Iron (Fe) is the element with atomic number 26, symbol Fe. It has four stable isotopes. It is the most stable product of fusion in 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] which means the log of the ratio of iron atoms to hydrogen atoms (of the star or galaxy) minus the same for the Sun, i.e., "[Fe/H] = 0.0" means "same iron abundance as the Sun".
Shock can excited iron [Fe II] forbidden lines at 1.257 micron and 1.644 micron, which can be observed.