It is well known that practically all the elements in the periodic table (save for hydrogen, some helium and some lithium) are born in the core of a star. When these stars die, these elements are released into the interstellar medium, from which new stars are born. Therefore, stars are composed of a number of different elements. Astronomers use the term "metallicity", or "chemical abundance" to describe the amount of a particular element found a star. A preliminary word of warning, "metallicity" can have multiple meanings -- some scientists use the term in reference to the iron (Fe) content of the star alone, whereas others may use it to refer to the average of the abundances of a number of elements, since, remember, any element heavier than helium may be called a metal!

When studying an unknown or poorly-understood object, it is preferable to have another object to compare it to. This is astronomy's version of running a controlled experiment, so to speak. Before we can begin to grasp the behavior or properties of what we are studying, we must be able to compare it to something whose behaviors or properties we already understand. Therefore, when astronomers measure the chemical abundances of a star, they compare them to the chemical abundances of a star they know quite well. Most often than not, that star is our Sun, since we know no other star better.

Because metallicity calculations made relative to the Sun are so common, a notation has been developed to summarize the results. This bracket notation, [A/B], represents the ratio of the logarithms (base 10) of the chemical abundance, A/B, of a star to that of the sun:


Astronomers discuss almost every number as a log, simply because the numbers they deal with (ages, distances, masses, etc.) are so very large.

You may ask why there is an A and a B here, when this is the measurement of the abundance of one particular element? Well, this is a subtlety of the notation. A star's chemical abundance is actually made relative to its abundance of hydrogen or its abundance of iron. As an example, a star's oxygen (O) abundance is often measured relative to its iron (Fe) abundance, and this is the ratio that is compared to that of O to Fe in the Sun within the bracket notation:


Say for example that the above [O/Fe] = -2.0. The means that the star's oxygen O abundance (again relative to its iron Fe abundance) is 100 x less than (10^-2) the oxygen abundance of the Sun (relative to the Sun's iron content). You may wonder why Fe is included at all here, since it only seems to confuse the issue. Well, it's there because most people find the ratios of elements in a star more interesting than the actual abundances themselves. This is because different mechanisms cause the release of different elements into the interstellar medium -- by knowing the ratio of different elements in a star, one gets an idea of what types of processes enriched the gas that the star formed from. So because iron is released into space by a particular process (a Type Ia supernova), and also because it is so easy to measure in stars, most element abundances are measured relative to it.

In the plot shown here, taken from one of Ivans's papers on M 5, [Na/Fe] is plotted versus [O/Fe]. Look at the star marked L4201 in the upper right. You can guess by eye that it has a [Na/Fe] ~ +0.31 and an [O/Fe] ~ +0.4. If you can remember your logs, 10^+0.31 ~ 2 (indicating that L4201 has a ratio of sodium to iron twice that of the Sun) and 10^+0.4 ~ 2.5 (meaning that its oxygen to iron ratio is 2.5x that of the Sun.)

Plots such as this that show the abundances of several stars together are common in papers and are useful for revealing trends and distributions of chemical abundances in groups of stars. More often than not, the motivation for a metallicity study of a star or cluster of stars is precisely this -- to gain a better understanding of the distributions of abundances of elements in, or the evolution of the chemical enrichment of, a large sample of stars.

-HRJ