APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4902
Joined: Fri Dec 04, 2009 3:27 am

APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by APOD Robot » Thu Jan 05, 2023 5:07 am

Image Messier 45: The Daughters of Atlas and Pleione

Explanation: Hurtling through a cosmic dust cloud a mere 400 light-years away, the lovely Pleiades or Seven Sisters open star cluster is well-known for its striking blue reflection nebulae. It lies in the night sky toward the constellation Taurus and the Orion Arm of our Milky Way galaxy. The sister stars are not related to the dusty cloud though. They just happen to be passing through the same region of space. Known since antiquity as a compact grouping of stars, Galileo first sketched the star cluster viewed through his telescope with stars too faint to be seen by eye. Charles Messier recorded the position of the cluster as the 45th entry in his famous catalog of things which are not comets. In Greek myth, the Pleiades were seven daughters of the astronomical titan Atlas and sea-nymph Pleione. Their parents names are included in the cluster's nine brightest stars. This well-processed, color-calibrated telescopic image features pin-point stars and detailed filaments of interstellar dust captured in over 9 hours of exposure. It spans more than 20 light-years across the Pleiades star cluster.

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

User avatar
Ann
4725 Å
Posts: 12475
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Ann » Thu Jan 05, 2023 7:49 am

What can I say? We are used to seeing emission nebulas without stars (or with suppressed stars):


But today is the first time that I have seen the Pleiades with suppressed stars!

The Pleiades Antoine and Dalia Grelin.png
The Pleiades with stars. Credit: Antoine and Dalia Grelin.

The Pleiades look so different with suppressed stars that I hardly recognized them! But it is a beautiful picture, to be sure. :ssmile:

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

MelvzLuster
Asternaut
Posts: 3
Joined: Tue Oct 18, 2022 2:16 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by MelvzLuster » Thu Jan 05, 2023 8:11 am

Great & wonderful!

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1526
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by JohnD » Thu Jan 05, 2023 9:22 am

Ann,
You give us two pics of the Pleiades, one with "suppressed stars", but there seems to be no congruence in the nebulae either. They could be pics of two different objects. What am I missing?

John

User avatar
Ann
4725 Å
Posts: 12475
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Ann » Thu Jan 05, 2023 10:43 am

JohnD wrote: Thu Jan 05, 2023 9:22 am Ann,
You give us two pics of the Pleiades, one with "suppressed stars", but there seems to be no congruence in the nebulae either. They could be pics of two different objects. What am I missing?

John
I think this is what we are seeing:

APOD 5 January 2023 annotated.png

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

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1526
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by JohnD » Thu Jan 05, 2023 12:34 pm

AH! So rotate the left pic
You do not have the required permissions to view the files attached to this post.

User avatar
orin stepanek
Plutopian
Posts: 7887
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by orin stepanek » Thu Jan 05, 2023 1:54 pm

APOD20180704.jpg
Atlas & Pleione watch as the daughters dance! :D
You do not have the required permissions to view the files attached to this post.
Orin

Smile today; tomorrow's another day!

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Thu Jan 05, 2023 10:09 pm

JohnD wrote: Thu Jan 05, 2023 12:34 pm AH! So rotate the left pic
I temper my seemingly daily annoyance that all astronomical photographs aren't always rotated the same way, with the thought that the mental gymnastics needed in order to see that two such differently oriented (or even differently scaled, or differently filtered!) photos really are of the same stuff, is serving to steer my mind away from premature senescence. :ssmile:
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by VictorBorun » Sat Jan 07, 2023 3:22 am

what are the colours?
I expected the sky shine palette:
orange and yellow for threads angularly close to a background Pleiades
cyan and blue for threads angularly distant from a background Pleiades

I think I see some of it in images with bright stars but no yellow core in images suppressing stars

User avatar
Ann
4725 Å
Posts: 12475
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Ann » Sat Jan 07, 2023 7:18 am

VictorBorun wrote: Sat Jan 07, 2023 3:22 am what are the colours?
I expected the sky shine palette:
orange and yellow for threads angularly close to a background Pleiades
cyan and blue for threads angularly distant from a background Pleiades

I think I see some of it in images with bright stars but no yellow core in images suppressing stars
All the bright stars of the Pleiades are blue (or, as Chris would say, blue-white). They all belong to spectral class B, where stars are always intrinsically blue, and they all have solid negative B-V indices (which is a defining characteristic of blue stars).

However:

All the bright stars of the Pleiades are swathed in nebulosity. The stars of the Pleiades are not hot enough to ionize a red emission nebula, so all the nebulas surrounding the stars are blue reflection nebulas.

