iron

Iron (Fe) is the element with atomic number 26, symbol Fe. The common isotope has a mass number of 56, but mass numbers 54, 57, and 58 are also stable. It is the most stable product of fusion in stars, but produced in only in the final fusion of the most massive post-main-sequence stars. Because iron can be detected at a distance, the ratio of iron to hydrogen is used as an approximate measure of metallicity. This is expressed as the numerical value [Fe/H], i.e., in bracket notation indicating the log of the ratio of iron atoms to hydrogen atoms (e.g., of a star, cloud, or galaxy) minus the same for the Sun, i.e., "[Fe/H] = 0.0" means "same iron abundance as the Sun".

Ionized iron forbidden lines, such as [Fe II] at 1.257 micron and 1.644 micron, are observable signs of shock.

Iron lines are a target of solar observation: various solar emission lines, including many from ionized iron, are used in observations of the Sun's surface and corona; the many lines from iron ionization states up into the twenties provide clues regarding the temperature, which may reveal which atmospheric layer produced them, and their Doppler shift may reveal radial velocity, a clue to details of solar activity.

In extremely hot environments, such as some active galactic nuclei (AGNs), iron may be ionized to the point that it has a few or no electrons (such as Fe XXVI missing 25 and Fe XXVII missing all, i.e., fully ionized) and spectral lines associated with these states are of interest.

In planetary science and geology, the means by which Earth's crust acquired iron is of interest. The iron present at the formation of the Earth itself is naturally at the core due to its high density. Isotopic ratios of iron found on Earth and elsewhere in the solar system are not identical, leading competing theories how this came about. It is considered plausible that all the crust's iron is from meteorites.