APOD: NGC 7023: The Iris Nebula (2021 Sep 03)

[APOD Robot]

NGC 7023: The Iris Nebula

Explanation: These cosmic clouds have blossomed 1,300 light-years away, in the fertile starfields of the constellation Cepheus. Called the Iris Nebula, NGC 7023 is not the only nebula to evoke the imagery of flowers. Still, this deep telescopic image shows off the Iris Nebula's range of colors and symmetries, embedded in surrounding fields of interstellar dust. Within the Iris itself, dusty nebular material surrounds a hot, young star. The dominant color of the brighter reflection nebula is blue, characteristic of dust grains reflecting starlight. Central filaments of the reflection nebula glow with a faint reddish photoluminesence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light. Infrared observations indicate that this nebula contains complex carbon molecules known as PAHs. The dusty blue petals of the Iris Nebula span about six light-years.

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[Ann]

Ah, dear old Iris Nebula! It's a beautiful APOD, too!

Irish_RC8_LHaRGB1024[1].png

The Iris Nebula. Photo: Satwant Kumar

Perhaps I shouldn't have used the word "old" in relation to the Iris Nebula, because the nebula, or at least the illumination star (HD 200775) appears to be really young indeed. Simbad calls it a Herbig Ae/Be star.

Wikipedia wrote:

A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young (<10Myr) star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks.[1] Hydrogen and calcium emission lines are observed in their spectra. They are 2-8 Solar mass (M☉) objects, still existing in the star formation (gravitational contraction) stage and approaching the main sequence (i.e. they are not burning hydrogen in their center). In the Hertzsprung–Russell diagram these stars are located to the right of the main sequence. They are named after the American astronomer George Herbig, who first distinguished them from other stars in 1960. The original Herbig criteria were:

*Spectral type earlier than F0 (in order to exclude T Tauri stars),

*Balmer emission lines in the stellar spectrum (in order to be similar to T Tauri stars),

*Projected location within the boundaries of a dark interstellar cloud (in order to select really young stars near their birthplaces),

*Illumination of a nearby bright reflection nebula (in order to guarantee physical link with star formation region).

So the Iris Nebula is really the birthplace of HD 200775, and the star is just emerging from its birth cocoon.

At spectral class B2, the star appears massive for a Herbig Ae/Be star. However, a spectral class of B2 appears to be absolutely ideal to create bright and beautiful blue reflection nebulas. Just look at these B2 stars and their fantastic nebulas:

M78, illuminated by B2 binary star HD 38563. Photo: ESO/Igor Chekalin.

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Rho Ophiuchi blue reflection nebula (top), illuminated by B2 binary star Rho Ophiuchi. Photo: Adam Block.

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So what happens if the illuminating star is a little hotter than spectral class B2? Well, then the nebula turns red!

Dull reddish Running Man set against a light blue background. Photo: Adam Block.

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Pi Scorpii (left) and Delta Scorpii) right. Photo: I don't know.

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Nebula NGC 1977, at left, is illuminated mainly by star 42 Orionis at spectral class B1. It is hot enough to ionize a somewhat dull red emission nebula, surrounded by blue reflection nebulosity. Other blue or bluish stars nearby contribute to the blue reflection nebula.

Star Delta Orionis, at right in the picture at right, consists of a B0III primary and an O9V secondary. This star is surrounded by an all-red emission nebula. But star Pi Orionis, at left in the picture at right, consists of a B1 primary and a B2 secondary. Pi Sco has a red emission nebula just like Del Sco, and Pi Sco also illuminates a prominent blue reflection nebula.

Ann

[Holger Nielsen]

Ann, thank you for the effort you have taken to produce this interesting supplement!
Holger

[orin stepanek]

Irish_RC8_LHaRGB.png

This is a very beautiful nebula and the Iris is a great name for it
because of it's semblance!

IMG_0528.JPG

[VictorBorun]

Irish_RC8_LHaRGB.pngThis is a very beautiful nebula and the Iris is a great name for it
because of it's semblance!