The nebulas of the Pleiades are bluer than the stars themselves. The nebulas are also much, much fainter than the stars themselves.

We see a similar phenomenon in our own skies:


Our clear sky is bluer than the Sun. Indeed, while our skies are blue, the Sun is not blue at all. (Or rather, the Sun's spectral class of G2V and its B-V index of +0.656 defines the Sun as a non-blue star.) At the same time, our blue skies are much, much fainter than the brilliant Sun.

So, back to the Pleiades:


When photographers bring out the blue reflection nebulosity of the Pleiades, they also overexpose the stars. Since the stars themselves are not as blue as the surrounding nebulas, the overexposed stars look white. In fact, they may even look yellow.

For all of that, and unlike our own Sun, the bright stars of the Pleiades are indeed blue.


This is one of my own favorite portraits of the Pleiades. The whole image looks like a winter wonderland, where the stars are suspended like glittering snowflakes against a magnificent whirling blue background.


Here is a widefield image, where the Pleiades are seen rising behind autumn leaves trees during Halloween:


I recommend this gallery of Pleiades pictures, where you can really see that the Pleiades look different when they are photographed by different photographers using different equipment and filters and processing their images differently.

Ann
Color Commentator

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Sat Jan 07, 2023 3:25 pm

Ann wrote: Sat Jan 07, 2023 7:18 am
VictorBorun wrote: Sat Jan 07, 2023 3:22 am what are the colours?
...
...
I recommend this gallery of Pleiades pictures, where you can really see that the Pleiades look different when they are photographed by different photographers using different equipment and filters and processing their images differently.

Ann
Wow - that gallery sure does present a pleasing panoply of Pleides portraits!
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by VictorBorun » Sat Jan 07, 2023 11:17 pm

Ann wrote: Sat Jan 07, 2023 7:18 am Our clear sky is bluer than the Sun. Indeed, while our skies are blue, the Sun is not blue at all.
but close to Sun sky shine must be pale yellowish and, closer still, even amber.
Because the light is not exactly filtered by the scattering; it is rather just separated. Shorter wavelengths are scattered by wider angles and can be seen further away from the star, are not they?

Maybe kids choose yellow to draw the sun and its rays because they recognize the pattern: the clear sky is blue everywhere but close to the sun

User avatar
Ann
4725 Å
Posts: 12475
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Ann » Sun Jan 08, 2023 5:49 am

VictorBorun wrote: Sat Jan 07, 2023 11:17 pm
Ann wrote: Sat Jan 07, 2023 7:18 am Our clear sky is bluer than the Sun. Indeed, while our skies are blue, the Sun is not blue at all.
but close to Sun sky shine must be pale yellowish and, closer still, even amber.
Because the light is not exactly filtered by the scattering; it is rather just separated. Shorter wavelengths are scattered by wider angles and can be seen further away from the star, are not they?
No, that's not how it works.

The true color of the Sun is white. And there is a very simple reason for it: It is white to us.

Sunlight has always been the brightest light that humans can see. Our eyes have evolved so that we see sunlight as white, which is the brightest "color".

But what happens on a clear day is that some of the blue light from the Sun is scattered away by the atmosphere. This effect causes the sky to be blue, but it also makes direct sunlight look a little yellower.

Let's look at two pictures:


Note that the parts of the ground that are in shadow look quite deeply blue, because they are illuminated by the blue sky only, not by direct sunlight. Note that the color of the shadows are a deeper blue than the color of the sky, particularly when compared to the color of the sky near the horizon. Also note that the shadows are much darker than the sunlit snow, because (as we all know) direct sunlight is so much brighter than the blue light of the shadows.


Now let's compare daylight on a sunny day with daylight on a cloudy day:


On a sunny day, the face and hair of the girl take on a warmer, slightly golden-yellow hue, even the parts that are actually in shadow. On a cloudy day, her face and hair become "paler" and more "silvery". Her face also becomes somewhat flat.

Note, too, that on the "cloudy photo", the green background becomes more colorful and vivid. The leafy background is in shadow in the "sunny picture", and it looks muted and dark. Also note that the girl's dress looks more intensely blue in the "cloudy picture", where the blue hue also looks somewhat "flat". On a cloudy day, the blue dress contrasts rather sharply with the girl's pale skin.

As you can see from the two pictures of the girl, sunlight itself is yellowish. But note that the illumination in the "cloudy picture" is daylight. When clouds make daylight look all uniform, the color of this daylight is, well, neutral. It is not yellowish. And daylight is, of course, sunlight.

It is a mistake to believe that the sky is yellow or amber near the disk of the Sun, when the Sun is high in the sky. That part of the sky is brilliantly bright, of course, and it is dangerous to look at it.

