<p>Unless you know all the stars in the diagram suffer the same extinction, then you should de-redden them individually.</p>

<p>If you do know they have the same extinction (e.g. they are all nearby stars and it is zero; or they are all stars well above the Galactic plane in a similar direction; or stars in a cluster at the same distance) then you can plot a diagram using the observed magnitudes. In the latter scenario of a cluster, this diagram can be used to estimate the reddening of the cluster.</p>

<p>If you want to compare with models, to estimate temperatures or masses, then you must de-redden (or redden the model!)</p>

CC BY-SA 4.0
2019-11-01T20:15:29.537Z
2019-11-01T20:15:29.537Z
ProfRob
<p>If you can correct them because you know <code>A_V</code> and <code>A_B</code> (which can be hard), it's probably better, although I don't think there is a general rule here.</p>

CC BY-SA 4.0
2019-10-02T14:25:18.633Z
2019-10-02T14:25:18.633Z
onekiloparsec
<p>I am reading my textbooks and a few "Astro101" websites, but I cannot find a final answer to this:</p>

<p>In the <span class="math-container">$B-V$</span> colour index that is plotted in an H-R diagram, are <span class="math-container">$B$</span> and <span class="math-container">$V$</span> the (observed) apparent magnitudes, or should they be corrected for extinction first, i.e. with known <span class="math-container">$A_V$</span> and <span class="math-container">$A_B$</span> values?</p>

<photometry>
How do I calculate B-V to make a Hertzsprung Russell diagram?
Curious_George
# How do I calculate B-V to make a Hertzsprung Russell diagram?

(Asked by: username_0 on 2019-10-02)

I am reading my textbooks and a few "Astro101" websites, but I cannot find a final answer to this:

In the $B-V$ colour index that is plotted in an H-R diagram, are $B$ and $V$ the (observed) apparent magnitudes, or should they be corrected for extinction first, i.e. with known $A\_V$ and $A\_B$ values?

**Tags:** photometry

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## 2 answers

### Answer by username_1 on 2019-11-01. Score: 3

Unless you know all the stars in the diagram suffer the same extinction, then you should de-redden them individually.

If you do know they have the same extinction (e.g. they are all nearby stars and it is zero; or they are all stars well above the Galactic plane in a similar direction; or stars in a cluster at the same distance) then you can plot a diagram using the observed magnitudes. In the latter scenario of a cluster, this diagram can be used to estimate the reddening of the cluster.

If you want to compare with models, to estimate temperatures or masses, then you must de-redden (or redden the model!)


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### Answer by username_2 on 2019-10-02. Score: 0

If you can correct them because you know `A_V` and `A_B` (which can be hard), it's probably better, although I don't think there is a general rule here.


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