APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

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APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by APOD Robot » Sun Feb 19, 2023 6:14 am

Image Seven Dusty Sisters in Infrared

Explanation: Is this really the famous Pleiades star cluster? Known for its iconic blue stars, the Pleiades is shown here in infrared light where the surrounding dust outshines the stars. Here three infrared colors have been mapped into visual colors (R=24, G=12, B=4.6 microns). The base images were taken by NASA's orbiting Wide Field Infrared Survey Explorer (WISE) spacecraft. Cataloged as M45 and nicknamed the Seven Sisters, the Pleiades star cluster is by chance situated in a passing dust cloud. The light and winds from the massive Pleiades stars preferentially repel smaller dust particles, causing the dust to become stratified into filaments, as seen. The featured image spans about 20 light years at the distance of the Pleiades, which lies about 450 light years distant toward the constellation of the Bull (Taurus).

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by AVAO » Sun Feb 19, 2023 7:37 am

APOD Robot wrote: Sun Feb 19, 2023 6:14 am Image Seven Dusty Sisters in Infrared
This APOD was already published on 2018 May 28. viewtopic.php?t=38331
Without a description of how the image was created, the result is highly questionable.
This also applies to other APODs by Francesco Antonucci such as https://apod.nasa.gov/apod/ap170507.html
The author's Astrobin account has not shown any entries since 2015.
Various links in today's APOD lead to Nirvana.

"I find this problematic, because they (the stars) are not at all overwhelmed by dust in the original data. They seem to have been shrunk with some kind of filter. Fake glowing effects, hazy rings, and way-too-crisp fake diffraction spikes spoil the image. I'm not sure about the colors. They seem overworked. I don't think they actually closely match with the data, though there is some beautiful variation for this nebula in the infrared spectrum. Like, it hasn't just had its saturation increased, colors were totally shifted or unevenly balanced. I personally think it's leaning too heavily on the creative side, but to each their own. Aside from the odd use of color, there are lots of mosaic seams and various filters have been heavily applied."

Comment by Geck in the then APOD 2018.

It's ok if really great pictures are used again - after all it's Sunday.
But it would be good if the discussion at the time and their comments were also taken into account in the selection.

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Ann » Sun Feb 19, 2023 9:06 am

AVAO wrote: Sun Feb 19, 2023 7:37 am
APOD Robot wrote: Sun Feb 19, 2023 6:14 am Image Seven Dusty Sisters in Infrared
This APOD was already published on 2018 May 28. viewtopic.php?t=38331
Without a description of how the image was created, the result is highly questionable.
This also applies to other APODs by Francesco Antonucci such as https://apod.nasa.gov/apod/ap170507.html
The author's Astrobin account has not shown any entries since 2015.
Various links in today's APOD lead to Nirvana.

"I find this problematic, because they (the stars) are not at all overwhelmed by dust in the original data. They seem to have been shrunk with some kind of filter. Fake glowing effects, hazy rings, and way-too-crisp fake diffraction spikes spoil the image. I'm not sure about the colors. They seem overworked. I don't think they actually closely match with the data, though there is some beautiful variation for this nebula in the infrared spectrum. Like, it hasn't just had its saturation increased, colors were totally shifted or unevenly balanced. I personally think it's leaning too heavily on the creative side, but to each their own. Aside from the odd use of color, there are lots of mosaic seams and various filters have been heavily applied."

Comment by Geck in the then APOD 2018.

It's ok if really great pictures are used again - after all it's Sunday.
But it would be good if the discussion at the time and their comments were also taken into account in the selection.
Thanks, AVAO!

I too find the colors sometimes very odd. Most problematic, to me, is the fact that the bright stars of the Pleiades themselves (which you can find with the help of the overlay) are definitely weirdly colored. For example, Atlas and Pleione - the two stars in the "handle" of the Pleiades - are shown as red. That makes absolutely no sense to me. And Alcyone, brightest star of the Pleiades, is shown as yellow-orange. That is also totally weird.

However, a good thing about today's APOD is that it singles out an optically faint star and makes it shine brightly in this infrared image. I'm talking, of course, about the brilliant orange-looking star at about 8 o'clock.

SY Tau infrared annotated.png
SY Tau annotated.png

It turns out that this star is SY Tau, otherwise known as BD+23 555 or TYC 1800-2105-1. The star has a tiny Gaia parallax of 0.7001 [0.0387] milliarcseconds, which puts it at a distance of some ~4,600 light-years. This is more than ten times as far away as the Pleiades cluster.

Very interestingly, SY Tau's (apparent) V magnitude is around 9.5. Yes, but its infrared magnitude is much, much brighter. Its K magnitude (and I think that refers to mid infrared, although I'm too lazy to google) is ~3.58. Now what absolute K magnitude is a star with whose apparent K brightness at a distance of ~4,600 light-years is ~3.58?

If I used my google calculations correctly, the absolute K magnitude of such a star would be -7, with a K luminosity of some 60,000 solar. Except it would be a lot brighter than that, if we compare it with the Sun, because the Sun's infrared brightness is comparatively modest. The blackbody curve of the Sun peaks at optical wavelengths, but stars like SY Tau peak in the infrared.

Spectrum of Betelgeuse Christophe Pellier.png
Spectrum of a red giant star (Betelgeuse).
Credit: Christophe Pellier

The long and short of it is that SY Tau is a brilliantly bright, very distant red giant star, and this APOD has revealed its brilliance to us. And, as a Color Commentator, I might add that it is quite right that it looks orange, too, since it is particularly bright at H (mapped as yellow) and K (mapped as red) wavelengths.

Approved by the Color Commentator!

Ann
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Last edited by Ann on Mon Feb 20, 2023 4:22 am, edited 1 time in total.
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by AstroLux » Sun Feb 19, 2023 3:13 pm

What happened to this image, the spikes on stars have been added in post and poorly done with literal crosses over the actual star cores. This is no artistic choice this is just wrong and poorly executed especially since you can also see the stitching line artefacts on the image. The person who processed this has clearly no line for integrity and realism of actual data captured by NASA WISE, he clearly made this only to "look cool". Image

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by MoreInput » Sun Feb 19, 2023 9:03 pm

Hi!

Regarding the posted details from Ann of SY Tau (infrared annotated.png). A little above the star there is a tiny round structure visible.
It this an artifact or could it be a real structure, like a very distant planetary nebulae?
Best regards,
Stefan

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by orin stepanek » Sun Feb 19, 2023 9:14 pm

Pleiades_WiseAntonucci_960_Roll.jpg
Anew view of the sisters! 8-)
pleiades_lane_960.jpg
Dusty Sisters! Awesome!
dogs-on-joyrides-16__605.jpg
I often wondered why dogs like the wind so much! :roll:
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Smile today; tomorrow's another day!

