This chapter contains two sections, which deal with metals and rare earth elements respectively. The behavior of both groups of elements presents certain general characteristics, which would be completely lost if they were described in the sections on individual elements.

The behavior of metals

The term ‘metals’ is used by astronomers in an ill-defined way. It may refer to ‘all elements other than He’ or to ‘elements with Z>8’ or to ‘elements with Z> 21’. One should always try to ascertain what a given author means when he uses the word ‘metals’. If the term ‘metals’ is ill-defined then the same happens with ‘metal abundances’. The abundances of metals are usually defined with respect to hydrogen, in the sense N(m)/N(H), normalized with respect to the same ratio in the sun. More often one uses the decimal logarithm of this quantity in order to have small numbers without exponents. So for instance [Fe/H] = −0.6 means that the abundance of iron with respect to hydrogen is four times less than in the sun. Usually this is abbreviated to −0.6 dex.

Another possible definition, due to Kudritzki (1987), is to consider the ratio N(m)/N(H)+4N(He). This has the advantage that the denominator remains constant if the star is in its hydrogen burning phase

The two preceding definitions are clear and unambiguous. However, very often the discussions based upon them become ambiguous because authors speak of ‘metals’ when in reality they are referring to the abundance of one individual element.