason that certain absorption and emission lines, if strong enough, would alter the colour of a star to a perceivable extent.

How far could a star's visual colour deviate from the black body axis as depicted in the first picture?

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  • $\begingroup$ You are correct, absorption/emission lines can alter colors. Also sunspots can cause slight deviations if you take the mean of all it’s colors. At brown-dwarf temperatures, emission lines and atmospheric composition dominates blackbody radiation, hence many brown dwarfs can be magenta, for instance $\endgroup$
    – WarpPrime
    Commented Sep 4, 2021 at 23:03
  • $\begingroup$ @fasterthanlight Good point about the sunspots. Re: brown dwarfs - I was under the impression that brown dwarfs only appear magenta when they're lit up by some other light source. $\endgroup$
    – Ingolifs
    Commented Sep 4, 2021 at 23:06
  • $\begingroup$ Brown dwarfs can shine on their own, if not for a few hundred million years $\endgroup$
    – WarpPrime
    Commented Sep 4, 2021 at 23:17
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    $\begingroup$ @uhoh I guess what is observed from earth. $\endgroup$
    – Ingolifs
    Commented Sep 5, 2021 at 19:47
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    $\begingroup$ To answer the opposite question: How close to a blackbody can a stars spectrum be? see iopscience.iop.org/article/10.3847/1538-3881/aac88b. From aasnova.org/2018/10/31/perfect-blackbodies-in-the-sky. $\endgroup$
    – TazAstroSpacial
    Commented Sep 21, 2021 at 6:08

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