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by AVAO » Sun Feb 19, 2023 10:23 pm

MoreInput wrote: Sun Feb 19, 2023 9:03 pm Hi!

Regarding the posted details from Ann of SY Tau (infrared annotated.png). A little above the star there is a tiny round structure visible.
It this an artifact or could it be a real structure, like a very distant planetary nebulae?
Best regards,
Stefan
Basically, I would orientate myself on the original processing or on very good interpretations.
But these strange fragments are not visible there.

https://www.nasa.gov/mission_pages/WISE ... 13121.html
https://www.flickr.com/photos/geckzilla ... otostream/

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by johnnydeep » Mon Feb 20, 2023 12:43 am

AVAO wrote: Sun Feb 19, 2023 10:23 pm
MoreInput wrote: Sun Feb 19, 2023 9:03 pm Hi!

Regarding the posted details from Ann of SY Tau (infrared annotated.png). A little above the star there is a tiny round structure visible.
It this an artifact or could it be a real structure, like a very distant planetary nebulae?
Best regards,
Stefan
Basically, I would orientate myself on the original processing or on very good interpretations.
But these strange fragments are not visible there.

https://www.nasa.gov/mission_pages/WISE ... 13121.html
https://www.flickr.com/photos/geckzilla ... otostream/
Wow. So that little ring that I would have sworn was a real thing is just a processing artifact? Make me wonder how much of the rest of the structure we see is real!
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by AVAO » Mon Feb 20, 2023 4:41 am

johnnydeep wrote: Mon Feb 20, 2023 12:43 am
AVAO wrote: Sun Feb 19, 2023 10:23 pm
MoreInput wrote: Sun Feb 19, 2023 9:03 pm Hi!

Regarding the posted details from Ann of SY Tau (infrared annotated.png). A little above the star there is a tiny round structure visible.
It this an artifact or could it be a real structure, like a very distant planetary nebulae?
Best regards,
Stefan
Basically, I would orientate myself on the original processing or on very good interpretations.
But these strange fragments are not visible there.

https://www.nasa.gov/mission_pages/WISE ... 13121.html
https://www.flickr.com/photos/geckzilla ... otostream/
Wow. So that little ring that I would have sworn was a real thing is just a processing artifact? Make me wonder how much of the rest of the structure we see is real!
So. I think there are differences. These ring-shaped objects are also visible with two other bright stars in the image, and they are all located at the top left of the star - but exactly on the axis of the original spikes. So this indicates that these are with high probability an artificially caused optical phenomenon (artifact) of the original optics - in contrast to artificially and amateurishly inserted pseudo-spikes by the author of the picture. Why these are not visible in the original processings of the WISE data, but here they are, is probably due to the excessive use of effect filters or the fact that additional wavelengths were used. What exactly remains in the dark. But what can be ruled out due to the multiple repetition of this effect is that they are not planetary nebulae for sure.

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Ann » Mon Feb 20, 2023 5:28 am

AVAO wrote: Mon Feb 20, 2023 4:41 am
johnnydeep wrote: Mon Feb 20, 2023 12:43 am
AVAO wrote: Sun Feb 19, 2023 10:23 pm

Basically, I would orientate myself on the original processing or on very good interpretations.
But these strange fragments are not visible there.

https://www.nasa.gov/mission_pages/WISE ... 13121.html
https://www.flickr.com/photos/geckzilla ... otostream/
Wow. So that little ring that I would have sworn was a real thing is just a processing artifact? Make me wonder how much of the rest of the structure we see is real!
So. I think there are differences. These ring-shaped objects are also visible with two other bright stars in the image, and they are all located at the top left of the star - but exactly on the axis of the original spikes. So this indicates that these are with high probability an artificially caused optical phenomenon (artifact) of the original optics - in contrast to artificially and amateurishly inserted pseudo-spikes by the author of the picture. Why these are not visible in the original processings of the WISE data, but here they are, is probably due to the excessive use of effect filters or the fact that additional wavelengths were used. What exactly remains in the dark. But what can be ruled out due to the multiple repetition of this effect is that they are not planetary nebulae for sure.

There is another ring near another red giant star in this APOD, HD 23712:

APOD 19 Feb 2023 detail HD 23712.png
APOD 19 Feb 2023 infrared HD 23712.png

If you ask me, I'd say that these rings are reflections in the optics of the detector. Note that the rings are only visible in the infrared, where these two red giants are particularly bright.

I have seen similar rings in other pictures, even in pictures taken with professional instruments. So I'd say that, yes, they are reflections. Therefore, if you see a little ring in a picture near a bright star, your first thought should be that this is probably just a photographic artifact.

Personally I'm wondering why HD 23712 doesn't look brighter than it does in infrared light. Why doesn't it look brighter than SY Tau, for example? SY Tau is intrinsically brighter than HD 23712, but SY Tau is five times farther away, and the apparent K magnitude of HD 23712 is around 1.8, while that of SY Tau is around 3.6. That's a big difference.

Also, why does HD 23712 look white, when SY Tau look orange? Both peak at mid infrared wavelengths, and are fainter at all shorter wavelengths.

Anyway. Note that the little ring near SY Tau is orange, the same (mapped) color as SY Tau, while the little ring near HD 23712 is whitish, more or less the same mapped color as HD 23712.

Personally I'm more interested in this "little guy":

APOD 19 Feb 2023 little man.png

What is it? To me it looks like a little man, standing upright with raised arms. There is an arched roof above him.

Could the "arms and legs" of this "little man" be some sort of jets emitted by a very young star? And could the arched roof have been lifted by outflows from this very young star?

Inquiring minds want to know!

Ann
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Sat Feb 25, 2023 8:52 pm

Ann wrote: Sun Feb 19, 2023 9:06 am
AVAO wrote: Sun Feb 19, 2023 7:37 am This APOD was already published on 2018 May 28. viewtopic.php?t=38331
Without a description of how the image was created, the result is highly questionable.
This also applies to other APODs by Francesco Antonucci such as https://apod.nasa.gov/apod/ap170507.html
The author's Astrobin account has not shown any entries since 2015.
Various links in today's APOD lead to Nirvana.

"I find this problematic, because they (the stars) are not at all overwhelmed by dust in the original data. They seem to have been shrunk with some kind of filter. Fake glowing effects, hazy rings, and way-too-crisp fake diffraction spikes spoil the image. I'm not sure about the colors. They seem overworked. I don't think they actually closely match with the data, though there is some beautiful variation for this nebula in the infrared spectrum. Like, it hasn't just had its saturation increased, colors were totally shifted or unevenly balanced. I personally think it's leaning too heavily on the creative side, but to each their own. Aside from the odd use of color, there are lots of mosaic seams and various filters have been heavily applied."

