<p>I think you are talking about color index and temperature. So, I will briefly explain under that circumstance.</p>

<p>Color index, e.g., B-V is the difference in brightness (magnitude) of the light observed by filters B and V. (The following discussion can be generalized to other color indeces, e.g., U-B or g-i). Filters B and V have effective wavelengths 4450 (bluer) and V 5510 (redder) A (see <a href="https://en.wikipedia.org/wiki/Photometric_system" rel="nofollow noreferrer">Wikipedia</a>), respectively, therefore B-V increases when the object becomes red. (Recall that magnitude is more negative for being brighter). The temperature which is derived from the color index is called "color temperature;" this is different to blackbody temperature, or kinetic temperature, or other types of inferred temperatures (see [this <a href="https://www.physics.byu.edu/faculty/christensen/Physics%20427/FTI/Measures%20of%20Temperature.htm" rel="nofollow noreferrer">2</a>).</p>

<p>For the color temperature, this is very observational inference. To map from the color index to the color temperature, it is inferred from other well-known object. See <a href="https://en.wikipedia.org/wiki/Color_index" rel="nofollow noreferrer">Wikipedia</a> for this.</p>

<p>Another way to work around is to transform the color index into the flux ratio, then applying the blackbody function as the inference. For example, from B-V >>> flux(4450A)/flux(5510A) >>> blackbody temperature corresponding to the flux ratio.</p>

CC BY-SA 4.0
2018-05-25T13:10:09.167Z
2018-05-25T13:10:09.167Z
Kornpob Bhirombhakdi
<p>How do you determine the temperature of a star by color?</p>

CC BY-SA 4.0
2018-05-25T00:15:35.197Z
You got it backwards. Stars are black body radiators, so their color is determined mostly by their temperature. Coldest stars are red. Intermediate temperatures are: orange, yellow, and white. Hottest stars are blue.
Florin Andrei
CC BY-SA 4.0
2018-05-25T14:46:20.623Z
The question is a good one, but we frown on doing people's homework for them.
Carl Witthoft
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2018-05-25T14:46:41.907Z
I'm voting to close this question as off-topic because it's homework (and time-sensitive)
Carl Witthoft
CC BY-SA 4.0
2018-05-25T14:47:46.327Z
@FlorinAndrei  that's a bit too simplified, seeing as "white" is not a color.  As you know, the temperature calculation is based on the peak output wavelength.
Carl Witthoft
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2018-05-25T18:26:39.497Z
@CarlWitthoft It's a 3-line comment, of course it's simplified.
Florin Andrei
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2018-05-26T13:26:05.833Z
@JanDoggen at the time I wrote that,  the text said something like "Please answer ASAP because worksheet is due"
Carl Witthoft
CC BY-SA 4.0
2018-05-26T13:32:27.030Z
Sometimes I really don't understand how this site is operated. Even if it is a homework question, it is still a question.
Kornpob Bhirombhakdi
<star><fundamental-astronomy><temperature>
How can the temperature of stars can be determined by their color?
user23179
# How can the temperature of stars can be determined by their color?

(Asked by: username_0 on 2018-05-25)

How do you determine the temperature of a star by color?

**Tags:** star, fundamental-astronomy, temperature
> **Comments:**
> **username_1**: You got it backwards. Stars are black body radiators, so their color is determined mostly by their temperature. Coldest stars are red. Intermediate temperatures are: orange, yellow, and white. Hottest stars are blue.  
> **username_2**: The question is a good one, but we frown on doing people's homework for them.  
> **username_2**: I'm voting to close this question as off-topic because it's homework (and time-sensitive)  
> **username_2**: @FlorinAndrei  that's a bit too simplified, seeing as "white" is not a color.  As you know, the temperature calculation is based on the peak output wavelength.  
> **username_1**: @CarlWitthoft It's a 3-line comment, of course it's simplified.  
> **username_2**: @JanDoggen at the time I wrote that,  the text said something like "Please answer ASAP because worksheet is due"  
> **username_3**: Sometimes I really don't understand how this site is operated. Even if it is a homework question, it is still a question.  

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## 1 answers

### Answer by username_3 on 2018-05-25. Score: 3

I think you are talking about color index and temperature. So, I will briefly explain under that circumstance.

Color index, e.g., B-V is the difference in brightness (magnitude) of the light observed by filters B and V. (The following discussion can be generalized to other color indeces, e.g., U-B or g-i). Filters B and V have effective wavelengths 4450 (bluer) and V 5510 (redder) A (see [Wikipedia](https://en.wikipedia.org/wiki/Photometric_system)), respectively, therefore B-V increases when the object becomes red. (Recall that magnitude is more negative for being brighter). The temperature which is derived from the color index is called "color temperature;" this is different to blackbody temperature, or kinetic temperature, or other types of inferred temperatures (see [this [2](https://www.physics.byu.edu/faculty/christensen/Physics%20427/FTI/Measures%20of%20Temperature.htm)).

For the color temperature, this is very observational inference. To map from the color index to the color temperature, it is inferred from other well-known object. See [Wikipedia](https://en.wikipedia.org/wiki/Color_index) for this.

Another way to work around is to transform the color index into the flux ratio, then applying the blackbody function as the inference. For example, from B-V >>> flux(4450A)/flux(5510A) >>> blackbody temperature corresponding to the flux ratio.


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