What causes Ha, Hb, OIII and SII emission lines?

[NightCal]

I presently researching narrow band filters and I'm keen to find out what actually causes H alpha, H beta, OIII and SII emission lines we see in nebula? I'm after a simple ex-explanation, not a doctoral thesis. I also want to know why some nebula emit in Hb and others in OIII or Ha.
Thanks

[Terry B]

Essentially it is hydrogen in clouds absorbing energised photons usually from a nearby star emitting UV light. This makes one of the electrons "excite" & change its orbit. As the electron drops back to its non excited state it will release a photon at a particular wavelength. This is the emission line. This has to do with quantum physics and photons only being emitted at particular energy levels.
This is a very simple explanation and I'm sure others will have better.
Cheers

[NightCal]

Terry thanks for that.

Now some objects emit in H alpha and other in H beta, is this to do with the abundance of different isotopes of hydrogen in the nebula, with the abundance of different isotopes of hydrogen in the star, differences in the energy emissions of the star or some other mechanism?

Similarly. for nebula prominent in OIII and SII line filters, is this because of different energy emissions from the star, the relative abundance of these elements in the star. or is it to do with the relative abundance of these elements in the gas cloud that is emitting or reflecting the light from the star?

Or have I completely misunderstood your answer?!!!

[Terry B]

The H beta is just a different energy level. Not related to isotopes but different quantum levelsof energy.
The O and S levels are the same process fron oxygen and sulphur atoms in the clouds. These are more common in planetary nebulas because these elements have been made in the star and then shed off.
The primordial hydrogen clouds that stars condense from will be relatively poor in heavier elements so mostly emit just hydrogen lines.

[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.