IMG_0528.JPG

I can see some difference here: the hue of an iris flower is just violet (and some places are pale, hinting at a white cover over deep violet pigment capsules) while the hue of the Iris Nebula is shifting, within cool range, from blue cyan (and pale) near the protostar in the center to violet blue (and dark) far from the center.

Wiki says this coloring is the same as a low sun sky (the same scattering process that gives us blue skies and red sunsets). I would like to elaborate.

Day's skyshine has a warm color region of red to orange to yellow; reflection nebulae do not.
Day's skyshine is blue at >10° from the sun; reflection nebulae are blue at <1° from the star.
Day's skyshine scatters photons with N₂, O₂, CO₂ and H₂O molecules; reflection nebulae scatter photons with C, Fe or Ni-based dust particles >10 times the size of a molecule.
Still those dust particles are smaller than 0.4 to 0.7 micrometers range of visible photons' wavelength. Hence the similarity:
deep cool hues are far from the center, not so cool hues are close to the center.

There are examples of the opposite: coronae made with >1 micrometer water particles showing the reversed order of the hues.

Click to view full size image

[Chris Peterson]

Irish_RC8_LHaRGB.pngThis is a very beautiful nebula and the Iris is a great name for it
because of it's semblance!

IMG_0528.JPG

I can see some difference here: the hue of an iris flower is just violet (and some places are pale, hinting at a white cover over deep violet pigment capsules) while the hue of the Iris Nebula is shifting, within cool range, from blue cyan (and pale) near the protostar in the center to violet blue (and dark) far from the center.

Wiki says this coloring is the same as a low sun sky (the same scattering process that gives us blue skies and red sunsets). I would like to elaborate.

Day's skyshine has a warm color region of red to orange to yellow; reflection nebulae do not.

Daytime skyshine is not warm. It is blue, just like reflection nebulas. Both are created by the same scattering physics.

[neufer]

Wiki says this coloring is the same as a low sun sky (the same scattering process that gives us blue skies and red sunsets). I would like to elaborate.

Day's skyshine has a warm color region of red to orange to yellow; reflection nebulae do not.

Daytime skyshine is not warm. It is blue, just like reflection nebulas.

Both are created by the same scattering physics.

Both are created by similar Rayleigh scattering physics:

• 1) Reflection nebulas are mostly due to isolated solid dust particles scattering light incoherently.
  
  2) Daytime skyshine are mostly due to isolated gas density fluctuations scattering light incoherently.

• Why is the sky blue?

<<Rayleigh scattering: When scatterering particles are much smaller than the wavelength of light the process is known as Rayleigh scattering after Lord Rayleigh, John William Strutt, (1842 - 1919) who first described it mathematically. The scattering is inversely proportional to the fourth power of the wavelength. For example, blue light of 450nm wavelength is scattered 4.4X more strongly than 650nm red light. The wavelength dependence come from the extent of coupling between the frequencies associated with bound electrons within the atoms and the oscillating electric field of the light waves. Coupling increases as the oscillation frequencies get more similar.

Rayleigh scattering requires that there be no coherence between the individual scatterers. In dense gases when molecules are closer together this condition is not satisfied and light is predominantly scattered forwards rather than in all directions. In dense gases and liquids another process can operate, Einstein-Smoluchowski scattering. Molecular motion and collisions produce exceedingly transient local density and refractive index fluctuations that act as scattering centres. The wavelength dependence is the same as for Rayleigh scattering.>>

[Chris Peterson]

Wiki says this coloring is the same as a low sun sky (the same scattering process that gives us blue skies and red sunsets). I would like to elaborate.

Day's skyshine has a warm color region of red to orange to yellow; reflection nebulae do not.

Daytime skyshine is not warm. It is blue, just like reflection nebulas.

Both are created by the same scattering physics.

Both are created by similar Rayleigh scattering physics:

• 1) Reflection nebulas are mostly due to isolated solid dust particles scattering light incoherently.
  
