# 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, [$\alpha$/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

## 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