Quote:

---

Hi all, this is an open question about 500.7nm emmission lines. I am wondering if HII regions are mostly hydrogen with the strong 656.3nm Balmer line why do they show up better in an OIII filter? In other words, why is the oxygen emmission line brighter? Thanks for any response!

---

Well, the human eye generally has very poor sensitivity to the 6563 Angstrom (656.3nm) H-alpha line, as the eye's peak sensitivity is in the yellow-green part of the spectrum around 5500 Angstroms. Thus, while the H-alpha line is considerably stronger in absolute strength, the OIII lines (4959 and 5007 Angstroms) sit close to our visual peak, so they tend to be perceived a lot better visually than the H-Alpha line does. With a large enough aperture, one can begin to see some of the faint pinks of the H-alpha contribution, but in most cases, the OIII and H-Beta emission dominate the color (one reason the nebula generally appears a bluish-green color). An OIII filter doesn't generally let through *any* H-alpha, so you will only see the bluish-green emission from the fobidden OIII lines. Still, there is enough excited Oxygen in the nebula to allow it to be easily seen from its OIII emissions alone. I have seen some faint pinks in M42 using a DGM Optics NPB filter, but they were rather faint (the bright core region remains a pale bluish-green color). Clear skies to you.

--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

Post Extras:

Quote:

---

Thank you for that explanation it answered my question exactly. About what percentage of the HII region is actually made of oxygen?

---

Most of the HII region is Hydrogen (around 71% by mass) followed by about 27% Helium and less than two percent of everything else. Oxygen is probably the third most abundant element in the sun (about 0.97 percent by mass), so in nebulae, it is probably on the same order of abundance. Clear skies to you.

--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

Post Extras: