Starship Asterisk*

NGC3314 wrote: ↑Tue Aug 17, 2021 8:01 pm

Ann wrote: ↑Tue Aug 17, 2021 1:40 pm
Well, 658 nm is the wavelength of ionized sulfur. It's seems relatively straightforward that sulfur should be ionized and made to emit photons in the relative presence of an extremely hot white dwarf. Similarly, 501 nm is the wavelength of doubly ionized oxygen. Doubly ionized oxygen represents a higher degree of ionization that ionized sulfur.

I do find the 469 nm ionized helium mystifying. I checked the emission lines of helium, and there appears to be no helium line at 469 nm. There is one at 471.3 nm, however.

The 469nm (4686 A) emission line is from He II (with a single electron), most often seen when that single electron hasjust recombined with a bare helium nucleus. It is analogous to Paschen-alpha of hydrogen at four times the wavelength (hydrogen ions with Z protons have their spectral lines at Z² times the energy of hydrogen). It takes 54.6 electron volts (four times the value 13.6 for hydrogen) to fully ionize helium, and only the very hottest stars can manage that over significant volumes. (In contrast, sulfur is practically always ionized at least once, since its first ionization energy is less than that of hydrogen so space is full of radiation that can ionize S.) A nonintuitive feature in the structures of nebulae is that the photons just above an ionization threshold are absorbed more effectively than photons at higher energies, so the average photon energy (say between the edges of H and He) increases going outward because the lower-energy ones were absorbed first. He II and the hydrogen lines come directly from recombination , while such "forbidden" lines as [O III], [N II], [ S II] etc. are collisionally excited as the loose electrons rattle around before actually binding again to a nucleus (so each electron can excite many forbidden-line photons during its free escapades. (Disclosure - I have become a big fan of He II emission as a way to trace photoionization by active galactic nuclei even when the gas has such low oxygen and nitrogen fraction that normal tests might suggest that it's ionized by hot stars. Cough, Hanny's Voorwerp and its kin, cough.)

(Hmm - I just learned that you have to put an extra space in [ S II] to keep BBS from interpreting that as strikethrough markup).

Ann wrote: ↑Tue Aug 17, 2021 1:40 pm
Well, 658 nm is the wavelength of ionized sulfur. It's seems relatively straightforward that sulfur should be ionized and made to emit photons in the relative presence of an extremely hot white dwarf. Similarly, 501 nm is the wavelength of doubly ionized oxygen. Doubly ionized oxygen represents a higher degree of ionization that ionized sulfur.

I do find the 469 nm ionized helium mystifying. I checked the emission lines of helium, and there appears to be no helium line at 469 nm. There is one at 471.3 nm, however.

JohnD wrote: ↑Tue Aug 17, 2021 2:23 pm Thank you, Ann and Chris!

My next assignment is clear! Meanwhile, and if I may, isn't a photon a photon? For photons to have different energies, they will have different frequencies? Sure, near the star, an atom is more likely to receive a photon more frequently, perhaps leading to the doubly ioniosed oxygen?

JOhn

orin stepanek wrote: ↑Tue Aug 17, 2021 12:55 pm
Ah! Looks like a Flower!
So this is how the sun might go?

• Where have all the graveyards gone?
  Long time passing.
  Where have all the graveyards gone?
  Long time ago.
  Where have all the graveyards gone?
  They're covered with flowers every one.
  Oh, When will they ever learn?
  Oh, When will they ever learn?

JohnD wrote: ↑Tue Aug 17, 2021 2:23 pm Thank you, Ann and Chris!

My next assignment is clear! Meanwhile, and if I may, isn't a photon a photon? For photons to have different energies, they will have different frequencies? Sure, near the star, an atom is more likely to receive a photon more frequently, perhaps leading to the doubly ioniosed oxygen?

JOhn

JohnD wrote: ↑Tue Aug 17, 2021 1:29 pm Chris,
OK, I imagined a simple explanation, and was wrong, but I can do complexity! You will be as short of time as any busy academic - please refer me to some source that will explain the mechanism?

