<p>The primary driver of a star's colour is its temperature, by way of black-body radiation.
<a href="https://i.sstatic.net/TbA51.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/TbA51.png" alt="enter image description here" /></a></p>
<p>Black body radiation restricts stellar colour to a narrow slice of the full RGB spectrum.</p>
<p>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.
<a href="https://i.sstatic.net/7auQH.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/7auQH.png" alt="Sun's spectrum compared to an idealised blackbody spectrum at 5777 K" /></a>
(All images from wikimedia commons)</p>
<p>There are some mentions in Wikipedia articles of unusual coloured stars. For instance, <a href="https://en.wikipedia.org/wiki/Mu_Cephei" rel="nofollow noreferrer">Mu Cephei</a> is otherwise known as <strong>Herschel's Garnet Star</strong>, and <a href="https://en.wikipedia.org/wiki/Carbon_star" rel="nofollow noreferrer">Carbon stars</a> are described as being 'ruby red'.</p>
<p>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.</p>
<p>It stands to reason that certain absorption and emission lines, if strong enough, would alter the colour of a star to a perceivable extent.</p>
<p><strong>How far could a star's visual colour deviate from the black body axis as depicted in the first picture?</strong></p>

CC BY-SA 4.0
2021-09-04T23:03:05.083Z
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
WarpPrime
CC BY-SA 4.0
2021-09-04T23:06:07.710Z
@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.
Ingolifs
CC BY-SA 4.0
2021-09-04T23:17:46.337Z
Brown dwarfs can shine on their own, if not for a few hundred million years
WarpPrime
CC BY-SA 4.0
2021-09-05T00:43:55.513Z
@fasterthanlight I know, but isn't that colour a dim red? In order to appear purple it would have to emit a decent quantity of blue light in the first place.
Ingolifs
CC BY-SA 4.0
2021-09-05T10:17:16.013Z
Interesting question! If a star is inside other material for which it is formed, do you ask about the spectrum of light exiting the photosphere, or being observed at Earth?
uhoh
CC BY-SA 4.0
2021-09-05T19:47:01.393Z
@uhoh I guess what is observed from earth.
Ingolifs
CC BY-SA 4.0
2021-09-08T10:07:57.267Z
I think one should focus on a level more intrinsic to the stars. Otherwise there could be dramatic changes, like at sunset.
Alchimista
CC BY-SA 4.0
2021-09-21T06:08:29.513Z
To answer the opposite question: How close to a blackbody can a stars spectrum be? see https://iopscience.iop.org/article/10.3847/1538-3881/aac88b. From https://aasnova.org/2018/10/31/perfect-blackbodies-in-the-sky/.
TazAstroSpacial
<star><spectra><black-body><stellar-classification><color>
How far can a star's visual colour deviate from black body radiation?
Ingolifs
# How far can a star's visual colour deviate from black body radiation?

(Asked by: username_0 on 2021-09-04)

The primary driver of a star's colour is its temperature, by way of black-body radiation.
[![enter image description here](https://i.sstatic.net/TbA51.png)](https://i.sstatic.net/TbA51.png)

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](https://i.sstatic.net/7auQH.png)](https://i.sstatic.net/7auQH.png)
(All images from wikimedia commons)

There are some mentions in Wikipedia articles of unusual coloured stars. For instance, [Mu Cephei](https://en.wikipedia.org/wiki/Mu_Cephei) is otherwise known as **Herschel's Garnet Star**, and [Carbon stars](https://en.wikipedia.org/wiki/Carbon_star) 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?**

**Tags:** star, spectra, black-body, stellar-classification, color
> **Comments:**
> **username_1**: 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  
> **username_0**: @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.  
> **username_1**: Brown dwarfs can shine on their own, if not for a few hundred million years  
> **username_0**: @fasterthanlight I know, but isn't that colour a dim red? In order to appear purple it would have to emit a decent quantity of blue light in the first place.  
> **username_2**: Interesting question! If a star is inside other material for which it is formed, do you ask about the spectrum of light exiting the photosphere, or being observed at Earth?  
> **username_0**: @uhoh I guess what is observed from earth.  
> **username_3**: I think one should focus on a level more intrinsic to the stars. Otherwise there could be dramatic changes, like at sunset.  
> **username_4**: To answer the opposite question: How close to a blackbody can a stars spectrum be? see https://iopscience.iop.org/article/10.3847/1538-3881/aac88b. From https://aasnova.org/2018/10/31/perfect-blackbodies-in-the-sky/.  

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## 0 answers

