bracket notation

A bracket notation is used to characterize abundances of nuclear species within an astronomical object, specifically, the ratio of the abundances of two specific elements (chemical abundance ratios or abundance ratios). The notation's general form is [x/y] for chemical species x and y. For example, a particular body's [Fe/He] is a measure of the ratio of its number-count of iron atoms with that of its helium atoms. The meaning (using this example):

[Fe/He] means
log10(NFe/NHe)body - log10(NFe/NHe)Sun

• N_Fe - number count of iron atoms.
• N_He - number count of helium atoms.
• body - the object for which we are specifying an abundance ratio.
• log₁₀(A)_B - for body B, the base-10 log of the value A.

If a body's [Fe/He] were 0, then its ratio of iron to helium would be identical to that of the Sun. If it were 1, then its ratio would be ten times that of the Sun, and if it were -3, then its ratio would be a thousandth of that of the Sun. Sometimes rather than a specific element, a symbol for some set of elements is specified; for example, M or Z for all metals, or α (alpha) for all alpha elements.

An unrelated bracket notation commonly used for spectral line designation within astrophysics is to indicate a forbidden line; for example, [CII] indicates a forbidden spectral line of singly-ionized carbon.