APOD: Colors: Ring Nebula versus Stars (2021 Jul 21)

[APOD Robot]

Colors: Ring Nebula versus Stars

Explanation: What if you could see, separately, all the colors of the Ring? And of the surrounding stars? There's technology for that. The featured image shows the Ring Nebula (M57) and nearby stars through such technology: in this case, a prism-like diffraction grating. The Ring Nebula is seen only a few times because it emits light, primarily, in only a few colors. The two brightest emitted colors are hydrogen (red) and oxygen (blue), appearing as nearly overlapping images to the left of the image center. The image just to the right of center is the color-combined icon normally seen. Stars, on the other hand, emit most of their light in colors all across the visible spectrum. These colors, combined, make a nearly continuous streak -- which is why stars appear accompanied by multicolored bars. Breaking object light up into colors is scientifically useful because it can reveal the elements that compose that object, how fast that object is moving, and how distant that object is.

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[Ann]

Colors: Ring Nebula versus Stars. Image Credit: Robert Vanderbei (Princeton U.).

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Very nice! The picture readily explains why planetary nebulas may look green, cyan or blue to the human eye, but never red.

Planetary nebulas basically emit just two wavelengths of visible light: Hydrogen alpha at 656 nm, and OIII at 500.7 nm.

The human eye is relatively insensitive to wavelengths longer than perhaps 630 nm. Hα is definitely so deep into the red part of the spectrum, and all Hα nebulas in space have such low surface brightnesses, that the human eye can't detect the red color of hydrogen alpha nebulas.

500 nm, by contrast, is square in the middle of the greatest color sensitivity of the human eye. Therefore, if an OIII nebula has a sufficiently high surface brightness, it should be possible for humans to detect its cyan color. And planetary nebulas are often really bright in OIII.

Ann

[VictorBorun]

ring.jpg (42.03 KiB) Viewed 4219 times

red is a wide range and farely common, but cyan is narrow range and rare

And for us trichromates the cyan hue is exactly complement to red.

So the mix is gray or reddish gray or cyanish gray; no new hues can come out of a complementary pair.

The same goes for a comlexion of a human with no skin pigment.
There can be some red from red blood cells, there is always some cyan from cell's membranes.
The mix is blush or dusty pale or cyanish depending on skin capillars behavior.

Can I turn pale? Reddish? Cyanish? Yes, I can. Can I turn greenish? Only if I get myself some pigment, like bilirubin from some clotted blood in an old hematoma.

Then how could the Ring Nebula get those vertical slightly fucshia sectors and those horizontal slightly lime sectors?
They must be some artifacts.

[orin stepanek]

Like a burning ring of fire! To me the Ring Nebula kinda looks like a
hole in the universe! Opening of a wormhole; there I go
Sify stuff!

Awe; Kitty love!

[smitty]

What are all the little white dots?

[Chris Peterson]

ring.jpgred is a wide range and farely common, but cyan is narrow range and rare

And for us trichromates the cyan hue is exactly complement to red.

So the mix is gray or reddish gray or cyanish gray; no new hues can come out of a complementary pair.

A mix of red and green light appears yellow to the human eye. A mix of red and blue light appears magenta. The actual color we will see with some combination of the narrow bands, however, depends on their relative intensities. We create cyan on screens by mixing green and blue, but the "cyan" of the oxygen emission is not such a mix, and our eyes treat it very differently than what looks like that color on a screen. It's complicated.

[Chris Peterson]

Planetary nebulas basically emit just two wavelengths of visible light: Hydrogen alpha at 656 nm, and OIII at 500.7 nm.

They also commonly have a strong emission from sulfur. But at 672 nm, this is very near the end of our visible range, and largely indistinguishable from the Ha emission. Visually, the two overlap.

[E Fish]

Colors: Ring Nebula versus Stars. Image Credit: Robert Vanderbei (Princeton U.).

---

Very nice! The picture readily explains why planetary nebulas may look green, cyan or blue to the human eye, but never red.

Colors of planetary nebulas annotated.png

Planetary nebulas basically emit just two wavelengths of visible light: Hydrogen alpha at 656 nm, and OIII at 500.7 nm.

The human eye is relatively insensitive to wavelengths longer than perhaps 630 nm. Hα is definitely so deep into the red part of the spectrum, and all Hα nebulas in space have such low surface brightnesses, that the human eye can't detect the red color of hydrogen alpha nebulas.

500 nm, by contrast, is square in the middle of the greatest color sensitivity of the human eye. Therefore, if an OIII nebula has a sufficiently high surface brightness, it should be possible for humans to detect its cyan color. And planetary nebulas are often really bright in OIII.

Ann

Wow. Thanks, Ann. I had never thought to correlate the wavelengths we can see to those particular spectra, although I should have. Kind of a "duh" moment for me. Thanks for the extra information. This is why I started reading these discussion threads.

[neufer]

Click to play embedded YouTube video.

[VictorBorun]

A mix of red and green light appears yellow to the human eye.

Doubly ionized oxygen (O III in spectroscopic notation) emits at 500.7 nm and 495.9 nm.
That's not exactly cyan (493 nm), that's greenish cyan.
So there can be pale mixtures with such hues as orange to yellow to lime to green.
No chance for fucshia (pink) though.

