<p>I think the misunderstanding (which I've been asked about before) is because Population II stars are redder <em>as a population</em>, even though for a given mass, a main-sequence Pop II (i.e. metal-poor) star would be bluer.  That is, if you compare representative samples of Pop I and Pop II stars, the Pop II stars will on average be redder.</p>

<p>This because Pop II stars are generally old so the blue stars have already died as supernovae or evolved into compact remnants. So looking at the whole population, you'll see more of the longer-lived, lower-mass stars (including red giants), which are redder.</p>

<p>The same argument applies to galaxies that aren't forming stars appearing redder than those galaxies that are still forming stars.  Since new stars are being born, some are blue, but they live fast and die young, so unless the galaxy keeps forming stars, it will become red overall.  Again, this isn't because of the stars being redder for a given property but because the population lacks young, hot, blue stars.</p>

CC BY-SA 3.0
2017-03-02T09:20:45.320Z
2017-03-02T09:20:45.320Z
Warrick
<p>I have found in numerous places such as this website: <a href="http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Structure/metals.html" rel="nofollow noreferrer">http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Structure/metals.html</a> or in "Introduction to Stellar Astrophysics" by B. Carroll, that state:</p>

<blockquote>
  <p>Population I: <br>
  metal rich [Fe/H] > -1 <br>
  disk stars <br>
  open clusters <br>
  metal rich makes stars redder</p>
  
  <p>Population II<br>
  metal poor [Fe/H] &lt; -1  <br>
  halo stars<br>
  globular clusters <br>
  metal poor makes stars bluer</p>
</blockquote>

<p>And I also found (from: <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html" rel="nofollow noreferrer">http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html</a>): </p>

<blockquote>
  <p>Population I stars include the sun and tend to be luminous, hot and young, concentrated in the disks of spiral galaxies. They are particularly found in the spiral arms. With the model of heavy element formation in supernovae, this suggests that the gas from which they formed had been seeded with the heavy elements formed from previous giant stars. About 2% of the total belong to Population I.</p>
  
  <p>Population II stars tend to be found in globular clusters and the nucleus of a galaxy. They tend to be older, less luminous and cooler than Population I stars. They have fewer heavy elements, either by being older or being in regions where no heavy-element producing predecessors would be found. Astronomers often describe this condition by saying that they are "metal poor", and the "metallicity" is used as an indication of age.</p>
</blockquote>

<p><b>If Population I stars are metal rich and therefore appear red, how can they be luminous, hot, and young, and concentrated in disk (the region of star formation)? Shouldn't they appear blue? How does color relate to metallicity?</b></p>

<p>Hot stars should be blue, shouldn't they? As stated here for example:</p>

<blockquote>
  <p>A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K. (<a href="http://www.webexhibits.org/causesofcolor/18B.html" rel="nofollow noreferrer">http://www.webexhibits.org/causesofcolor/18B.html</a>)</p>
</blockquote>

<p>Also, this question is a little similar but not quite: <a href="https://astronomy.stackexchange.com/questions/11057/metal-rich-or-metal-poor">Metal rich or metal poor?</a></p>

<p>There is a graph: <a href="http://science.psu.edu/alert/images/SDSSmetals.jpg/image_view_fullscreen" rel="nofollow noreferrer">http://science.psu.edu/alert/images/SDSSmetals.jpg/image_view_fullscreen</a></p>

<p>I'm not sure if the colors there correspond to the colors of the stars, but they used blue to refer to old stars. Why?</p>

CC BY-SA 3.0
2017-02-28T22:39:54.823Z
They don't "appear red".  They "appear redder".  Big difference.  They shift the color slight towards the red side of the spectrum.  Metals don't universally make everything straight-up red.
zibadawa timmy
CC BY-SA 3.0
2017-03-01T07:14:20.643Z
The piece you quote is just poorly written. Pop II stars are bluer for the same mass. This means they appear to the left of Pop I stars, of the same mass, in the HR diagram.
ProfRob
CC BY-SA 3.0
2017-03-01T15:39:29.087Z
@RobJeffries Just to check that I understand correctly, Pop I can appear blue but Pop II are bluer when you compare the two classes by same mass?
Guest
<star><stellar-evolution><metallicity>
Relationship between metallicity and color? Should Pop. I stars be blue?
Guest
# Relationship between metallicity and color? Should Pop. I stars be blue?

(Asked by: username_0 on 2017-02-28)

I have found in numerous places such as this website: <http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Structure/metals.html> or in "Introduction to Stellar Astrophysics" by B. Carroll, that state:

> Population I:   
> metal rich [Fe/H] > -1   
> disk stars   
> open clusters   
> metal rich makes stars redder
>
> Population II  
> metal poor [Fe/H] < -1   
> halo stars  
> globular clusters   
> metal poor makes stars bluer

And I also found (from: <http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html>):

> Population I stars include the sun and tend to be luminous, hot and young, concentrated in the disks of spiral galaxies. They are particularly found in the spiral arms. With the model of heavy element formation in supernovae, this suggests that the gas from which they formed had been seeded with the heavy elements formed from previous giant stars. About 2% of the total belong to Population I.
>
> Population II stars tend to be found in globular clusters and the nucleus of a galaxy. They tend to be older, less luminous and cooler than Population I stars. They have fewer heavy elements, either by being older or being in regions where no heavy-element producing predecessors would be found. Astronomers often describe this condition by saying that they are "metal poor", and the "metallicity" is used as an indication of age.

**If Population I stars are metal rich and therefore appear red, how can they be luminous, hot, and young, and concentrated in disk (the region of star formation)? Shouldn't they appear blue? How does color relate to metallicity?**

Hot stars should be blue, shouldn't they? As stated here for example:

> A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K. (<http://www.webexhibits.org/causesofcolor/18B.html>)

Also, this question is a little similar but not quite: [Metal rich or metal poor?](https://astronomy.stackexchange.com/questions/11057/metal-rich-or-metal-poor)

There is a graph: <http://science.psu.edu/alert/images/SDSSmetals.jpg/image_view_fullscreen>

I'm not sure if the colors there correspond to the colors of the stars, but they used blue to refer to old stars. Why?

**Tags:** star, stellar-evolution, metallicity

> **Comments:**
> **username_1**: They don't "appear red".  They "appear redder".  Big difference.  They shift the color slight towards the red side of the spectrum.  Metals don't universally make everything straight-up red.  
> **username_2**: The piece you quote is just poorly written. Pop II stars are bluer for the same mass. This means they appear to the left of Pop I stars, of the same mass, in the HR diagram.  
> **username_0**: @RobJeffries Just to check that I understand correctly, Pop I can appear blue but Pop II are bluer when you compare the two classes by same mass?  

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## 1 answers

### Answer by username_3 on 2017-03-02. Score: 4 | Accepted answer

I think the misunderstanding (which I've been asked about before) is because Population II stars are redder *as a population*, even though for a given mass, a main-sequence Pop II (i.e. metal-poor) star would be bluer. That is, if you compare representative samples of Pop I and Pop II stars, the Pop II stars will on average be redder.

This because Pop II stars are generally old so the blue stars have already died as supernovae or evolved into compact remnants. So looking at the whole population, you'll see more of the longer-lived, lower-mass stars (including red giants), which are redder.

The same argument applies to galaxies that aren't forming stars appearing redder than those galaxies that are still forming stars. Since new stars are being born, some are blue, but they live fast and die young, so unless the galaxy keeps forming stars, it will become red overall. Again, this isn't because of the stars being redder for a given property but because the population lacks young, hot, blue stars.


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