[AGarvin]

In the quantum world, electrons in an atom can exist only at specific energy levels. The lowest principle quantum number n is 1, the lowest energy state. Then 2, 3, etc .... I believe the highest level on the periodic table is 7. As an electrom drops down to a lower energy level it will emit a photon, producing an emission line.

Ha & Hb lines are part of the hydrogen Balmer series of emission lines that are produced when an electron drops down to n=2. Ha is produced when the electron drops from n=3 to n=2, and Hb when n=4 to n=2. These are both in the visible spectrum. H gamma is produced when n=5 to n=2, but this is in the UV spectrum.

The terminology of using Roman numerals after an element (as in OIII) designates atom electron "count". The Roman numeral represents 1 plus the number of any electrons lost (ionised), so OIII is an ionised oxygen atom that has lost 2 electrons. You'll often hear of emission nebula refered to as HII regions, meaning ionised Hydrogen atoms that have lost 1 electron (since neutral hydrogen (HI) has only one electron, there is no such thing as HIII).

Andrew.