Comment by Geck in the then APOD 2018.

It's ok if really great pictures are used again - after all it's Sunday.
But it would be good if the discussion at the time and their comments were also taken into account in the selection.
Thanks, AVAO!

I too find the colors sometimes very odd. Most problematic, to me, is the fact that the bright stars of the Pleiades themselves (which you can find with the help of the overlay) are definitely weirdly colored. For example, Atlas and Pleione - the two stars in the "handle" of the Pleiades - are shown as red. That makes absolutely no sense to me. And Alcyone, brightest star of the Pleiades, is shown as yellow-orange. That is also totally weird.

However, a good thing about today's APOD is that it singles out an optically faint star and makes it shine brightly in this infrared image. I'm talking, of course, about the brilliant orange-looking star at about 8 o'clock.

SY Tau infrared annotated.png
SY Tau annotated.png

It turns out that this star is SY Tau, otherwise known as BD+23 555 or TYC 1800-2105-1. The star has a tiny Gaia parallax of 0.7001 [0.0387] milliarcseconds, which puts it at a distance of some ~4,600 light-years. This is more than ten times as far away as the Pleiades cluster.

Very interestingly, SY Tau's (apparent) V magnitude is around 9.5. Yes, but its infrared magnitude is much, much brighter. Its K magnitude (and I think that refers to mid infrared, although I'm too lazy to google) is ~3.58. Now what absolute K magnitude is a star with whose apparent K brightness at a distance of ~4,600 light-years is ~3.58?

If I used my google calculations correctly, the absolute K magnitude of such a star would be -7, with a K luminosity of some 60,000 solar. Except it would be a lot brighter than that, if we compare it with the Sun, because the Sun's infrared brightness is comparatively modest. The blackbody curve of the Sun peaks at optical wavelengths, but stars like SY Tau peak in the infrared.


The long and short of it is that SY Tau is a brilliantly bright, very distant red giant star, and this APOD has revealed its brilliance to us. And, as a Color Commentator, I might add that it is quite right that it looks orange, too, since it is particularly bright at H (mapped as yellow) and K (mapped as red) wavelengths.

Approved by the Color Commentator!

Ann
This talk made me wonder why we expect stars' hue or any thermal lamp's hue to be either an orange or a blue (and not a lime, an aqua, a purple or a magenta). And the second question is whether such limitation should give way to the whole rainbow spectrum or even to the whole colour wheel when we use artificial colour scheme to represent a really wide electromagnetic band.

1) We are used to orange&blue limitation for the evening sky, but that have nothing to do with thermal lamps. The sky colours the low sun's light by Raleigh's scattering which creates monotonic spectra: one orange closer to the sun and one blue further from the sun

2) A thermal lamp at T° radiates a spectrum with a peak, a preferable photon's energy about kT. If an observer drew his white balance from a flat spectrum etalon, they would see Sun as an aqua, a green-cyan hue. But we usually have a T=5000°K thermal lamp etalon for a white balance, e. g. the sun, the moon, an artificial light with light diodes or Hg + luminophores. So we usually see Sun as white and every star or any thermal lamp as Sun-like, or hotter, or cooler. This means we would see a flat spectrum radiation as a magenta hue and any lamp we perceive as through a magenta filter, absorbing some of the green and cyan photons. That's what turns a peak into a slope — an orange or a blue.

3) IMHO it's not wise to keep this limited hues when representing wide band pictures

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Sat Feb 25, 2023 9:00 pm

Ann wrote: Sun Feb 19, 2023 9:06 am
AVAO wrote: Sun Feb 19, 2023 7:37 am This APOD was already published on 2018 May 28. viewtopic.php?t=38331
Without a description of how the image was created, the result is highly questionable.
This also applies to other APODs by Francesco Antonucci such as https://apod.nasa.gov/apod/ap170507.html
The author's Astrobin account has not shown any entries since 2015.
Various links in today's APOD lead to Nirvana.

"I find this problematic, because they (the stars) are not at all overwhelmed by dust in the original data. They seem to have been shrunk with some kind of filter. Fake glowing effects, hazy rings, and way-too-crisp fake diffraction spikes spoil the image. I'm not sure about the colors. They seem overworked. I don't think they actually closely match with the data, though there is some beautiful variation for this nebula in the infrared spectrum. Like, it hasn't just had its saturation increased, colors were totally shifted or unevenly balanced. I personally think it's leaning too heavily on the creative side, but to each their own. Aside from the odd use of color, there are lots of mosaic seams and various filters have been heavily applied."

Comment by Geck in the then APOD 2018.

It's ok if really great pictures are used again - after all it's Sunday.
But it would be good if the discussion at the time and their comments were also taken into account in the selection.
Thanks, AVAO!

I too find the colors sometimes very odd. Most problematic, to me, is the fact that the bright stars of the Pleiades themselves (which you can find with the help of the overlay) are definitely weirdly colored. For example, Atlas and Pleione - the two stars in the "handle" of the Pleiades - are shown as red. That makes absolutely no sense to me. And Alcyone, brightest star of the Pleiades, is shown as yellow-orange. That is also totally weird.

However, a good thing about today's APOD is that it singles out an optically faint star and makes it shine brightly in this infrared image. I'm talking, of course, about the brilliant orange-looking star at about 8 o'clock.

SY Tau infrared annotated.png
SY Tau annotated.png

It turns out that this star is SY Tau, otherwise known as BD+23 555 or TYC 1800-2105-1. The star has a tiny Gaia parallax of 0.7001 [0.0387] milliarcseconds, which puts it at a distance of some ~4,600 light-years. This is more than ten times as far away as the Pleiades cluster.

Very interestingly, SY Tau's (apparent) V magnitude is around 9.5. Yes, but its infrared magnitude is much, much brighter. Its K magnitude (and I think that refers to mid infrared, although I'm too lazy to google) is ~3.58. Now what absolute K magnitude is a star with whose apparent K brightness at a distance of ~4,600 light-years is ~3.58?

If I used my google calculations correctly, the absolute K magnitude of such a star would be -7, with a K luminosity of some 60,000 solar. Except it would be a lot brighter than that, if we compare it with the Sun, because the Sun's infrared brightness is comparatively modest. The blackbody curve of the Sun peaks at optical wavelengths, but stars like SY Tau peak in the infrared.


The long and short of it is that SY Tau is a brilliantly bright, very distant red giant star, and this APOD has revealed its brilliance to us. And, as a Color Commentator, I might add that it is quite right that it looks orange, too, since it is particularly bright at H (mapped as yellow) and K (mapped as red) wavelengths.