And the main artifact now happens to be the cyan Ring monochromatic image. It should have been greenish cyan.

[johnnydeep]

What are all the little white dots?

Those are the stars whose smeared out spectral images appear about 2 inches to the left, just like the combined image of the ring nebula is 2 inches to the right of the red and blue parts of its smeared out spectra:

Ring Nebula and Stars and their Spectra

[MarkBour]

What are all the little white dots?

Those are the stars whose smeared out spectral images appear about 2 inches to the left, just like the combined image of the ring nebula is 2 inches to the right of the red and blue parts of its smeared out spectra . . .

I'm wondering about the distance between the "combined" images and the streaks. The spread-out spectrum of a typical star in today's APOD (large-res image, rendered on my computer screen) is just under 4 cm in length, and goes from red at the ieft to blue-violet at the right. Then, at least another 6 cm intervenes between it and the normal image. So, in that gap there would be space for some ultraviolet spectrum (and beyond the red edge of the smear, could be room for infrared spectrum). I wonder if the instrument used in producing today's APOD actually does pick up such information, though we can't see it in the image.

I have read that the Balmer series and Lyman series both go into the ultraviolet, so this information would seem to be of some importance to astronomers.

>>>>Wikimedia: https://upload.wikimedia.org/wikipedia/ ... drogen.jpg<<<<

[smitty]

Thank you for the answer about the little white dots! Much appreciated!

[johnnydeep]

What are all the little white dots?

Those are the stars whose smeared out spectral images appear about 2 inches to the left, just like the combined image of the ring nebula is 2 inches to the right of the red and blue parts of its smeared out spectra . . .

I'm wondering about the distance between the "combined" images and the streaks. The spread-out spectrum of a typical star in today's APOD (large-res image, rendered on my computer screen) is just under 4 cm in length, and goes from red at the ieft to blue-violet at the right. Then, at least another 6 cm intervenes between it and the normal image. So, in that gap there would be space for some ultraviolet spectrum (and beyond the red edge of the smear, could be room for infrared spectrum). I wonder if the instrument used in producing today's APOD actually does pick up such information, though we can't see it in the image.

I have read that the Balmer series and Lyman series both go into the ultraviolet, so this information would seem to be of some importance to astronomers.

>>>>Wikimedia: https://upload.wikimedia.org/wikipedia/ ... drogen.jpg<<<<
Capture.png

To be honest, I don't really understand where the "combined image" is coming from. Is it generated at the same time as the spectra and by the same diffraction grating (somehow). The "featured image" link has these details about the hardware used:

22:19-23:55 EDT July 15, 2021.
Canon T2i DSLR on 10" RC at f/9.
RSpec Star Analyser 100 diffraction grating, 87 min (unguided 60 second subexposures).
Nonlinear stretch.

[bobaz]

One powerful application of objective prism spectroscopy was not mentioned: finding high-redshift galaxies by noting the apparent wavelength of the Balmer break at 3646 Å, or more extremely, the Lyman break at 912 Å, where the intensity of the continuous spectrum (the color streak) drops significantly due to the complete ionization of Hydrogen from the 2nd (Balmer) or 1st (Lyman) energy level. At high-Z (redshift) these breaks appear at much longer wavelengths and are detectable by visual inspection, not requiring exhaustive, one-by-one spectroscopy of individual galaxies.

[Chris Peterson]

To be honest, I don't really understand where the "combined image" is coming from. Is it generated at the same time as the spectra and by the same diffraction grating (somehow). The "featured image" link has these details about the hardware used:

22:19-23:55 EDT July 15, 2021.
Canon T2i DSLR on 10" RC at f/9.
RSpec Star Analyser 100 diffraction grating, 87 min (unguided 60 second subexposures).
Nonlinear stretch.

Nothing is "combined" (other than the routine stacking to create a long exposure). This is what things look like through a diffraction grating. The individual light sources pass directly through, as if there was no grating. This is the zero order image, which we see as the ordinary nebula and the stars around it. The first order of diffraction is to the left of the zero order images. These are the individual spectra of the stars and nebula. (With a simple grating there would be a mirror of the first order diffraction on the other side, but this grating is blazed- designed in a way that optimizes most of the light into just one of the first order directions.)

[johnnydeep]

To be honest, I don't really understand where the "combined image" is coming from. Is it generated at the same time as the spectra and by the same diffraction grating (somehow). The "featured image" link has these details about the hardware used:

22:19-23:55 EDT July 15, 2021.
Canon T2i DSLR on 10" RC at f/9.
RSpec Star Analyser 100 diffraction grating, 87 min (unguided 60 second subexposures).
Nonlinear stretch.

Nothing is "combined" (other than the routine stacking to create a long exposure). This is what things look like through a diffraction grating. The individual light sources pass directly through, as if there was no grating. This is the zero order image, which we see as the ordinary nebula and the stars around it. The first order of diffraction is to the left of the zero order images. These are the individual spectra of the stars and nebula. (With a simple grating there would be a mirror of the first order diffraction on the other side, but this grating is blazed- designed in a way that optimizes most of the light into just one of the first order directions.)

Ok. I only used the term "combined" because the descriptive text used the mysterious phrase "color-combined icon". So then, it's not really combined, but undiffracted I suppose.

<sigh> Diffraction gratings are yet another aspect of optics that eludes my understanding. </sigh>