Metallicity and Star Populations

No, star populations are not the number of stars in a specific galaxy or the universe or whatnot. No, it has nothing to do with numbers of stars. In fact, these “populations” are actually classes of stars.

To begin with, we need to discuss metallicity. As you (probably) know, stars are almost completely made up of hydrogen and helium. So any other element found in a star would be considered a “metal”. In astronomy, carbon, neon, and fluorine are all considered metals. (And also every other element not hydrogen or helium.) So when you study astronomy, you really must forget all those silly little things taught to you in chemistry class (assuming you have ever had a chemistry class before, of course). Metallicity is essentially the amount of a star that isn’t made up of hydrogen or helium, and the symbol for metallicity is generally Z.

Anyways, the amount of metal found in a star is its metallicity. And metallicity is also divided up into several classes (called populations), depending on the amount of metal in the star. And that, friends, is what a star population is.

There are three categories of metallicity: Population I, Population II, and Population III. Each population, respectively, has decreasing metallic content and increasing age (theoretically—I don’t think anyone has ever tried to go out and sample stuff from a star…). To make all this a lot clearer, I’m going to try to explain in the next few paragraphs.

First off, let’s talk about Population I stars. These are the young stars, having more metal than the other populations (because of the development of heavier elements over billions and billions of years). In a galaxy, they are usually located towards the centre of the galaxy, usually in the disk of the galaxy. For these stars, Z~0.02, and up to 0.03.

The next class is called Population II stars. They are the old stars, with very little metal (because they were formed in a time when heavier elements were nonexistent). They are found more at the outer edges of galaxies, significantly above or below the disk. The metallicity content of these stars is about Z~0.001. And since the kinematics, positions, and chemical compositions of Population I and II stars are different, they provide us with a wealth of information about the Milky Way.

Between Population I and Population II stars, we have the intermediate, or disk population. Those are the stars that just kind of loiter around somewhere in the galaxy, somewhat between the Population I and II stars. They have a medium amount of metal, and they just kind of float around the place. These are your average, every-day, middle-age stars. In fact, they are so average that they get a special name, just to make them feel better. Although sometimes, astronomers like to just put these stars in Population I or II categories, which personally I think is just idiotic. They don’t match the definition, in any case. Do note, though, that just like middle-aged people, they’re there…but the other people on either side of their age range are just as numerous. They don’t have a set metallicity or even a set range; they just are.

And then we have Population III stars. We don’t know if these exist. That’s right, folks, even the existence of this class of stars is purely theoretical. These are the stars that are thought to contain no metal at all. So their metallicity is essentially Z~0. Cool, huh? But the thing is…we’ve never found a Population III star. Those would have to have been formed right at the Big Bang, and with such high mass and energy, they would have burned out fairly (very) quickly. Theoretically, at the time of the big bang, there was only hydrogen and helium, with trace amounts of lithium and beryllium. There were no heavier elements, and apparently lithium and beryllium didn’t really make its way into the stars. So these stars contain just about no metal, are indefinably old, and are purely theoretical. My personal favourite, really. So if on a test they ask you “What population is <star>?”, don’t put Population III. They don’t exist (at least, not definitely).

Our galaxy is actually mostly Population I stars (assuming the classification of disk population into Populations I and II, of course), because Population II stars are very old and most of them have burned out. Really, the only Population II stars that are easily visible to the eye are the globular star clusters, and I’m sure that all of you have studied globular star clusters very extensively.

By percentage, our sun has a 1.8% of metals. But in metallicity, that’s apparently not relevant. The Sun is a standard of comparison for any other star’s metallicity (the sun’s metallicity being equal to 0). By using the metallicity calculations (which are underneath this paragraph), any star with a metallicity<0 is automatically Population II, and any star with a metallicity>0 is automatically Population I. (Again, this irritates me to no end because of the fact that there is a disk population.) Because we are egotistical gits, everything has to be based on our sun, so the range of metallicities in our galaxy ranges from -5.4 to 0.6. Bit skewed? Yes. Obviously, our sun is on the metal-rich side, and yet, we still call it “neutral”.

Then we have the calculations for metallicity. They are complicated, insane, and involve higher mathematics. If you have not taken Algebra 2 (or know what logs are), you might want to stay away from this section. It’s like the equilibrium constant stuff, just not quite as complex. But if you don’t know logs and enjoy torturing yourself greatly, go ahead and read this.

The measurement of metallicity actually comes from the amount of iron in the star. Not every bit of non-hydrogen/helium in the star, but simply iron. It serves as a basis for comparing the ratios of iron to hydrogen in relation to our sun, and it’s the general way to figure metallicity. Now, iron is not the most abundant metal in stars (or even close to it, really), but it’s among the easiest to measure with the technology that we have now. So since we decided to be lazy, this is what we get.

VERY IMPORTANT: The equation below is a RATIO of how much metal is in the star. It is the most commonly used form, but it is NOT a value. The value is what is in the paragraphs above. I hope that clears up any confusion.

Now, the general formula for deriving metallicity is this:

If all of you know the law of logs that states that  , then you can simplify that thing up there to:

which, by the way, is much, much more helpful to solve. The whole  thing is in fact not division, but actually a representation of the logarithmic ratio of the abundance of iron in a star in comparison to the Sun.  means the number of iron atoms in a given amount of volume, and  is the number of hydrogen atoms in a given amount of volume.

According to the age-metallicity relation, a higher amount of iron means it’s a younger star. However, the universe doesn’t like simple laws that are set in stone, and therefore has to make it more confusing. Because the age of a star is basically based on the amount of iron it has, the universe uses Einstein’s probability and statistics laws to make everything very misleading. For example, Type Ia supernovae (abbreviated as SN Ia) are responsible for the vast majority of iron production, and significant numbers of them don’t even appear until about 10,000,000,000 years after star formation begins. So there’s really not that much iron going around the interstellar medium. And even after the SN Ia events occur, it’s not going to mix evenly throughout everything. So essentially, one region may get lots of iron, and another region? Not so much. So even though the stars in both those regions are the same age, the stars in region 2 seem older. Bit problematic, right?

And that concludes metallicity and star populations. If you have any questions, you can (attempt) to email me.


TL;DR – Metallicity is the metal content of a star (in astronomy, a metal is anything that’s not H or He). Stars are divided into populations based on their metallicity, with Pop I stars being metal-rich, Pop II stars being metal-poor, and theoretical Pop III stars having no metals at all. The equations for metallicity compare the amounts of iron and hydrogen in the star to the amounts in the sun (because we base everything off our own star). Metallicity doesn’t necessarily show the age of a star, since Type Ia supernovae scatter iron unequally throughout their surroundings.


I like typing in British English. A lot. So if some things look misspelled…check to see if it’s the British spelling first before calling me out.


Q: How many astronomers does it take to change a light bulb?

A: What’s a light bulb?

This entry was posted in General.

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