Metallicities and Stellar Populations

Remember that a star's chemical composition can be characterized by

	fraction by mass	solar value
hydrogen content	X	0.70
helium content	Y	0.28
everything else (C, O, Mg, Si, Fe, etc: "metals")	Z	0.02

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:

[Fe/H] = +1.0
[Fe/H] = -2.0

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:

Iron, built up in massive stars and also later in Type Ia (white dwarf) supernovae: prompt and delayed release after stars are formed, involve high and low mass stars
Alphas, built up in massive stars: prompt release, high mass stars

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:

Globular clusters are generally metal-poor
Disk stars span a range of metallicities
Open clusters are generally more metal-rich

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

Why?

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