Approved by the Color Commentator!

Ann
This talk made me wonder why we expect stars' hue or any thermal lamp's hue to be either an orange or a blue (and not a lime, an aqua, a purple or a magenta). And the second question is whether such limitation should give way to the whole rainbow spectrum or even to the whole colour wheel when we use artificial colour scheme to represent a really wide electromagnetic band.

1) We are used to orange&blue limitation for the evening sky, but that have nothing to do with thermal lamps. The sky colours the low sun's light by Raleigh's scattering which creates monotonic spectra: one orange closer to the sun and one blue further from the sun

2) A thermal lamp at T° radiates a spectrum with a peak, a preferable photon's energy about kT. If an observer drew his white balance from a flat spectrum etalon, they would see Sun as an aqua, a green-cyan hue. But we usually have a T=5000°K thermal lamp etalon for a white balance, e. g. the sun, the moon, an artificial light with light diodes or Hg + luminophores. So we usually see Sun as white and every star or any thermal lamp as Sun-like, or hotter, or cooler. This means we would see a flat spectrum radiation as a magenta hue and any lamp we perceive as through a magenta filter, absorbing some of the green and cyan photons. That's what turns a peak into a slope — an orange or a blue.

3) IMHO it's not wise to keep this limited hues when representing wide band pictures

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Sat Feb 25, 2023 9:01 pm

Ann wrote: Sun Feb 19, 2023 9:06 am
AVAO wrote: Sun Feb 19, 2023 7:37 am This APOD was already published on 2018 May 28. viewtopic.php?t=38331
Without a description of how the image was created, the result is highly questionable.
This also applies to other APODs by Francesco Antonucci such as https://apod.nasa.gov/apod/ap170507.html
The author's Astrobin account has not shown any entries since 2015.
Various links in today's APOD lead to Nirvana.

"I find this problematic, because they (the stars) are not at all overwhelmed by dust in the original data. They seem to have been shrunk with some kind of filter. Fake glowing effects, hazy rings, and way-too-crisp fake diffraction spikes spoil the image. I'm not sure about the colors. They seem overworked. I don't think they actually closely match with the data, though there is some beautiful variation for this nebula in the infrared spectrum. Like, it hasn't just had its saturation increased, colors were totally shifted or unevenly balanced. I personally think it's leaning too heavily on the creative side, but to each their own. Aside from the odd use of color, there are lots of mosaic seams and various filters have been heavily applied."

Comment by Geck in the then APOD 2018.

It's ok if really great pictures are used again - after all it's Sunday.
But it would be good if the discussion at the time and their comments were also taken into account in the selection.
Thanks, AVAO!

I too find the colors sometimes very odd. Most problematic, to me, is the fact that the bright stars of the Pleiades themselves (which you can find with the help of the overlay) are definitely weirdly colored. For example, Atlas and Pleione - the two stars in the "handle" of the Pleiades - are shown as red. That makes absolutely no sense to me. And Alcyone, brightest star of the Pleiades, is shown as yellow-orange. That is also totally weird.

However, a good thing about today's APOD is that it singles out an optically faint star and makes it shine brightly in this infrared image. I'm talking, of course, about the brilliant orange-looking star at about 8 o'clock.

SY Tau infrared annotated.png
SY Tau annotated.png

It turns out that this star is SY Tau, otherwise known as BD+23 555 or TYC 1800-2105-1. The star has a tiny Gaia parallax of 0.7001 [0.0387] milliarcseconds, which puts it at a distance of some ~4,600 light-years. This is more than ten times as far away as the Pleiades cluster.

Very interestingly, SY Tau's (apparent) V magnitude is around 9.5. Yes, but its infrared magnitude is much, much brighter. Its K magnitude (and I think that refers to mid infrared, although I'm too lazy to google) is ~3.58. Now what absolute K magnitude is a star with whose apparent K brightness at a distance of ~4,600 light-years is ~3.58?

If I used my google calculations correctly, the absolute K magnitude of such a star would be -7, with a K luminosity of some 60,000 solar. Except it would be a lot brighter than that, if we compare it with the Sun, because the Sun's infrared brightness is comparatively modest. The blackbody curve of the Sun peaks at optical wavelengths, but stars like SY Tau peak in the infrared.


The long and short of it is that SY Tau is a brilliantly bright, very distant red giant star, and this APOD has revealed its brilliance to us. And, as a Color Commentator, I might add that it is quite right that it looks orange, too, since it is particularly bright at H (mapped as yellow) and K (mapped as red) wavelengths.

Approved by the Color Commentator!

Ann
This talk made me wonder why we expect stars' hue or any thermal lamp's hue to be either an orange or a blue (and not a lime, an aqua, a purple or a magenta). And the second question is whether such limitation should give way to the whole rainbow spectrum or even to the whole colour wheel when we use artificial colour scheme to represent a really wide electromagnetic band.

1) We are used to orange&blue limitation for the evening sky, but that have nothing to do with thermal lamps. The sky is colouring the low sun's light by Raleigh's scattering which creates monotonic spectra: one orange closer to the sun and one blue further from the sun

2) A thermal lamp at T° radiates a spectrum with a peak, a preferable photon's energy about kT. If an observer drew his white balance from a flat spectrum etalon, they would see Sun as an aqua, a green-cyan hue. But we usually have a T=5000°K thermal lamp etalon for a white balance, e. g. the sun, the moon, an artificial light with light diodes or Hg + luminophores. So we usually see Sun as white and every star or any thermal lamp as Sun-like, or hotter, or cooler. This means we would see a flat spectrum radiation as a magenta hue and any lamp we perceive as through a magenta filter, absorbing some of the green and cyan photons. That's what turns a peak into a slope — an orange or a blue.

3) IMHO it's not wise to keep this limited hues when representing wide band pictures

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Ann » Sun Feb 26, 2023 7:39 am

VictorBorun wrote: Sat Feb 25, 2023 9:01 pm This talk made me wonder why we expect stars' hue or any thermal lamp's hue to be either an orange or a blue (and not a lime, an aqua, a purple or a magenta). And the second question is whether such limitation should give way to the whole rainbow spectrum or even to the whole colour wheel when we use artificial colour scheme to represent a really wide electromagnetic band.