Neufer,
Thank you! So the blue is due to Oxygen, not Helium? But why is that fluorescing near the star, while Nitrogen is, further out?
John

JohnD wrote: ↑Tue Aug 17, 2021 9:53 am The intense spectrum of colours across the Ring Nebula represent the different elements being ionised and then fluorescing at a distinct frequnency and colour. So the spectrum represents a gradient of elements in the nebula that depends on their distance from the star. How does this gradient form?
If the colours are produced by the elements suggested by Ann (F658N ([N II], red), F501N ([O III], green), and F469N (He II, blue), then it cannot be the atoms' masses, as blue - Helium, by far the lightest - is in the middle, nearest the star and apparently least affected by the solar winds and expansion of its outer layers. What other atomic property has so effectively differentiated the population of different atoms?

JOhn

JohnD wrote: ↑Tue Aug 17, 2021 9:53 am
If the colours are produced by the elements suggested by Ann (F658N ([N II], red), F501N ([O III], green), and F469N (He II, blue), then it cannot be the atoms' masses, as blue - Helium, by far the lightest - is in the middle, nearest the star and apparently least affected by the solar winds and expansion of its outer layers.

https://en.wikipedia.org/wiki/Ring_Nebula wrote:

M57, The Ring Nebula (Hubble Space Telescope)

---

<<M57 is 2,570 light-years from Earth. Photographs taken over a period of 50 years show the rate of nebula expansion is roughly 1 arcsecond per century, which corresponds to spectroscopic observations as 20–30 km s⁻¹. M57 is illuminated by a central white dwarf or planetary nebula nucleus (PNN) of 15.75v visual magnitude.

All the interior parts of this nebula have a blue-green tinge that is caused by the doubly ionized oxygen emission lines at 495.7 and 500.7 nm. These observed so-called "forbidden lines" occur only in conditions of very low density containing a few atoms per cubic centimeter.

In the outer region of the ring, part of the reddish hue is caused by hydrogen emission at 656.3 nm, forming part of the Balmer series of lines. Forbidden lines of ionized nitrogen or N II contributes to the reddishness at 654.8 and 658.3 nm.>>

JohnD wrote: ↑Tue Aug 17, 2021 9:53 am The intense spectrum of colours across the Ring Nebula represent the different elements being ionised and then fluorescing at a distinct frequnency and colour. So the spectrum represents a gradient of elements in the nebula that depends on their distance from the star. How does this gradient form?
If the colours are produced by the elements suggested by Ann (F658N ([N II], red), F501N ([O III], green), and F469N (He II, blue), then it cannot be the atoms' masses, as blue - Helium, by far the lightest - is in the middle, nearest the star and apparently least affected by the solar winds and expansion of its outer layers. What other atomic property has so effectively differentiated the population of different atoms?

Except for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial circle.

NASA wrote:

The Blue Ring Nebula, which perplexed scientists for over a decade, appears to be the youngest known example of two stars merged into one...

While merged star systems are thought to be fairly common, they are nearly impossible to study immediately after they form because they're obscured by debris the collision kicks up. Once the debris has cleared – at least hundreds of thousands of years later – they're challenging to identify because they resemble non-merged stars. The Blue Ring Nebula appears to be the missing link: Astronomers are seeing the star system only a few thousand years after the merger, when evidence of the union is still plentiful. It appears to be the first known example of a merged star system at this stage.
...
The team concluded that the nebula was the product of a relatively fresh stellar merger that likely occurred between a star similar to our Sun and another star only about one-tenth that size (or about 100 times the mass of Jupiter). Nearing the end of its life, the Sun-like star began to swell, creeping closer to its companion. Eventually, the smaller star fell into a downward spiral toward its larger companion. Along the way, the larger star tore the smaller star apart, wrapping itself in a ring of debris before swallowing the smaller star entirely.

This was the violent event that led to the formation of the Blue Ring Nebula. The merger launched a cloud of hot debris into space that was sliced in two by the gas disk. This created two cone-shaped debris clouds, their bases moving away from the star in opposite directions and getting wider as they travel outward. The base of one cone is coming almost directly toward Earth and the other almost directly away. They are too faint to see alone, but the area where the cones overlap (as seen from Earth) forms the central blue ring GALEX observed.

Millennia passed. The expanding debris cloud cooled and formed molecules and dust, including hydrogen molecules that collided with the interstellar medium, the sparse collection of atoms and energetic particles that fill the space between stars. The collisions excited the hydrogen molecules, causing them to radiate in a specific wavelength of far-UV light. Over time, the glow became just bright enough for GALEX to see.