Numbers and Notation

Stellar spectroscopists have evolved a standard notation for specifying the chemical composition of a stellar atmosphere. By abundance of the element E is meant - usually - the ratio of the number density n(E) to that of hydrogen, i.e., A(E) = n(E)/n(H). The 

Frequently, the stellar and solar abundance of an element may be compared using the square-bracket notation , i.e., [E/H] = loge(E)star — loge(E)sun- For example, we may speak of Galactic metal-poor stars as those with [Fe/H] — 1, that is stars with less than one-tenth the Fe abundance of the Sun. The notation is readily extended to a ratio of two elements Ei and E2 for example, a star with [Si/Fe] = +0.5 has a Si/Fe abundance ratio that is three times that of the Sun. Two notes of caution (i) when [X] is quoted check that the Sun was adopted as the reference object, and (ii) A few authors may define [X] quite differently  

David L. Lambert Stellar Nucleosynthesis - an observer s view 

On occasions, a stellar composition will be expressed in terms of mass fractions. Stellar structure calculations commonly use mass fractions. It is the universal custom to refer to the H and He mass fractions by the letters X and Y, respectively. The mass fraction of all other elements - metals in astronomical parlance - is denoted by Z. Today, Z may be broken into parts Z(C), Z(O), etc. Of course, X + Y + Z = 1. Chemical evolution of the Universe from the Big Bang to the Sun could be summarized as the change from (X,Y,Z) (0.76, 0.24, 0.00) immediately following the Big Bang to (0.73, 0.25, 0.02), which is representative of the Sun and the present solar neighbourhood - a few per cent change in 10 billion years. Tales of stellar nucleosynthesis are coded in the few per cent rise of Z. 

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Note that the numbering and notation areentirely arbitrary. We now generate the closed-loop transfer function, and check that it has the same closed-loop poles ... 

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