1) We are used to orange&blue limitation for the evening sky, but that have nothing to do with thermal lamps. The sky is colouring the low sun's light by Raleigh's scattering which creates monotonic spectra: one orange closer to the sun and one blue further from the sun

2) A thermal lamp at T° radiates a spectrum with a peak, a preferable photon's energy about kT. If an observer drew his white balance from a flat spectrum etalon, they would see Sun as an aqua, a green-cyan hue. But we usually have a T=5000°K thermal lamp etalon for a white balance, e. g. the sun, the moon, an artificial light with light diodes or Hg + luminophores. So we usually see Sun as white and every star or any thermal lamp as Sun-like, or hotter, or cooler. This means we would see a flat spectrum radiation as a magenta hue and any lamp we perceive as through a magenta filter, absorbing some of the green and cyan photons. That's what turns a peak into a slope — an orange or a blue.

3) IMHO it's not wise to keep this limited hues when representing wide band pictures
Let's start with the question of why stars are not green. The short answer is that stars are not green because the Sun is green, and the Sun doesn't look green to us.

Okay, but surely the Sun isn't green?


Actually, yes, it is. The Sun is as green as a star can get, because its blackbody curve peaks in the green (or blue-green) part of the spectrum.

The Sun is way, way too bright for us to be able to look at it directly without permanently damaging our retinas, but sometimes it is possible to look at it when it is shining through fog:


The Sun actually looks a bit yellowish in the picture where it is seen shining through fog, but we must remember that the Earth's atmosphere is always reddening the color of the Sun. From above the Earth's atmosphere, the Sun would look more definitely white - and even more brilliantly bright, if seen from above planet Earth.

The reason why the Sun doesn't look green to us, even though its blackbody spectrum peaks in the green part of the spectrum, is precisely because the Sun, like all stars, emits its energy as a blackbody spectrum whose peak and slope depend on its temperature. As you can see from the first picture I posted, the Sun emits light of all the colors of the rainbow, plus some ultraviolet and a lot of infrared light. So the GREEN light from the Sun has COMPETITION!

That's the way it is with all stars. They are all blackbody emitters, and any dominant wavelength they emit has competition.

The blackbody curves of hot stars are "taller", and peak at shorter wavelengths than cooler stars:


Hot stars peak in the ultraviolet. Cool stars peak in the infrared. Stars like the Sun peak in the visible part of the spectrum, near the green part of it. But these stars don't look green, because of the competition with other wavelengths. Stars whose spectra rise sharply "up" through the visible part of the spectrum, typically stars whose temperature is near 10,000 K to 12,000 K, look bluish (at least through a telescope) because their blackbody curves rise sharply through the visible spectrum towards the blue and purple end of it. Cool stars whose temperatures are near 3,000 K to 4,000 K produce so little blue light that our eyes don't react to it, but we do pick up the green and yellow light that they produce, as well as their red light. Their "overall light" looks yellow-orange.

But there are objects that are green, because they are narrowband emitters. Comet comas are almost always green, and planetary nebulas are typically green:

Comet 46P Wirtanen The Pleiades The Hyades a Meteor and Interstellar Dust Roger Clark.png
Comet 46P Wirtanen, The Pleiades, The Hyades, a Meteor and Interstellar Dust.
Credit: Roger N, Clark

Comet comas emit "spikes" of light, and since our eyes are most sensitive to blue-green, green and yellow light, we react strongly to the isolated spikes of green light:

Spectrum of Comet Lovejoy David Boyd.png
Spectrum of Comet Lovejoy. Credit: David Boyd.

Comet comas emit strong green light from molecular carbon, C2, at around 520 nm and 470 nm (around 5200 Å and 4700 Å).


Planetary nebulas of emit a very strong spike of light at around 500 nm (5000 Å), which is in the blue-green part of the spectrum:

Spectrum of planetary nebula M78.png

Stars normally look more or less white to the eye, because our color vision is not very sensitive to faint light. We are reasonably good at seeing faint yellow light, which is why stars like Betelgeuse, Antares and Aldebaran as well as planet Mars will pop out to us in the night sky. But they don't truly look red to us. If we think they do, it is because the contrast between these cool stars and the hot stars that often surround them is great.

A few stars do look truly red, or at least deeply reddish orange. These are carbon stars, such as T Lyrae:


Our own Sun is reddened by the Earth's atmosphere, and particularly so at sunrise and sunset, when the light of the Sun has to pass through particularly thick layers of the Earth's atmosphere.

Carbon stars are reddened, or so I think, by "soot" that they have deposited in their own atmosphere. I suggest you google the exact mechanism of carbon stars reddening, but whatever the exact mechanism, these stars are indeed self-reddened, and therefore they look redder than any other (nearby) stars. They have lost all their purple, blue and green light, and they emit very little yellow light. Hence their orange-red color.

But to summarize: No, there are no stars that would look lime, aqua, purple or magenta to our eyes or to the RGB filters that we typically use for "true-color" photography. And the reason for this is that no stars produce strong spikes in their spectra in such a way that their light would look lime, aqua, purple or magenta to our eyes. Or green, for that matter.

Personally I would not like to see mapped colors for stars, so that, for example, hot stars like the ones of Orion's Belt would be colored purple (because they emit most of their light at ultraviolet wavelengths), or stars relatively similar to the Sun being mapped in shades of green.

Ann
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Mon Feb 27, 2023 4:46 pm

Ann wrote: Sun Feb 26, 2023 7:39 am Personally I would not like to see mapped colors for stars, so that, for example, hot stars like the ones of Orion's Belt would be colored purple (because they emit most of their light at ultraviolet wavelengths), or stars relatively similar to the Sun being mapped in shades of green.
Ann
this APOD is colouring warm clouds using full Richard Of York Gave Battle In Vain colour range, is it?

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Ann » Mon Feb 27, 2023 6:36 pm

VictorBorun wrote: Mon Feb 27, 2023 4:46 pm
Ann wrote: Sun Feb 26, 2023 7:39 am Personally I would not like to see mapped colors for stars, so that, for example, hot stars like the ones of Orion's Belt would be colored purple (because they emit most of their light at ultraviolet wavelengths), or stars relatively similar to the Sun being mapped in shades of green.
Ann
this APOD is colouring warm clouds using full Richard Of York Gave Battle In Vain colour range, is it?
I'm not quite sure what you mean here, but... I guess I agree with you, why not? :wink:

Anyway. Coloring stars according to their spectral classes would be too much of a bother, I think. I tried to do it with the brightest stars of Auriga.

Auriga Eckhard Slawik.png
Auriga. Credit: Eckhard Slawik
Auriga Eckhard Slawik colored stars.png
Auriga with stars colored (more or less correctly)
according to their spectral class

These are the nine brightest stars of constellation Auriga (well, one of the stars technically belongs to constellation Taurus), moving clockwise from Capella:

1) Capella. This is a binary star consisting of one star of spectral class K0III and one star of spectral class G1III. Capella is a bit yellower than the Sun. I made it look pale yellow.