  2) Daytime skyshine are mostly due to isolated gas density fluctuations scattering light incoherently.

Exactly. Rayleigh scattering. The same physics.

[neufer]

Daytime skyshine is not warm. It is blue, just like reflection nebulas.

Both are created by the same scattering physics.

Both are created by similar Rayleigh scattering physics:

• 1) Reflection nebulas are mostly due to isolated solid dust particles scattering light incoherently.
  
  2) Daytime skyshine are mostly due to isolated gas density fluctuations scattering light incoherently.

Exactly. Rayleigh scattering. The same physics.

Your blue skies in Guffey are a lot bluer than simple Rayleigh scattering would
suggest thanks to low pressure Einstein-Smoluchowski scattering.

• Why is the sky blue?

<<Rayleigh scattering requires that there be no coherence between the individual scatterers. In dense gases when molecules are closer together this condition is not satisfied and light is predominantly scattered forwards rather than in all directions. In dense gases and liquids another process can operate, Einstein-Smoluchowski scattering. Molecular motion and collisions produce exceedingly transient local density and refractive index fluctuations that act as scattering centres. The wavelength dependence is the same as for Rayleigh scattering.>>

Marian Smoluchowski

---

Click to play embedded YouTube video.

<<Marian Smoluchowski (28 May 1872 – 5 September 1917) was a Polish physicist who worked in the Polish territories of the Austro-Hungarian Empire. He was a pioneer of statistical physics, and an avid mountaineer.

Born into an upper-class family in Vorder-Brühl, near Vienna, Smoluchowski studied physics at the University of Vienna. After several years at other universities (Paris, Glasgow, Berlin), in 1899 Smoluchowski moved to Lwów (present-day Lviv), where he took a position at the University of Lwów. He was president of the Polish Copernicus Society of Naturalists, 1906–7. His non-professional interests included skiing, mountain climbing in the Alps and the Tatra Mountains, watercolor painting, and playing the piano.

Smoluchowski conducted fundamental research on the kinetic theory of matter. In 1904 he discovered density fluctuations in the gas phase, and in 1908 he was the first physicist to ascribe the phenomenon of critical opalescence to large density fluctuations. In 1906, shortly after Albert Einstein, he independently explained Brownian motion. Smoluchowski presented an equation which became a basis for the theory of stochastic processes. His investigations explained the blue color of the sky as a consequence of light scattering in the atmosphere.>>

[VictorBorun]

Your blue skies in Guffey are a lot bluer than simple Rayleigh scattering would
suggest thanks to low pressure Einstein-Smoluchowski scattering.

• Why is the sky blue?

I wonder:

1) what the numbers are for pure Rayleigh vs. Einstein-Smoluchowski modelling of skyshine on Earth. I used to think Rayleigh's model was good enough to suggest the size of an air molecule and let Robert Millikan plan his hunt for the electron's charge. (In 1865, Loschmidt was the first to estimate the size of the molecules that make up the air:[6] his result was only twice the true size, a remarkable feat given the approximations he had to make.)

2) what is the thing with blue cyan to violet blue hues of the central to peripheral regions of Iris Nebula? Do cyan photons squeeze between dust particles more easily than violet photons?

[alter-ego]

Your blue skies in Guffey are a lot bluer than simple Rayleigh scattering would
suggest thanks to low pressure Einstein-Smoluchowski scattering.

I wonder:

1) what the numbers are for pure Rayleigh vs. Einstein-Smoluchowski modelling of skyshine on Earth. I used to think Rayleigh's model was good enough to suggest the size of an air molecule and let Robert Millikan plan his hunt for the electron's charge. (In 1865, Loschmidt was the first to estimate the size of the molecules that make up the air:[6] his result was only twice the true size, a remarkable feat given the approximations he had to make.)

2) what is the thing with blue cyan to violet blue hues of the central to peripheral regions of Iris Nebula? Do cyan photons squeeze between dust particles more easily than violet photons?

