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

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4407
Joined: Fri Dec 04, 2009 3:27 am

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

Post by APOD Robot » Wed Jul 21, 2021 4:05 am

Image 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.

<< Previous APOD This Day in APOD Next APOD >>

User avatar
Ann
4725 Å
Posts: 11520
Joined: Sat May 29, 2010 5:33 am

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

Post by Ann » Wed Jul 21, 2021 5:16 am


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
You do not have the required permissions to view the files attached to this post.
Color Commentator

VictorBorun
Science Officer
Posts: 313
Joined: Fri Oct 16, 2020 10:25 pm

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

Post by VictorBorun » Wed Jul 21, 2021 7:09 am

ring.jpg
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.
You do not have the required permissions to view the files attached to this post.

User avatar
orin stepanek
Plutopian
Posts: 6802
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

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

Post by orin stepanek » Wed Jul 21, 2021 12:15 pm

ring-nebula-full_jpg.jpg
Like a burning ring of fire! To me the Ring Nebula kinda looks like a
hole in the universe! :mrgreen: Opening of a wormhole; there I go
Sify stuff!😉
snuggle_01.jpg.optimal.jpg
Awe; Kitty love! 😍
You do not have the required permissions to view the files attached to this post.
Orin

Smile today; tomorrow's another day!

smitty
Science Officer
Posts: 117
Joined: Tue Jul 04, 2006 9:57 am

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

Post by smitty » Wed Jul 21, 2021 12:30 pm

What are all the little white dots?

User avatar
Chris Peterson
Abominable Snowman
Posts: 16139
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

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

Post by Chris Peterson » Wed Jul 21, 2021 1:08 pm

VictorBorun wrote:
Wed Jul 21, 2021 7:09 am
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

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
Chris Peterson
Abominable Snowman
Posts: 16139
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

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

Post by Chris Peterson » Wed Jul 21, 2021 1:19 pm

Ann wrote:
Wed Jul 21, 2021 5:16 am
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.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

E Fish
Science Officer
Posts: 117
Joined: Thu Mar 09, 2017 2:29 pm

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

Post by E Fish » Wed Jul 21, 2021 1:29 pm

Ann wrote:
Wed Jul 21, 2021 5:16 am

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.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18354
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

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

Post by neufer » Wed Jul 21, 2021 2:14 pm

Click to play embedded YouTube video.
Art Neuendorffer

VictorBorun
Science Officer
Posts: 313
Joined: Fri Oct 16, 2020 10:25 pm

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

Post by VictorBorun » Wed Jul 21, 2021 2:33 pm

Chris Peterson wrote:
Wed Jul 21, 2021 1:08 pm
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.
CIE RGB..png
You do not have the required permissions to view the files attached to this post.

User avatar
johnnydeep
Commander
Posts: 820
Joined: Sun Feb 20, 2011 8:57 pm

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

Post by johnnydeep » Wed Jul 21, 2021 3:56 pm

smitty wrote:
Wed Jul 21, 2021 12:30 pm
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.JPG
You do not have the required permissions to view the files attached to this post.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
MarkBour
Subtle Signal
Posts: 1180
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

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

Post by MarkBour » Wed Jul 21, 2021 6:23 pm

johnnydeep wrote:
Wed Jul 21, 2021 3:56 pm
smitty wrote:
Wed Jul 21, 2021 12:30 pm
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.
You do not have the required permissions to view the files attached to this post.
Mark Goldfain

smitty
Science Officer
Posts: 117
Joined: Tue Jul 04, 2006 9:57 am

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

Post by smitty » Wed Jul 21, 2021 6:37 pm

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

User avatar
johnnydeep
Commander
Posts: 820
Joined: Sun Feb 20, 2011 8:57 pm

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

Post by johnnydeep » Wed Jul 21, 2021 8:46 pm

MarkBour wrote:
Wed Jul 21, 2021 6:23 pm
johnnydeep wrote:
Wed Jul 21, 2021 3:56 pm
smitty wrote:
Wed Jul 21, 2021 12:30 pm
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.
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.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

bobaz
Asternaut
Posts: 1
Joined: Wed Jul 21, 2021 8:51 pm

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

Post by bobaz » Wed Jul 21, 2021 9:04 pm

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.

User avatar
Chris Peterson
Abominable Snowman
Posts: 16139
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

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

Post by Chris Peterson » Thu Jul 22, 2021 4:17 am

johnnydeep wrote:
Wed Jul 21, 2021 8:46 pm
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.)
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
johnnydeep
Commander
Posts: 820
Joined: Sun Feb 20, 2011 8:57 pm

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

Post by johnnydeep » Thu Jul 22, 2021 12:53 pm

Chris Peterson wrote:
Thu Jul 22, 2021 4:17 am
johnnydeep wrote:
Wed Jul 21, 2021 8:46 pm
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>
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."