2) Epsilon Auriga, a binary star whose components are F0 Iab (or II-III) + ~B5V. Epsilon Auriga is a little bit bluer than the Sun. I chose to make it white.

3) Zeta Auriga, a binary star whose components are K5 II + B7 V. The combined color of this star is more like a star of spectral class K0, so I chose to make it look bright yellow.

4) Eta Auriga is a star of spectral class B3 V. It is the bluest and hottest of the bright stars of Auriga. I made it light blue.

5) Iota Auriga. It is a star of spectral class K3 II, and it is sufficiently reddish that I chose to make it orange.

6) Beta Tauri. This star officially belongs to constellation Taurus. It is a star of spectral class B7III. It is neither as blue nor as hot as Eta Aur, and I chose to make is a slightly paler shade of blue than Eta.

7) Theta Auriga. This is a binary star whose components are spectral classes A0pSi + F2-5V. The A0pSi stars are classified as A-type stars because of some peculiarities in their spectra, but in my opinion, these stars should rightfully be classified as B-type stars. They are both brighter and bluer than any "normal" A-type star. I chose to make Theta Aur light blue, the same shade of blue as Beta Tau (although I could have made it a little paler still).

8) Beta Auriga. This is a binary star consisting of two virtually identical stars of spectral class A1m IV. (No, don't ask me about the "m".) The two components have just evolved off the main sequence and become a little brighter than they were before, so that each of them is about twice as bright as Sirius. In other respects, they are a lot like Sirius. I chose to make them white.

9). Pi Auriga. This is a star of spectral class M3 II. It is the reddest and the coolest of the bright stars of Auriga. I chose to make it orange.

Well, conclusion:

I frankly think it is too much work to color all the stars in astronomical photographs according to their spectral classes 😵, particularly in view of the fact that it is not always clear how every star should be defined spectroscopically. What about Zeta Auriga, a binary whose components are spectral classes K5 II + B7 V, but whose combined color makes it look more like a star of spectral class K0 or K2? 🤔

So let's just take normal RGB pictures of the sky and see what the stars come out looking like, okay?

Ann
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by johnnydeep » Mon Feb 27, 2023 6:56 pm

VictorBorun wrote: Mon Feb 27, 2023 4:46 pm
Ann wrote: Sun Feb 26, 2023 7:39 am Personally I would not like to see mapped colors for stars, so that, for example, hot stars like the ones of Orion's Belt would be colored purple (because they emit most of their light at ultraviolet wavelengths), or stars relatively similar to the Sun being mapped in shades of green.
Ann
this APOD is colouring warm clouds using full Richard Of York Gave Battle In Vain colour range, is it?
Hah - I did a double take there. Never heard of that mnemonic for ROYGBIV!
https://www.phrases.org.uk/meanings/richard-of-york-gave-battle-in-vain.html wrote:The best known Richard of York who did badly in battle (and there was more than one) was the English king Richard III. It is probably him who is being referred to with ROYGBIV. Richard was the last English king to die in battle, in 1485 at the Battle of Bosworth Field. He was the king who Shakespeare gave the line 'A horse, a horse, my kingdom for a horse'.
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"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Ann » Mon Feb 27, 2023 7:21 pm

johnnydeep wrote: Mon Feb 27, 2023 6:56 pm
VictorBorun wrote: Mon Feb 27, 2023 4:46 pm
Ann wrote: Sun Feb 26, 2023 7:39 am Personally I would not like to see mapped colors for stars, so that, for example, hot stars like the ones of Orion's Belt would be colored purple (because they emit most of their light at ultraviolet wavelengths), or stars relatively similar to the Sun being mapped in shades of green.
Ann
this APOD is colouring warm clouds using full Richard Of York Gave Battle In Vain colour range, is it?
Hah - I did a double take there. Never heard of that mnemonic for ROYGBIV!
https://www.phrases.org.uk/meanings/richard-of-york-gave-battle-in-vain.html wrote:The best known Richard of York who did badly in battle (and there was more than one) was the English king Richard III. It is probably him who is being referred to with ROYGBIV. Richard was the last English king to die in battle, in 1485 at the Battle of Bosworth Field. He was the king who Shakespeare gave the line 'A horse, a horse, my kingdom for a horse'.
Richard Of York Gave Battle In Vain? ROYGBIV? I have never come across that mnemonic at all!

Let's see: Richard = red.
Of = orange.
York = yellow.
Gave = green.
Battle = blue.
In = I suppose that's meant to be indigo, but today "I" seems to mean infrared!
Vain = violet.

Thanks for teaching me!

Ann
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by johnnydeep » Mon Feb 27, 2023 7:35 pm

Ann wrote: Mon Feb 27, 2023 7:21 pm
johnnydeep wrote: Mon Feb 27, 2023 6:56 pm
VictorBorun wrote: Mon Feb 27, 2023 4:46 pm

this APOD is colouring warm clouds using full Richard Of York Gave Battle In Vain colour range, is it?
Hah - I did a double take there. Never heard of that mnemonic for ROYGBIV!
https://www.phrases.org.uk/meanings/richard-of-york-gave-battle-in-vain.html wrote:The best known Richard of York who did badly in battle (and there was more than one) was the English king Richard III. It is probably him who is being referred to with ROYGBIV. Richard was the last English king to die in battle, in 1485 at the Battle of Bosworth Field. He was the king who Shakespeare gave the line 'A horse, a horse, my kingdom for a horse'.
Richard Of York Gave Battle In Vain? ROYGBIV? I have never come across that mnemonic at all!

Let's see: Richard = red.
Of = orange.
York = yellow.
Gave = green.
Battle = blue.
In = I suppose that's meant to be indigo, but today "I" seems to mean infrared!
Vain = violet.

Thanks for teaching me!

Ann
I find "Roy G. Biv" easy enough to remember as a fictional man's name (likely an astronomer!) easy enough to remember without the need of a longer mnemonic!
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Tue Feb 28, 2023 10:03 am

offtopic
Ann wrote: Mon Feb 27, 2023 7:21 pm Battle = blue.
In = I suppose that's meant to be indigo, but today "I" seems to mean infrared!
I think Newton's blew meant cyan and indigo Maxwell's blue.
In times astronomers discovered light beyond the visible range they were still using Newtonian red and violet terms and coined neologisms ultraviolet and infrared.
But later, in the XIX century, when James Clerk Maxwell started working on trichromatic systems, he used Blue meaning indigo and use it instead of violet for the short waves filter. Maxwell's RGB became so dominant, that in the XX century 3 quark's kinds of charge were called red/green/blue rather than red/green/violet and Doppler or gravitational shifts were called red/blue instead of red/violet.

