Metallicities and Stellar Populations

Remember that a star's chemical composition can be characterized by
fraction by mass
solar value
hydrogen content
helium content
everything else 
(C, O, Mg, Si, Fe, etc: "metals")

and we define these quantities such that X+Y+Z=1

A different way of measuring a star's chemical composition is by the Iron(Fe)-to-Hydrogen(H) ratio:

for the sun,  so that for every Iron atom there are 20,000 Hydrogen atoms.

We measure this value for other stars relative to the sun using a quantity called [Fe/H]:

Defined this way the Sun has a metallicity 

Question: What does it mean to have a metallicity:

Stars can span a wide range of metallicity: -4.5 < [Fe/H] < +1.0

Why would stars have different metallicities? What is metallicity tracking?

A second way to characterize metallicity is through the alpha-to-iron ratio, [α/Fe], which involve elements built by combining helium nuclei, such as Oxygen, Silicon, Neon, etc.

Two channels to build up metallicity enhancements:

High alpha/iron: stars formed in a rapid burst of star formation, which quickly ceased. Lower alpha/iron: stars formed more continuously over time.

Stellar Populations

Observations: what does this mean?

In the 1940s, Walter Baade introduced the concept of stellar populations:

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

So any model for how the Galaxy formed and evolved must explain why there are different populations of stars in different parts of the Galaxy.

Note that metallicity also correlates with color:

  • metal poor makes stars bluer
  • metal rich makes stars redder


  • Line blanketing: lots of metals (particularly Fe) in the atmospheres of stars absorb preferentially blue light, so the star looks a bit redder.

  • Opacity: more metals absorb energy from the interior of the star, making red giants "swell up" even more, and give them cooler (redder) temperatures.

So lots of things affect colors:
  • Age (young stars are blue)
  • Metallicity (metal rich stars are redder than metal poor stars)
  • Dust (reddens stars)
What a mess! But careful measurements of colors and spectra can detangle these effects, at least crudely...

Example of line blanketing