Metallicity is a measure of the proportion of 'heavy elements' or 'metals' (in astronomy, elements heavier than hydrogen or helium) that a star contains. Usually, metallicity is given in term of the relative amount of iron and hydrogen present, as determined by analyzing absorption lines in a stellar spectrum, compared with the solar value. The ratio of the amount of iron to the amount of hydrogen in the object is divided by the ratio of the amount of iron to the amount of hydrogen in the Sun. This value, denoted as [Fe/H], is calculated from the following logarithmic formula.
For example, if the metallicity [Fe/H] = –1 then the abundance of heavy elements in the star is one tenth that found in the Sun; if [Fe/H] = +1, the heavy element abundance is 10 times the solar value. Measurements for thousands of stars have established that the range of values for [Fe/H] is from –4 (very metal-poor) to +1 (very metal-rich).
Some general observations about the characteristics of stars as indicated by their metallicities: 1) the disk portion of a galaxy has a range of metallicities, with Population I stars having values > √1, i.e., towards smaller negative numbers to positive numbers less than +1, whereas Population II stars have negative values beyond –1; 2) globular clusters and the galactic halo are metal-poor (values more negative than –1); 3) metal-rich stars are in the red segment of the color index and metal-poor stars are blue; 4) although there can be complexities, in general metal-poor stars are young in appearance (either near the outer limits of the Universe which show stars that formed in the first few billion years after the Big Bang or stars formed more recently from gas clouds that have had little contribution of heavier elements from supernovae) and short-lived; 5) metal-rich stars from F, G, K, and M positions on the main sequence are redder than stars of similar sizes (masses); and 6) dust around a star will make it redder.
Overall, the rule of thumb is just that a star will show a metallicity that depends on prior processes that have changed the composition of the interstellar gas in the neighborhood in which it forms. This is a function mainly of the number of supernovae that have occurred previous to the formation of the star and the amounts of metals each ejected that then became mixed into the cloud that supplies the star (and other stars growing from this cloud). Since, over time the gas composition in the interstellar medium should progressively enrich in metals, then those stars that are metal-rich tend to have organized in later stages of a galaxy's history.
From the above it follows that stars that are extremely metal-poor are likely to be first-generation and thus primitive. Moreover, only stars that have a fairly high metallicity (roughly solar or greater) are possible candidates for planetary systems, since the cores of planets are formed from metals such as iron and nickel.