Placidus
19-11-2014, 02:26 PM
G'day,
What can we amateurs learn about the astrophysics of an object using say 3nM H-alpha, OIII, SII, and NII filters?
I understand the basic physics of say the Balmer series, forbidden transitions, excitation by photons versus shock energy. I broadly understand that material ejected from the surface of primordial stars will be mostly H/He, while material dredged up or exploded from the very depths of 3rd generation stars will contain more "metals" and therefore O, S, and N (without which one cannot see OIII, SII, and NII emissions).
Putting it together a tiny bit, H-alpha is easy to find, because it will show the location of fairly dense gas excited by even modest UV, since the relevant excited state is easy to produce, and the relevant transition occurs very quickly before thermal collisions destroy it. Conversely, OIII, SII, and NII emission will require several things: the presence of dredged up or older or more processed material, more energetic UV to produce the required excited state, and very low gas pressure so that collisions don't destroy the state before the forbidden transition occurs.
Putting it together a tiny bit more, that would explain why we find OIII emission closer to a white dwarf, WR, or OB star, where there's plenty of hard UV, whereas H-alpha predominates further out. Similar arguments might explain why we find utterly negligible SII in the outermost shock fronts of the Norma Bipola Nebula, but we find more SII in some PN's (eg dumbell) and some SNR's (eg Pencil Nebula).
When we actually go out there and start photographing, we find that OIII is ubiquitous, but SII and NII are pretty hard to find, and need very long exposures. Why? What, in more detail, do they tell us, that might make it worth the amateur's while to bother?
Anyone know any good books (or people) that can (gently) take me the next steps in understanding that are relevant to what we can photograph without a high resolution spectrograph?
Anyone know any objects (or lists of objects) that are especially rewarding or informative to photograph in SII or NII?
Best,
Mike
What can we amateurs learn about the astrophysics of an object using say 3nM H-alpha, OIII, SII, and NII filters?
I understand the basic physics of say the Balmer series, forbidden transitions, excitation by photons versus shock energy. I broadly understand that material ejected from the surface of primordial stars will be mostly H/He, while material dredged up or exploded from the very depths of 3rd generation stars will contain more "metals" and therefore O, S, and N (without which one cannot see OIII, SII, and NII emissions).
Putting it together a tiny bit, H-alpha is easy to find, because it will show the location of fairly dense gas excited by even modest UV, since the relevant excited state is easy to produce, and the relevant transition occurs very quickly before thermal collisions destroy it. Conversely, OIII, SII, and NII emission will require several things: the presence of dredged up or older or more processed material, more energetic UV to produce the required excited state, and very low gas pressure so that collisions don't destroy the state before the forbidden transition occurs.
Putting it together a tiny bit more, that would explain why we find OIII emission closer to a white dwarf, WR, or OB star, where there's plenty of hard UV, whereas H-alpha predominates further out. Similar arguments might explain why we find utterly negligible SII in the outermost shock fronts of the Norma Bipola Nebula, but we find more SII in some PN's (eg dumbell) and some SNR's (eg Pencil Nebula).
When we actually go out there and start photographing, we find that OIII is ubiquitous, but SII and NII are pretty hard to find, and need very long exposures. Why? What, in more detail, do they tell us, that might make it worth the amateur's while to bother?
Anyone know any good books (or people) that can (gently) take me the next steps in understanding that are relevant to what we can photograph without a high resolution spectrograph?
Anyone know any objects (or lists of objects) that are especially rewarding or informative to photograph in SII or NII?
Best,
Mike