<p>You can never tell the luminosity of a star from knowledge of its color X-Y alone, that only tells you something about the surface temperature.  The problem is not solved by knowing the distance modulus, that only tells you how far the star is, but not how big it is.  If you want luminosity from those things, you need to know how bright the star looks, not just its color and distance.  Or you could look at the width of the spectral lines to infer luminosity class, but that brings in line resolution.  But if you know the distance modulus and Lx and Ly, not just Lx/Ly, then you can get the luminosity because you know its color, how bright it looks, and its distance.</p>

CC BY-SA 3.0
2016-10-07T19:04:56.757Z
Just to be clear though: Lx and Ly are some sort of flux, but the normalization is unknown to me. I do have two separate values and (so) I have their ratio which gives me the colour. However, they are unitless and for as far as I know, to calculate the luminosity I'd need to know the proper scaling. Am i wrong?
user1991
CC BY-SA 3.0
2016-10-08T00:14:57.823Z
That sounds right, if you know Lx and Ly independently of each other, you still have to know some sort of scale for them in order to know the actual luminosity.  If you don't know the scale of Lx and Ly, then it means nothing to know them independently, your knowledge is the same as their ratio.
Ken G
CC BY-SA 3.0
2016-10-07T17:19:29.653Z
2016-10-07T17:19:29.653Z
Ken G
<p>Given two colorindices LX and LY, such that the color X-Y is given by</p>

<pre><code>X-Y = -2.5 log(LX/LY)
</code></pre>

<p>is there a way to infer the luminosity in X (or Y)? I'm think not because any information on units has fallen out through this definition, but I could be wrong. I'm just not sure how to compute the luminosity straightforwardly otherwise, as this color index is the only output (along with the distance modulus) from an interpolation scheme of stellar templates.</p>

<p>Help would be much appreciated.</p>

<galaxy><luminosity>
From color index to luminosity
user1991
# From color index to luminosity

(Asked by: username_0 on 2016-10-07)

Given two colorindices LX and LY, such that the color X-Y is given by
X-Y = -2.5 log(LX/LY)

is there a way to infer the luminosity in X (or Y)? I'm think not because any information on units has fallen out through this definition, but I could be wrong. I'm just not sure how to compute the luminosity straightforwardly otherwise, as this color index is the only output (along with the distance modulus) from an interpolation scheme of stellar templates.
Help would be much appreciated.

**Tags:** galaxy, luminosity

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## 1 answers

### Answer by username_1 on 2016-10-07. Score: 1

You can never tell the luminosity of a star from knowledge of its color X-Y alone, that only tells you something about the surface temperature. The problem is not solved by knowing the distance modulus, that only tells you how far the star is, but not how big it is. If you want luminosity from those things, you need to know how bright the star looks, not just its color and distance. Or you could look at the width of the spectral lines to infer luminosity class, but that brings in line resolution. But if you know the distance modulus and Lx and Ly, not just Lx/Ly, then you can get the luminosity because you know its color, how bright it looks, and its distance.

> **Comments:**
> **username_0**: Just to be clear though: Lx and Ly are some sort of flux, but the normalization is unknown to me. I do have two separate values and (so) I have their ratio which gives me the colour. However, they are unitless and for as far as I know, to calculate the luminosity I'd need to know the proper scaling. Am i wrong?  
> **username_1**: That sounds right, if you know Lx and Ly independently of each other, you still have to know some sort of scale for them in order to know the actual luminosity.  If you don't know the scale of Lx and Ly, then it means nothing to know them independently, your knowledge is the same as their ratio.  

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