1) If you're asking about what the difference in molecule sizes are between Rayleigh and ES modelling, the short answer is it's not the molecule size that matters here, it's the separation between the molecules.
For Rayleigh, the atmosphere was assumed homogenous, and, within kilometers near sea level, the number density is too high to satisfy incoherent scattering from each molecule. For ES, a more accurate atmosphere model is based on larger molecular volumes of varying number densities from thermal gradients (e.g. altitude) and Brownian motion. Still less the wavelength of light, these local density variations affect the coherence / incoherence scattering efficiencies, and help explain the blue sky better. It seems to me the two scattering models do have some overlap, and I think the following is worth mentioning:

The sky is still far from completely understood, but the milestone work done by Rayleigh and Einstein leads to
an explanation complete enough for a quantitative description of daylight.

2) I'm dubious of inferring any truthful scattering phenomena from the colors you're referencing in this image.

[Ann]

I wonder:

2) what is the thing with blue cyan to violet blue hues of the central to peripheral regions of Iris Nebula? Do cyan photons squeeze between dust particles more easily than violet photons?

Good catch, Victor, even though I'm just going to talk about the hints of violet in the Iris Nebula. After all, the blue and cyan hues are just normal blue reflection nebulosity, caused by "forward scattering" of blue light by dust particles behind the light source. (If a substantive wall of dust is in front of the blue light source, the light will be "scattered away" by the dust, so that the shortwave blue and purple photons don't reach us.)

But you mentioned purple hues in the Iris Nebula. And in nebulas, purple color in RGB (+Hα) photography can only mean one thing: The presence of ionized hydrogen along with blue reflection nebulosity.

Let's compare today's APOD with a picture that really brings out the Hα in the Iris Nebula:

The Iris Nebula. Photo: Satwant Kumar.

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The Iris Nebula Adam Block.png

The Iris Nebula. Photo: Adam Block.

Note that Adam Block's image is "upside down" compared to Satwant Kumar's. Also note that Adam Block is clearly using an Hα filter for his image, which Satwant Kumar probably is not.

But where is the Hα in Adam Block's image? Let's take another look:

The Iris Nebula Adam Block annotated.png

Iris Nebula Adam Block annotated with hollows.png

The purple arrows point at pinkish areas that contain red Hα emission. This emission is quite faint. Very many color pictures of the Iris Nebula don't show the Hα at all.

Note that the typical "background color" of cosmic dust is brown. When such dust is "neutrally illuminated" (by white light), it will look light brown and sometimes a little reddish. It can be hard to see the difference between normal illuminated dust and faint reddish Hα emission.

Note that the Hα is seen along the edges of two "hollows" that have been carved in the dust. One of the hollows in the dust has been carved by HD 200775, the star illuminating the Iris Nebula. But there is another "hollow" above it, and one edge, near the purple question mark that I added, looks like it could contain Hα. If Hα is really there, where does it come from? What ionizes the hydrogen here? I'd say that the only answer is that the ionization, if it is really there, is caused by the illuminating star of the Iris Nebula, HD 200775 of spectral class B2.

All right. But where does the "upper hollow" come from? Actually, it has been carved by the cluster inside it, NGC 7023. Yes, that's a cluster! I'm not allowed to add more than three attachments, so I'll show you the cluster in another post!

So anyway, cluster NGC 7023 carved the hollow in the dust, and bright and relatively hot star HD 200775 faintly ionizes one of its edges. Probably!

Ann

[Ann]

Iris Nebula with cluster NGC 7023.png

Okay, I promised you: That's star cluster NGC 7023 (top) in the Iris Nebula! The cluster is responsible for the "top hat cavity" at top.

I can see that the arrows I drew make it look as if some force is pushing the dust inwards, toward the middle. Of course, the opposite is true. It is the wind and ultraviolet light from the hot bright star that pushes the dust outwards, creating both the cavity around the star and and the edges of gas and dust that get ionized. But the triangular cavity at top was made by the cluster.