Of course Maxwell did not have LED lamps. He was filtering light from thermal lamps, and not very hot at that.

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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Fred the Cat » Tue Feb 28, 2023 5:09 pm

Ann wrote: Mon Feb 27, 2023 6:36 pm
VictorBorun wrote: Mon Feb 27, 2023 4:46 pm
Ann wrote: Sun Feb 26, 2023 7:39 am Personally I would not like to see mapped colors for stars, so that, for example, hot stars like the ones of Orion's Belt would be colored purple (because they emit most of their light at ultraviolet wavelengths), or stars relatively similar to the Sun being mapped in shades of green.
Ann
this APOD is colouring warm clouds using full Richard Of York Gave Battle In Vain colour range, is it?
I'm not quite sure what you mean here, but... I guess I agree with you, why not? :wink:

Anyway. Coloring stars according to their spectral classes would be too much of a bother, I think. I tried to do it with the brightest stars of Auriga.

Auriga Eckhard Slawik.png
Auriga. Credit: Eckhard Slawik
Auriga Eckhard Slawik colored stars.png
Auriga with stars colored (more or less correctly)
according to their spectral class

These are the nine brightest stars of constellation Auriga (well, one of the stars technically belongs to constellation Taurus), moving clockwise from Capella:

1) Capella. This is a binary star consisting of one star of spectral class K0III and one star of spectral class G1III. Capella is a bit yellower than the Sun. I made it look pale yellow.

2) Epsilon Auriga, a binary star whose components are F0 Iab (or II-III) + ~B5V. Epsilon Auriga is a little bit bluer than the Sun. I chose to make it white.

3) Zeta Auriga, a binary star whose components are K5 II + B7 V. The combined color of this star is more like a star of spectral class K0, so I chose to make it look bright yellow.

4) Eta Auriga is a star of spectral class B3 V. It is the bluest and hottest of the bright stars of Auriga. I made it light blue.

5) Iota Auriga. It is a star of spectral class K3 II, and it is sufficiently reddish that I chose to make it orange.

6) Beta Tauri. This star officially belongs to constellation Taurus. It is a star of spectral class B7III. It is neither as blue nor as hot as Eta Aur, and I chose to make is a slightly paler shade of blue than Eta.

7) Theta Auriga. This is a binary star whose components are spectral classes A0pSi + F2-5V. The A0pSi stars are classified as A-type stars because of some peculiarities in their spectra, but in my opinion, these stars should rightfully be classified as B-type stars. They are both brighter and bluer than any "normal" A-type star. I chose to make Theta Aur light blue, the same shade of blue as Beta Tau (although I could have made it a little paler still).

8) Beta Auriga. This is a binary star consisting of two virtually identical stars of spectral class A1m IV. (No, don't ask me about the "m".) The two components have just evolved off the main sequence and become a little brighter than they were before, so that each of them is about twice as bright as Sirius. In other respects, they are a lot like Sirius. I chose to make them white.

9). Pi Auriga. This is a star of spectral class M3 II. It is the reddest and the coolest of the bright stars of Auriga. I chose to make it orange.

Well, conclusion:

I frankly think it is too much work to color all the stars in astronomical photographs according to their spectral classes 😵, particularly in view of the fact that it is not always clear how every star should be defined spectroscopically. What about Zeta Auriga, a binary whose components are spectral classes K5 II + B7 V, but whose combined color makes it look more like a star of spectral class K0 or K2? 🤔

So let's just take normal RGB pictures of the sky and see what the stars come out looking like, okay?

Ann
The history of the color wheel itself colorful but does it matter how you spin it?

Many tied iterations after Newton. These days we rely on Ann for the basics. :thumb_up: And some guy named Doppler to shift the spin. :yes:
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Ann » Tue Feb 28, 2023 7:59 pm

Fred the Cat wrote: Tue Feb 28, 2023 5:09 pm
The history of the color wheel itself colorful but does it matter how you spin it?

Many tied iterations after Newton. These days we rely on Ann for the basics. :thumb_up: And some guy named Doppler to shift the spin. :yes:
Thanks a billion for this link, Fred! :D

I particularly love this illustration of star colors:

Digital color codes of stars Jan Vincent Harre Rene Heller.png
"True" colors of stars of different spectral classes.
Illustration: Jan-Vincent Harre, René Heller


I love the bluish color of the F-type star! That's exactly right!


Andrey Feldman wrote in Advanced Science News about the study by Harre and Heller:

As expected, some color representations of stars appeared to be a bit inaccurate — for example, red dwarf stars actually look orange to the human eye and old white dwarfs appear pale orange. On the other hand, the results of the analysis validated an absence of yellow, green, cyan, or purple stellar types that sometimes pop up in scientific books and articles.

The combination of the nature of the stellar spectra and the nature of the human color perception, as represented by the color matching functions, does not allow these colors,” Harre explained. “For non-humans, stars of these colors could exist, depending on their color perception.”
Exactly!!!
Jan-Vincent Harre and René Heller wrote:

Our color codes carry information about the chromaticity (hue and saturation) of stars but not about their brightness
That's interesting, because I think the high surface brightness of hot blue-white stars play a part in explaining why their colors appear washed-out to our eyes. I mean, have you ever seen the brilliantly blue hues of the O- and B-type stars of Orion? No?


Hot blue stars have a much higher surface brightness than stars like the Sun (of spectral classes G to K), and they have a much, much higher surface brightness than stars of spectral class M (like Betelgeuse, Antares, Proxima Centauri etc.). Anything that is "too bright" for our eyes tends to look brilliantly white rather than colored. The fact that hot blue stars have a very high surface brightness makes me slightly critical of the picture of star colors that I just posted, because it makes the blue stars look kind of "dark". These stars sure aren't dark in reality!


Also, I think that the human eye may be slightly less sensitive to blue light than to red or green. Look at this picture of the color receptors in our retinas:



As you can see, there are a lot more red and green-sensitive cones in our retinas than blue-sensitive ones. Moreover, there seems to be no blue receptors at all in the macula, the most sensitive part of our retinas. I think, therefore, that our retinas may be overwhelmed by the high surface brightness of the hot blue stars, while at the same time not being fully able to really pick out the blue color of the blue stars, due to the small numbers of blue-sensitive cones in our retinas.

Harre and Heller said somewhere in their paper that the human eye is more sensitive to blue light than to yellow light when we look at faint light sources such as stars in the night sky.

Well, I disagree. I think it is the color-blind rods of our eyes that are sensitive to blue light. This in turn means that when we look at hot blue stars, the color-blind rods of our eyes will see the blue light of blue stars as white, and their strong "white-light response" will overwhelm the relatively few blue-sensitive cones in our retinas. So our rods tell us that blue stars are white, but our cones aren't able to pick out much of the stars' blue hue.

