Relationship between metallicity and color? Should Pop. I stars be blue?

Question

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?

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?

Answer 1

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.