Starship Asterisk*

NoelC wrote:It's a great image of an interesting and certainly uncommon object!

Some obvious basic questions:

- Why should there be SO much oxygen? I thought oxygen was a byproduct of supernovae. Did a bunch of them explode all at once out there?

- Quasar or no, why should it be glowing as brightly as the entire galaxy next door? Could the amount of radiation really have been that great?

- Shouldn't there be even more hydrogen? Most gas clouds glow primarily reddish because hydrogen is simply more common...

- And oxygen emissions aren't this greenish-yellow color by themselves - they're more bluish-green. Is this object red-shifted?

- I have to assume someone's done spectral analysis of this cloud and characterized the emission line(s). What else is in there?

br1an boru wrote:What would be the effect, if any, on life here on earth if there was a quasar in the centre of the Milky Way? Would the radiation be excessive and destructive to us?

NGC3314 wrote:To my eye, [O III] at zero redshift is a remarkable pure emerald hue (based on my one experience doing visual acquisition with a 3-meter telescope.

Ann wrote:It is not clear what "emerald green" really means. I googled "emerald color" and got, among others, these two samples:

These two shades of green are very dissimilar. Personally I wouldn't even use the word "green" to describe the cut stone. To me, the piece of jewellery represents what I think of as the color of ionized oxygen. The other sample, which I may call emerald green, is very far from what I think of as the color of OIII emission.

Gachala Emerald

NoelC wrote:I hope to have such an experience some day, where I get to look through a telescope with enough light gathering power to actually see color in astronomical objects. Even on the very brightest nebulae, I'm just not seeing color visually with my 10" Meade

Ann wrote:It is not clear what "emerald green" really means.

NoelC wrote:I'd love to hear from more folks who have used large aperture scopes on this... Bill says he sees "emerald green" color, Chris says there should be no difference between tiny and large scopes up to 40x because of exit pupil size, but then goes on to say OIII color can be seen as anything from pale gray-green to bright green.

I know that actually using my dSLR OIII emissions look more like the teal green of the cut stones shown above than the more yellowish-green swatches shown, and the camera does a pretty good job of making accurate looking color in the daytime...

Chris Peterson wrote: We need to distinguish the possible range of perceived color when viewing an OIII source with the range that is possible when viewing an OIII astronomical source. The example of bright green I gave was looking at a reference tube in the lab.

zloq wrote:

Chris Peterson wrote:
We need to distinguish the possible range of perceived color when viewing an OIII source with the range that is possible when viewing an OIII astronomical source. The example of bright green I gave was looking at a reference tube in the lab.

You saw the green of [OIII] glowing in a tube in "the lab?"

Do you know where I can buy a tube like that?

zloq wrote:You saw the green of [OIII] glowing in a tube in "the lab?"

Do you know where I can buy a tube like that?

Chris Peterson wrote:

zloq wrote:You saw the green of [OIII] glowing in a tube in "the lab?"

Yes. I have a set of spectroscopic reference tubes, which are interchangeable in a lamp fixture.

Do you know where I can buy a tube like that?

IIRC, mine came from Melles Griot about 20 years ago. The lamps themselves have "Siemens" etched on their edges.

zloq wrote:OK - you are describing something that does not exist and never has.

http://en.wikipedia.org/wiki/Forbidden_mechanism wrote:
<<In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient (electric dipole) route. Although the transitions are nominally "forbidden", there is a small probability of their spontaneous occurrence, should an atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited atom will make a forbidden transition to a lower energy state per unit time; by definition this probability is much lower than that for any transition permitted by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so rather than choosing the forbidden route. Nevertheless, "forbidden" transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes of order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions.

Forbidden emission lines have only been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. Even the hardest laboratory vacuum on Earth is still too dense for forbidden line emission to occur before atoms are collisionally de-excited.

However, in space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.

Forbidden line transitions are noted by placing square brackets around the atomic or molecular species in question, e.g. [O III] or .

Forbidden lines of nitrogen ([N II] at 654.8 and 658.4 nm), sulfur ( at 671.6 and 673.1 nm), and oxygen ([O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm) are commonly observed in astrophysical plasmas. These lines are extremely important to the energy balance of such things as planetary nebulae and H II regions. Also, the forbidden 21-cm hydrogen line is of the utmost importance for radio astronomy as it allows very cold neutral hydrogen gas to be seen.>>

http://www.daviddarling.info/encyclopedia/F/forbidden_line.html wrote:

<<Forbidden line: An emission line found in the spectrum of a rarefied gas under special conditions, such as those found in some nebulae, the solar corona, and parts of active galactic nuclei. Although not seen on Earth – hence their name – forbidden lines may account for 90% or more of the total visual brightness of an object such as a planetary nebula.

A forbidden line arises when an electron in an excited (energized) atom jumps from a metastable state to a lower energy level. Under normal circumstances, when particle densities are higher (greater than about 10⁸ per cm³), such an electron would almost immediately be knocked out of its metastable state by collision and not be given time to emit a photon. But in an environment like that of a planetary nebula, the time between collisions averages 10 to 10,000 seconds. Consequently, when ions such as O+, O2+ (singly and doubly ionized oxygen), or N+ (singly ionized nitrogen) go into metastable states by allowed transitions from higher states, they remain there undisturbed until they radiate spontaneously. A large fraction of the more highly excited ions eventually drop into these states and, in a nebular environment, practically every ion goes from them to the ground state by forbidden radiation.

Forbidden lines are denoted by enclosing them in brackets. The strongest are two lines of doubly ionized oxygen [O III], in the green part of the spectrum (to which the human eye is most sensitive) at 495.9 nm and 500.7 nm. When these lines were first seen, in the spectra of planetary nebula in the 1860s, their true nature wasn't recognized and it was thought they might be due to a new element, which was dubbed "nebulium." More than half a century passed before Ira Bowen provided the right explanation. Besides forbidden lines of ionized oxygen, others of neon, nitrogen, and other relatively abundant elements are seen making up the light of nebulae, as well as the ordinary, permitted lines of hydrogen and helium.>>