There is a standard notation in astrophysics for various levels of ionization of an atom. As illustrated in the adjacent table, this notation uses increasing Roman numerals to indicate higher levels of ionization.

Degrees of Ionization
Suffix	Ionization	Examples	Chemist's Notation
I	Not ionized	H I, He I	H, He
II	Singly ionized	H II, He II	H+, He+
III	Doubly ionized	He III, O III	He++, O++

We also show the standard chemist's notation for such ions, which consists of placing a right superscript on the element symbol indicating the net electrical charge on the ion (in the neutral case the superscript "0" is usually omitted).

Thus, for example, we shall speak later of "H II regions", meaning volumes of space in which the radiation from nearby hot stars has completely ionized the hydrogen.