Ann

[Chris Peterson]

2) I'm dubious of inferring any truthful scattering phenomena from the colors you're referencing in this image.

I believe this is the proper view. What we get from color here is that shorter wavelengths are scattered, we see characteristic hues of dust, and there's some luminescence. And basically, that's it. Color in images like this does not provide much more. Between the characteristics of the filters used to acquire the data, non-linear processing for aesthetics, the characteristics of our display devices, and the nature of our eyes, it simply isn't possible to use "color" to tell us very much beyond broad strokes.

[orin stepanek]

WOW!

[neufer]

Okay, I promised you: That's star cluster NGC 7023 (top) in the Iris Nebula! The cluster is responsible for the "top hat cavity" at top.

I can see that the arrows I drew make it look as if some force is pushing the dust inwards, toward the middle. Of course, the opposite is true. It is the wind and ultraviolet light from the hot bright star that pushes the dust outwards, creating both the cavity around the star and and the edges of gas and dust that get ionized. But the triangular cavity at top was made by the cluster.

• Are you sure https://apod.nasa.gov/apod/ap180801.html

[Ann]

Okay, I promised you: That's star cluster NGC 7023 (top) in the Iris Nebula! The cluster is responsible for the "top hat cavity" at top.

I can see that the arrows I drew make it look as if some force is pushing the dust inwards, toward the middle. Of course, the opposite is true. It is the wind and ultraviolet light from the hot bright star that pushes the dust outwards, creating both the cavity around the star and and the edges of gas and dust that get ionized. But the triangular cavity at top was made by the cluster.

• Are you sure https://apod.nasa.gov/apod/ap180801.html

Pretty sure:

Anne's Astronomy wrote:

The Iris Nebula (LBN 487, VDB 139 and Caldwell 4) is a bright reflection nebula some 6 light-years across and about 1,300 light-years away in the constellation Cepheus. The nebula is often mistakenly labelled for its associated open star cluster NGC 7023, which is present in the triangular “top hat” just above center in the image.

Wikipedia wrote:

The Iris Nebula (also known as NGC 7023 and Caldwell 4) is a bright reflection nebula in the constellation Cepheus. The designation NGC 7023 refers to the open cluster within the larger reflection nebula designated LBN 487.

And when I asked my software Guide to take me to NGC 7023, it clearly took me to the "top hat cavity" of the nebulosity, not to the star HD 200775.

Ann

[alter-ego]

Okay, I promised you: That's star cluster NGC 7023 (top) in the Iris Nebula! The cluster is responsible for the "top hat cavity" at top.

I can see that the arrows I drew make it look as if some force is pushing the dust inwards, toward the middle. Of course, the opposite is true. It is the wind and ultraviolet light from the hot bright star that pushes the dust outwards, creating both the cavity around the star and and the edges of gas and dust that get ionized. But the triangular cavity at top was made by the cluster.

• Are you sure https://apod.nasa.gov/apod/ap180801.html

Pretty sure:

Anne's Astronomy wrote:

The Iris Nebula (LBN 487, VDB 139 and Caldwell 4) is a bright reflection nebula some 6 light-years across and about 1,300 light-years away in the constellation Cepheus. The nebula is often mistakenly labelled for its associated open star cluster NGC 7023, which is present in the triangular “top hat” just above center in the image.

Wikipedia wrote:

The Iris Nebula (also known as NGC 7023 and Caldwell 4) is a bright reflection nebula in the constellation Cepheus. The designation NGC 7023 refers to the open cluster within the larger reflection nebula designated LBN 487.

And when I asked my software Guide to take me to NGC 7023, it clearly took me to the "top hat cavity" of the nebulosity, not to the star HD 200775.

Ann

I can't find any substantive data to support the open cluster as NGC 7023. Most hits support the nebula. The ones that ID the cluster aren't trustworthy they don't have good supporting data, or are self-inconsistent. Although "Anne" says confidently says the nebula is mistakenly ID'd, the heavy-weight reputable databases don't support that. Even the Wiki reference is not self consistent.

