Ionization and

Atoms and molecules are electrically neutral in that the number of negatively charged electrons is exactly equal to the number of positively charged protons. Much of the "normal matter" that we find around us is in this form. However, particularly when there are energy sources available, atoms or molecules can gain or lose electrons and acquire a net electrical charge. This process is called ionization.

Ionization of Atoms

Ionization is the gain or loss of electrons. The loss of electrons, which is the more common process in astrophysical environments, converts an atom into a positively charged ion, while the gain of electrons converts an atom into a negatively charged ion. In the subsequent discussion, we will use the terms ionization and ionize in the sense of losing electrons to form positive ions.

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

Notation for Degrees of Ionization
Suffix Ionization Examples Chemist's
I Not ionized (neutral) 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.


If most of the atoms or molecules in a region are ionized, the resulting state of matter corresponds to a gas that is electrically neutral on a global scale, but composed microscopically of positively charged ions and the negatively charged electrons stripped from the atoms to form the ions. Such a state of matter is called a plasma. Most of the matter in stars is in a plasma state. Thus, although what we refer to as "normal matter" consists of atoms and molecules, the evidence is that the most abundant form of matter in the Universe is not atoms and molecules but rather the plasma state.