Ann
Color Commentator

George

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by George » Mon Jan 09, 2023 4:40 pm

THREE CHEERS for johnnydeep’s comment about astrophoto orientation! Good grief!

QUESTION: The dust responsible for M45’s blue nebulosity is either BEYOND M45’s stars, or—like the gas responsible for Earth’s blue skies—it is BETWEEN M45’s stars and Earth. IF the latter, then why isn’t M45’s nebulosity called “scattering” nebulosity, rather than “reflection” nebulosity?

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Chris Peterson » Mon Jan 09, 2023 5:27 pm

George wrote: Mon Jan 09, 2023 4:40 pm THREE CHEERS for johnnydeep’s comment about astrophoto orientation! Good grief!

QUESTION: The dust responsible for M45’s blue nebulosity is either BEYOND M45’s stars, or—like the gas responsible for Earth’s blue skies—it is BETWEEN M45’s stars and Earth. IF the latter, then why isn’t M45’s nebulosity called “scattering” nebulosity, rather than “reflection” nebulosity?
The dust is around the stars, and it is reflecting the light of those stars. Reflection, not scattering, is the dominant mechanism.
Chris

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

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Mon Jan 09, 2023 7:33 pm

Chris Peterson wrote: Mon Jan 09, 2023 5:27 pm
George wrote: Mon Jan 09, 2023 4:40 pm THREE CHEERS for johnnydeep’s comment about astrophoto orientation! Good grief!

QUESTION: The dust responsible for M45’s blue nebulosity is either BEYOND M45’s stars, or—like the gas responsible for Earth’s blue skies—it is BETWEEN M45’s stars and Earth. IF the latter, then why isn’t M45’s nebulosity called “scattering” nebulosity, rather than “reflection” nebulosity?
The dust is around the stars, and it is reflecting the light of those stars. Reflection, not scattering, is the dominant mechanism.
Hmm. So, the stars are actually embedded within the dust, correct? Light from the stars hitting the dust between them and us should still be scattered though, right? Or is it just that since there is much more dust behind and in indeed in all directions NOT on a straight line between us and them, the reflected light overwhelms any scattered light?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Chris Peterson » Mon Jan 09, 2023 7:36 pm

johnnydeep wrote: Mon Jan 09, 2023 7:33 pm
Chris Peterson wrote: Mon Jan 09, 2023 5:27 pm
George wrote: Mon Jan 09, 2023 4:40 pm THREE CHEERS for johnnydeep’s comment about astrophoto orientation! Good grief!

QUESTION: The dust responsible for M45’s blue nebulosity is either BEYOND M45’s stars, or—like the gas responsible for Earth’s blue skies—it is BETWEEN M45’s stars and Earth. IF the latter, then why isn’t M45’s nebulosity called “scattering” nebulosity, rather than “reflection” nebulosity?
The dust is around the stars, and it is reflecting the light of those stars. Reflection, not scattering, is the dominant mechanism.
Hmm. So, the stars are actually embedded within the dust, correct? Light from the stars hitting the dust between them and us should still be scattered though, right? Or is it just that since there is much more dust behind and in indeed in all directions NOT on a straight line between us and them, the reflected light overwhelms any scattered light?
My understanding is that the nebular component of the Pleiades is a dust cloud unrelated to the star cluster and which that cluster just happens to be passing through at the moment. I doubt anybody has much idea about the true shape of the dust cloud and exact position of the stars within it. But the close match between the star colors and the nebula strongly argues that most of the light we're seeing is simply reflected.
Chris

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

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Mon Jan 09, 2023 8:25 pm

Chris Peterson wrote: Mon Jan 09, 2023 7:36 pm
johnnydeep wrote: Mon Jan 09, 2023 7:33 pm
Chris Peterson wrote: Mon Jan 09, 2023 5:27 pm

The dust is around the stars, and it is reflecting the light of those stars. Reflection, not scattering, is the dominant mechanism.
Hmm. So, the stars are actually embedded within the dust, correct? Light from the stars hitting the dust between them and us should still be scattered though, right? Or is it just that since there is much more dust behind and in indeed in all directions NOT on a straight line between us and them, the reflected light overwhelms any scattered light?
My understanding is that the nebular component of the Pleiades is a dust cloud unrelated to the star cluster and which that cluster just happens to be passing through at the moment. I doubt anybody has much idea about the true shape of the dust cloud and exact position of the stars within it. But the close match between the star colors and the nebula strongly argues that most of the light we're seeing is simply reflected.
Ok, but if the stars were dead center in the middle of a spherical dust cloud (say), would what I said about the predominance of reflection over scattering of the star light be roughly correct?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Chris Peterson » Mon Jan 09, 2023 8:33 pm

johnnydeep wrote: Mon Jan 09, 2023 8:25 pm
Chris Peterson wrote: Mon Jan 09, 2023 7:36 pm
johnnydeep wrote: Mon Jan 09, 2023 7:33 pm