Searching for NGC 7023 using the best databases and search engines (NED, SIMBAD, AAS Worldwide Telescope,NGC/IC Project , VizieR, Aladin, and my 3-Volume Millenium Star Atlas), I find the ID NGC 7023 = Iris Nebula in all cases, not the open cluster. In the Wiki article, the SEDS and VizieR links list the star cluster, but I don't trust these. First, Wiki VizieR link references 4 catalogs, but only the oldest catalog (VII/1B, 1973) lists the cluster, and getting to this result is funky - If you input NGC 7023 ID, you also have to introduce a search radius large enough to overlap the cluster location. Then the result shows the new location for NGC 7023 (LIke I said, funky). The other 3 newer catalogs take you to the nebula without forcing a coordinate change via an unknown search radius. Second, I don't trust the Wiki SEDS link because several supporting links don't go anywhere, and the associated link to the Digitize Sky Survey image does not agree with survey image result made today. The present survey image shows NGC 7023 as the nebula.

Sorry Ann, I think Anne's Astronomy is lacking up-to-date data, or the rest of the databases need correcting.

[alter-ego]

2) I'm dubious of inferring any truthful scattering phenomena from the colors you're referencing in this image.

I believe this is the proper view. What we get from color here is that shorter wavelengths are scattered, we see characteristic hues of dust, and there's some luminescence. And basically, that's it. Color in images like this does not provide much more. Between the characteristics of the filters used to acquire the data, non-linear processing for aesthetics, the characteristics of our display devices, and the nature of our eyes, it simply isn't possible to use "color" to tell us very much beyond broad strokes.

I agree. My problem was I didn't see the definitive violet shade in the APOD as is clearly visible in Adam Block's image(s). Now, Victor's question makes more sense. The region of visible violet scattering does suggest dust density is attenuating the violet within the central region, but not the cyan. The violet scatters into view in appropriate density regime.

[VictorBorun]

the blue and cyan hues are just normal blue reflection nebulosity, caused by "forward scattering" of blue light by dust particles behind the light source. (If a substantive wall of dust is in front of the blue light source, the light will be "scattered away" by the dust, so that the shortwave blue and purple photons don't reach us.)

purple color in RGB (+Hα) photography can only ionized hydrogen along with blue reflection nebulosity.

Let's compare today's APOD with a picture that really brings out the Hα in the Iris Nebula:

The Iris Nebula. Photo: Satwant Kumar.

---

The Iris Nebula Adam Block.png

The Iris Nebula. Photo: Adam Block.

Note that Adam Block's image is "upside down" compared to Satwant Kumar's. Also note that Adam Block is clearly using an Hα filter for his image, which Satwant Kumar probably is not.

Note that the typical "background color" of cosmic dust is brown. When such dust is "neutrally illuminated" (by white light), it will look light brown and sometimes a little reddish. It can be hard to see the difference between normal illuminated dust and faint reddish Hα emission.
Ann

Wow.

But I fear I was not as clear as should.
1) Satwant Kumar's pic of Iris Nebula (in the post) shows little if any purple. Hα red glow must be too faint to notice without narrow-band filter imaging.
By the shift of the hue from blue-cyan to violet-blue I meant just usual range of cool hues ██ ██ ██ as opposed to warm hues ██ ██ ██
These are hues for monotonic spectra. There can be no green or pink; even bounding hues are impossible, that is cyan and violet around the cool range and red and yellow around the warm range, ██ ██ and ██ ██. Purple is violet-pink and a no-go as a scattering hue.

2) Dust can have a color when dust particles can partly reflect and partly absorb the illuminating light. A photon must hit the dust particle centrally or near-centrally; if a photon just brushes the edge of a dust particle it never gets absorbed, just forward-scattered a little, so the dust particle material has no chance to show its color. Gray, brown and orange dust clouds need other illumination than a point background light source <1° from the line of sight.