This at least partly explains why blue stars typically look so whitish to our eyes.

Oh, and as to why the really quite red M-type stars never look red but orange (I'd say yellow-orange) to our eyes is almost certainly because our green-sensitive cones are particularly sensitive and extremely able to pick up "green photons", much more so than our red-sensitive cones are able to pick up red photons. Why else do you think people use green lasers to try to blind other people?


So our eyes react disproportionately strongly to the low levels of green light that red stars of spectral classes M and late K produce. This dilutes our perceived color of their hue, making us see these stars as yellow-orange rather than red.

Ann
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by Chris Peterson » Tue Feb 28, 2023 8:37 pm

Ann wrote: Tue Feb 28, 2023 7:59 pm
Jan-Vincent Harre and René Heller wrote:
Our color codes carry information about the chromaticity (hue and saturation) of stars but not about their brightness
That's interesting, because I think the high surface brightness of hot blue-white stars play a part in explaining why their colors appear washed-out to our eyes. I mean, have you ever seen the brilliantly blue hues of the O- and B-type stars of Orion? No?
The problem, of course, is that color isn't a physical metric, but a biological one. Color is not simply defined by hue and saturation, but also by intensity. Two completely different colors can have the same hue and saturation and differ only because their intensity differs. So all of these tools for estimating blackbody colors need to be handled carefully. There is useful information there, but you aren't getting accurate colors except in very limited cases.

(The same is true for narrowband emissions. OIII presents as an infinite number of colors depending on its brightness.)
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Tue Feb 28, 2023 9:51 pm

Ann wrote: Mon Feb 27, 2023 6:36 pm Coloring stars according to their spectral classes would be too much of a bother, I think. I tried to do it with the brightest stars of Auriga.
Coloring stars..png
Coloring stars -text.png
...
Here is my attemt to edit the whole picture according to Ann's suggestions on several stars
Click to view full size image 1 or image 2
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Re: APOD: Seven Dusty Sisters in Infrared (2023 Feb 19)

Post by VictorBorun » Tue Feb 28, 2023 11:24 pm

Chris Peterson wrote: Tue Feb 28, 2023 8:37 pm The problem, of course, is that color isn't a physical metric, but a biological one. Color is not simply defined by hue and saturation, but also by intensity. Two completely different colors can have the same hue and saturation and differ only because their intensity differs. So all of these tools for estimating blackbody colors need to be handled carefully. There is useful information there, but you aren't getting accurate colors except in very limited cases.

(The same is true for narrowband emissions. OIII presents as an infinite number of colors depending on its brightness.)
offtopic on colours

A colour is a meet of a socially accepted interpretation with the physics of spectra and trichromatism (like a phoneme is a meet of a linguistically important sound system with the physics of articulation and hearing).

A colour is used to recognise parts of the same object we see in a scene.

There are two ways to describe the system of colours.

A short description goes like this.

a colour is
either one of the 6 primary colours of a circular sequence red—yellow—green—cyan—violet—pink—red
or one of the many colours in 6 in-between regions of oranges—limes—aquas—blues—purples—magentas

A long description goes like this.

a colour is a thing from Hue—Whiteness—Blackness system or from Thick-Tinting — Thin-Tinting — Thinness system. You can easily see how people keep switching between those two systems when
• at one scene they talk about dirty, dusty or dark version of reddish orange or orangish yellow
• at another scene they talk about deep or pale version of blonde, chestnut, redhead, chocolate, or coffee

What the two systems do agree on is the 6 primary colours from the short description.
What the two systems disagree on is what to recognise as parts of the same object in the scene and what to proclaim differently coloured things.

All said is about a colour of a thing and not about a colour of a shining lamp. Humans are at loss when they see coloured little lamps in a scene illuminated from other source. So the thing we recognise well is a spectral function of the reflection factor, and it's not as detailed as a spectral function of an audio event we can hear; our vision uses just 3-band analyser with the help of 3 types of cone cells in retina: red, or longwaves / green, or meanwaves / violet, or shortwaves. An observer at first glance is panning the new scene seeing nothing but giving each cone cell 100 milliseconds to quick-adapt to min and max levels of photon flow in different parts of this scene. At second glance the observer got reports on each pixel in the scene as having a reflection factor of 0 to 1 (as in black to as in white) for each of the 3 bands, L M S.

Mathematically speaking

Hue—Whiteness—Blackness system interprets a pixel as a mixture of white (i.e. a thing with reflecting factors of L=1, M=1 and S=1), black (i.e. a thing with LMS of 0 0 0), a primary colour like red (LMS of 1 0 0) and the next primary colour, in this case yellow (LMS 1 1 0).
A Whiteness > 0 feels the same as hiding a percent of the surface area of the thing behind white powder.
A Blackness > 0 feels the same as hiding a percent of the surface area of the thing in a black shadow.
An observer can judge the Whiteness and the Blackness because they are mathematically certain values:
Whiteness = Min(L, M, S)
Blackness = 1 — Max(L, M, S)
A Hue interprets the pure colour (i.e. a thing of zero Whiteness and zero Blackness) as a mixture of a pair of a primary colour and the next primary colour. So the orange hue is LMS of 1 .5 0 and the red-orange hue is LMS of 1 .25 0
You can't use this system to recognize a pile of oranges in a snapshot: there would be places between the spheres where photons would repeatedly bounce between the fruits passing one filter after another. Were you clinging to Hue—Whiteness—Blackness system, you would think the fruits turning redder at such places as if they are rotten there.

Thick-Tinting — Thin-Tinting — Thinness system interprets a pixel as a thick or thin application of an ink substance, and the substance has a way of more passing either red or green or violet and less passing one more of the three bands. A Thick-Tinting is either red or green or violet and stays detectable at the thick margin of the Thinness; Thin-Tinting stays detectable at the thin margin of the Thinness and can be:
anti-red (=cyan) only if Thick-Tinting is Green or Violet
anti-green (=pink) only if Thick-Tinting is Red or Violet
anti-violet (=yellow) only if Thick-Tinting is Red or Green.
An observer can get those because they are mathematically certain things. E.g. in the region of oranges
Red Thick-Tinting = Log[.5,Log[.5,L]]-Log[.5,Log[.5,M]]
Yellow Thin-Tinting = Log[.5,Log[.5,M]]-Log[.5,Log[.5,S]]
Thinness = Log[.5,Log[.5,M]]
Here are some examples of Red Thick-Tinting — Yellow Thin-Tinting pairs, or colours in the region of oranges:
оранжевый сектор..png
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