<p>The reddening line is the locus that a star would move along in a colour-colour diagram as the interstellar reddening towards it increases.</p>
<p>i.e. It is an equation on the colour-colour plot of the form (for this colour-colour plot)
<span class="math-container">$$E(U-B) = f(E(B-V)) + c\ ,$$</span>
where <span class="math-container">$E(U-B)$</span> is the colour excess in <span class="math-container">$U-B$</span> due to interstellar reddening, <span class="math-container">$E(B-V)$</span> is the colour excess in <span class="math-container">$B-V$</span> and <span class="math-container">$c$</span> is a constant chosen so that the locus would go through a star of the same spectral type but with no interstellar reddening.</p>
<p>Typically, for low reddening and a typical interstellar medium
<span class="math-container">$$ E(U-B) = 0.72 E(B-V) + 0.05 (E(B-V))^2\ ,$$</span>
which I imagine is the line drawn on the plot you show.</p>
<p>To establish the reddening to a particular star you can follow it back from its observed position in the colour-colour plot, along a reddening line, until it intersects the main sequence locus (if it <em>is</em> a main sequence star). The intersection would give its unreddened, intrinsic colours and the x-axis increment through which you traced it back would give its <span class="math-container">$E(B-V)$</span>.</p>

CC BY-SA 4.0
2021-05-04T20:12:07.747Z
Thank you sir for the amazing and apt answer.
Vampy
CC BY-SA 4.0
2021-05-04T17:56:21.247Z
2021-05-04T17:56:21.247Z
ProfRob
<p>The plot is from this <a href="http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1961ApJ...133..874A&amp;defaultprint=YES&amp;filetype=.pdf" rel="nofollow noreferrer">paper</a> (pg-8). <a href="https://i.sstatic.net/DeCTE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DeCTE.png" alt="Colo-Color diagram for Blackbody and stars" /></a></p>
<p>They have also talked(in the same <a href="http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1961ApJ...133..874A&amp;defaultprint=YES&amp;filetype=.pdf" rel="nofollow noreferrer">paper</a>) about how central stars from galactic longitude range <span class="math-container">$20^\circ-80^\circ$</span> will have a very unusual <em>Reddening</em> line (pg-9). The current plot is for central stars from the galactic longitude range <span class="math-container">$200^\circ-300^\circ$</span></p>
<blockquote>
<p>The group
of planetary nuclei plotted here are all those occurring in galactic longitudes 200o
-300°.
In the longitude range from 20° to 80° the points fall to the right of the reddening line in
Figure 3 (not plotted). If the latter energy-curves are well-behaved, a very unusual
reddening line would be needed to explain their position in the diagram. This is a puzzling result which is unexplained at present.</p>
</blockquote>
<p>My interpretation of <em>Reddening</em> line: It is showing that if we plot real star data where the sample contains all the stars from bluer end to redder end on this <strong>U-B vs B-V</strong> plot, we will get  the <em>Reddening</em>
line on the plot.</p>
<p>Is this interpretation correct?</p>

<star><nebula><black-body><color-index>
What does the Reddening line represents in this U-B vs B-V plot?
Vampy
# What does the Reddening line represents in this U-B vs B-V plot?

(Asked by: username_0 on 2021-05-04)

The plot is from this [paper](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1961ApJ...133..874A&defaultprint=YES&filetype=.pdf) (pg-8). [![Colo-Color diagram for Blackbody and stars](https://i.sstatic.net/DeCTE.png)](https://i.sstatic.net/DeCTE.png)

They have also talked(in the same [paper](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1961ApJ...133..874A&defaultprint=YES&filetype=.pdf)) about how central stars from galactic longitude range $20^\circ-80^\circ$ will have a very unusual *Reddening* line (pg-9). The current plot is for central stars from the galactic longitude range $200^\circ-300^\circ$

> The group
> of planetary nuclei plotted here are all those occurring in galactic longitudes 200o
> -300°.
> In the longitude range from 20° to 80° the points fall to the right of the reddening line in
> Figure 3 (not plotted). If the latter energy-curves are well-behaved, a very unusual
> reddening line would be needed to explain their position in the diagram. This is a puzzling result which is unexplained at present.

My interpretation of *Reddening* line: It is showing that if we plot real star data where the sample contains all the stars from bluer end to redder end on this **U-B vs B-V** plot, we will get the *Reddening*
line on the plot.

Is this interpretation correct?

**Tags:** star, nebula, black-body, color-index

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## 1 answers

### Answer by username_1 on 2021-05-04. Score: 0 | Accepted answer

The reddening line is the locus that a star would move along in a colour-colour diagram as the interstellar reddening towards it increases.

i.e. It is an equation on the colour-colour plot of the form (for this colour-colour plot)
$$E(U-B) = f(E(B-V)) + c\ ,$$
where $E(U-B)$ is the colour excess in $U-B$ due to interstellar reddening, $E(B-V)$ is the colour excess in $B-V$ and $c$ is a constant chosen so that the locus would go through a star of the same spectral type but with no interstellar reddening.

Typically, for low reddening and a typical interstellar medium
$$ E(U-B) = 0.72 E(B-V) + 0.05 (E(B-V))^2\ ,$$
which I imagine is the line drawn on the plot you show.

To establish the reddening to a particular star you can follow it back from its observed position in the colour-colour plot, along a reddening line, until it intersects the main sequence locus (if it *is* a main sequence star). The intersection would give its unreddened, intrinsic colours and the x-axis increment through which you traced it back would give its $E(B-V)$.

> **Comments:**
> **username_0**: Thank you sir for the amazing and apt answer.  

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