[Ann]

• Are you sure https://apod.nasa.gov/apod/ap180801.html

Pretty sure:

Anne's Astronomy wrote:

The Iris Nebula (LBN 487, VDB 139 and Caldwell 4) is a bright reflection nebula some 6 light-years across and about 1,300 light-years away in the constellation Cepheus. The nebula is often mistakenly labelled for its associated open star cluster NGC 7023, which is present in the triangular “top hat” just above center in the image.

Wikipedia wrote:

The Iris Nebula (also known as NGC 7023 and Caldwell 4) is a bright reflection nebula in the constellation Cepheus. The designation NGC 7023 refers to the open cluster within the larger reflection nebula designated LBN 487.

And when I asked my software Guide to take me to NGC 7023, it clearly took me to the "top hat cavity" of the nebulosity, not to the star HD 200775.

Ann

I can't find any substantive data to support the open cluster as NGC 7023. Most hits support the nebula. The ones that ID the cluster aren't trustworthy they don't have good supporting data, or are self-inconsistent. Although "Anne" says confidently says the nebula is mistakenly ID'd, the heavy-weight reputable databases don't support that. Even the Wiki reference is not self consistent.

Searching for NGC 7023 using the best databases and search engines (NED, SIMBAD, AAS Worldwide Telescope,NGC/IC Project , VizieR, Aladin, and my 3-Volume Millenium Star Atlas), I find the ID NGC 7023 = Iris Nebula in all cases, not the open cluster. In the Wiki article, the SEDS and VizieR links list the star cluster, but I don't trust these. First, Wiki VizieR link references 4 catalogs, but only the oldest catalog (VII/1B, 1973) lists the cluster, and getting to this result is funky - If you input NGC 7023 ID, you also have to introduce a search radius large enough to overlap the cluster location. Then the result shows the new location for NGC 7023 (LIke I said, funky). The other 3 newer catalogs take you to the nebula without forcing a coordinate change via an unknown search radius. Second, I don't trust the Wiki SEDS link because several supporting links don't go anywhere, and the associated link to the Digitize Sky Survey image does not agree with survey image result made today. The present survey image shows NGC 7023 as the nebula.

Sorry Ann, I think Anne's Astronomy is lacking up-to-date data, or the rest of the databases need correcting.

Thank you, alter-ego. I placed the NGC 7023 star cluster in the triangular cavity because of Anne's Astronomy and because my own software Guide so clearly placed NGC 7023 in that cavity. However, when I googled "NGC 7023 star cluster", I only got references to the nebula, apart from the link to Anne's Astronomy.

My main concern about placing a cluster in the triangular opening is that it doesn't look as if there is a cluster there. The stars inside look just like they are part of the background stars. Also, of course, if there is a cluster at the same distance as the Iris Nebula whose stars are so few, faint and scattered, the cluster would have to be way, way older - hundreds of millions of years older - than the illuminating star of the nebula, which seems decidedly odd.

So my claim that cluster NGC 7023 can be found in that triangular opening should be regarded as unsubstantiated and in all likelihood incorrect.

Ann

[Ann]

the blue and cyan hues are just normal blue reflection nebulosity, caused by "forward scattering" of blue light by dust particles behind the light source. (If a substantive wall of dust is in front of the blue light source, the light will be "scattered away" by the dust, so that the shortwave blue and purple photons don't reach us.)

purple color in RGB (+Hα) photography can only ionized hydrogen along with blue reflection nebulosity.

Let's compare today's APOD with a picture that really brings out the Hα in the Iris Nebula:

The Iris Nebula. Photo: Satwant Kumar.

---

The Iris Nebula Adam Block.png

The Iris Nebula. Photo: Adam Block.

Note that Adam Block's image is "upside down" compared to Satwant Kumar's. Also note that Adam Block is clearly using an Hα filter for his image, which Satwant Kumar probably is not.

