# 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.

• What is the level of metallicity of the central or nuclear region of the Milky Way, low or high? As we move away from the center does metallicity increase or decrease? – John Jun 21 '15 at 3:56
• The particular graphic you link is inconclusive on that. The stars it considers are approximately the same radial distance from the center as ourselves or further, and focuses in particular on stars above the galactic plane (the halo). Within the colored cone, stars have higher metallicity the closer you get to our sun, and lower metallicty ("older") as you head beyond and above us. – zibadawa timmy Jun 21 '15 at 4:06
• In general though not related to the graphic? – John Jun 21 '15 at 4:59
• @John If I was particulary sure I would have answered your other question on this topic. A cursory exploration suggests the opinion on the matter has changed with time. In the early 90's it looked like the bulge was on average twice as metal rich, but by the late 90's it looked like the bulge on average had about 70% of the sun's metal content (more metal poor). But that's a good 15+ years old, and I'm not sure what's happened since then. I'd consult HDE's links provided in his answer in your other question. – zibadawa timmy Jun 21 '15 at 5:03
• Thanks for your time I appreciate it. Do you think metallicity increases or decreases as we move away from the center? – John Jun 21 '15 at 5:06

Red is higher blue lower metalicity, so the halo has low metalicity and the disk higer.