# Metal rich or metal poor?

What does the image in the link mean? Are the stars in the halo high in metallicity or low? How about at the bottom by the disk?

The ones in the halo have blue colors, which is the "old" part of the spectrum. Each type of metallicity is usually measured using a form of logarithmic scale, since the flat percentage of metals (especially iron peak elements) of any star will be very small. In this case, we have $$[Fe/H] = \log(N_{\text{Fe}}/N_H)_{\text{star}} - \log(N_{\text{Fe}}/N_H)_{\text{sun}},$$ So we take the ratio of iron to hydrogen in the target star, measure it as well in our own sun, then take the ratio of those two and take the logarithm of that value. Since the $\log$ function is monotonic and has $\log(1)=0$, a negative value of $[Fe/H]$ means our Sun has more iron in it (relative to the amount of hydrogen). A value of $[Fe/H]=-1$ means our sun has an iron to hydrogen ratio that is 10 times greater than the given star's, for example. The more negative this value, the more metal-poor the star is (or, at least, iron poor) than our own sun, which is considered of a fairly recent generation. Lower metallicities are typical of earlier generations of stars, and so may be described as "old". Note, however, that they need not necessarily be old stars. In principle it is possible for a recently formed star to have formed in an unusually metal-poor region, or otherwise been unable to accrete the normal quantity of metals.