Note that the typical "background color" of cosmic dust is brown. When such dust is "neutrally illuminated" (by white light), it will look light brown and sometimes a little reddish. It can be hard to see the difference between normal illuminated dust and faint reddish Hα emission.
Ann

Wow.

But I fear I was not as clear as should.
1) Satwant Kumar's pic of Iris Nebula (in the post) shows little if any purple. Hα red glow must be too faint to notice without narrow-band filter imaging.
By the shift of the hue from blue-cyan to violet-blue I meant just usual range of cool hues ██ ██ ██ as opposed to warm hues ██ ██ ██
These are hues for monotonic spectra. There can be no green or pink; even bounding hues are impossible, that is cyan and violet around the cool range and red and yellow around the warm range, ██ ██ and ██ ██. Purple is violet-pink and a no-go as a scattering hue.

2) Dust can have a color when dust particles can partly reflect and partly absorb the illuminating light. A photon must hit the dust particle centrally or near-centrally; if a photon just brushes the edge of a dust particle it never gets absorbed, just forward-scattered a little, so the dust particle material has no chance to show its color. Gray, brown and orange dust clouds need other illumination than a point background light source <1° from the line of sight.

Victor, I was impressed by your colored squares. I can see that you used black squares and placed color hex markings around them.

But where did you find the black squares? I would love to use them, too!

Ann

[johnnydeep]

Victor, I was impressed by your colored squares. I can see that you used black squares and placed color hex markings around them.

But where did you find the black squares? I would love to use them, too!

Ann

I'm not Victor, but I thought they might just be unicode characters. His "square" is really two chars side-by-side. I thought they might be one of the "geometric shapes" shown here - https://www.unicodepedia.com/groups/geometric-shapes/ - but none exactly match what Victor used. Though the "black vertical rectagle" comes close: ▮, but it's not big enough! Can it be colorized with the "font color" tool in the post editor? Let's see: ▮▮▮. Yup! But there is also space between successive chars, which Victor's double black box chars don't have. I still suspect it's a unicode char however.

[Chris Peterson]

Victor, I was impressed by your colored squares. I can see that you used black squares and placed color hex markings around them.

But where did you find the black squares? I would love to use them, too!

Ann

I'm not Victor, but I thought they might just be unicode characters. His "square" is really two chars side-by-side. I thought they might be one of the "geometric shapes" shown here - https://www.unicodepedia.com/groups/geometric-shapes/ - but none exactly match what Victor used. Though the "black vertical rectagle" comes close: ▮, but it's not big enough! Can it be colorized with the "font color" tool in the post editor? Let's see: ▮▮▮. Yup! But there is also space between successive chars, which Victor's double black box chars don't have. I still suspect it's a unicode char however.

Sure. You can just look at it in the quoted text. It's Unicode U+2588, Full Block. In Windows you type it using ALT-219 (█) or a pair of those (██). As an ordinary character, you can apply whatever styling BBCODE offers: █.

[johnnydeep]

Victor, I was impressed by your colored squares. I can see that you used black squares and placed color hex markings around them.

But where did you find the black squares? I would love to use them, too!

Ann

I'm not Victor, but I thought they might just be unicode characters. His "square" is really two chars side-by-side. I thought they might be one of the "geometric shapes" shown here - https://www.unicodepedia.com/groups/geometric-shapes/ - but none exactly match what Victor used. Though the "black vertical rectagle" comes close: ▮, but it's not big enough! Can it be colorized with the "font color" tool in the post editor? Let's see: ▮▮▮. Yup! But there is also space between successive chars, which Victor's double black box chars don't have. I still suspect it's a unicode char however.

Sure. You can just look at it in the quoted text. It's Unicode U+2588, Full Block. In Windows you type it using ALT-219 (█) or a pair of those (██). As an ordinary character, you can apply whatever styling BBCODE offers: █.

Thanks. That's it alright. More here - http://www.unicode-symbol.com/block/Block_Elements.html

But how did you determine that is was U+2588? How did you "just look at in in the quoted text"? In my browser reply window it still shows up as a black block.