Hmm. So, the stars are actually embedded within the dust, correct? Light from the stars hitting the dust between them and us should still be scattered though, right? Or is it just that since there is much more dust behind and in indeed in all directions NOT on a straight line between us and them, the reflected light overwhelms any scattered light?
My understanding is that the nebular component of the Pleiades is a dust cloud unrelated to the star cluster and which that cluster just happens to be passing through at the moment. I doubt anybody has much idea about the true shape of the dust cloud and exact position of the stars within it. But the close match between the star colors and the nebula strongly argues that most of the light we're seeing is simply reflected.
Ok, but if the stars were dead center in the middle of a spherical dust cloud (say), would what I said about the predominance of reflection over scattering of the star light be roughly correct?
I think then you'd need to know something about the nature of the dust- composition and particle size- in order to decide how reflection and scatter balance out.
Chris

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

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Mon Jan 09, 2023 8:48 pm

Chris Peterson wrote: Mon Jan 09, 2023 8:33 pm
johnnydeep wrote: Mon Jan 09, 2023 8:25 pm
Chris Peterson wrote: Mon Jan 09, 2023 7:36 pm
My understanding is that the nebular component of the Pleiades is a dust cloud unrelated to the star cluster and which that cluster just happens to be passing through at the moment. I doubt anybody has much idea about the true shape of the dust cloud and exact position of the stars within it. But the close match between the star colors and the nebula strongly argues that most of the light we're seeing is simply reflected.
Ok, but if the stars were dead center in the middle of a spherical dust cloud (say), would what I said about the predominance of reflection over scattering of the star light be roughly correct?
I think then you'd need to know something about the nature of the dust- composition and particle size- in order to decide how reflection and scatter balance out.
Ok. So, as usual, reality is more complicated than I have a tendency to first assume. :ssmile:
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by VictorBorun » Sat Jan 14, 2023 12:48 pm

Chris Peterson wrote: Mon Jan 09, 2023 8:33 pm
johnnydeep wrote: Mon Jan 09, 2023 8:25 pm
Chris Peterson wrote: Mon Jan 09, 2023 7:36 pm
My understanding is that the nebular component of the Pleiades is a dust cloud unrelated to the star cluster and which that cluster just happens to be passing through at the moment. I doubt anybody has much idea about the true shape of the dust cloud and exact position of the stars within it. But the close match between the star colors and the nebula strongly argues that most of the light we're seeing is simply reflected.
Ok, but if the stars were dead center in the middle of a spherical dust cloud (say), would what I said about the predominance of reflection over scattering of the star light be roughly correct?
I think then you'd need to know something about the nature of the dust- composition and particle size- in order to decide how reflection and scatter balance out.
I wonder what do *reflect* and *scatter* mean here.
Is *reflect* what the dust particle far behind the star does; like make a photon go right back but a small angle we get in the picture as the angular distance between the star and a point in the cloud?
Is *scatter* what the dust particle in front of the faraway star does; like make a photon go astray by the same small angle?
But what do we call another scenario, like make a photon turn by 90° — this is what a dust particle would do were it at the same distance as the star?
A scatterer.png
You do not have the required permissions to view the files attached to this post.

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Chris Peterson » Sat Jan 14, 2023 2:49 pm

VictorBorun wrote: Sat Jan 14, 2023 12:48 pm
Chris Peterson wrote: Mon Jan 09, 2023 8:33 pm
johnnydeep wrote: Mon Jan 09, 2023 8:25 pm

Ok, but if the stars were dead center in the middle of a spherical dust cloud (say), would what I said about the predominance of reflection over scattering of the star light be roughly correct?
I think then you'd need to know something about the nature of the dust- composition and particle size- in order to decide how reflection and scatter balance out.
I wonder what do *reflect* and *scatter* mean here.
Is *reflect* what the dust particle far behind the star does; like make a photon go right back but a small angle we get in the picture as the angular distance between the star and a point in the cloud?
Is *scatter* what the dust particle in front of the faraway star does; like make a photon go astray by the same small angle?
But what do we call another scenario, like make a photon turn by 90° — this is what a dust particle would do were it at the same distance as the star?
A scatterer.png
They are different processes. In reflection, a photon bounces off a particle. In scattering, a photon is absorbed by a particle and a new one is emitted. Reflection is best understood by viewing photons as particles, scattering is best understood by considering the wave nature of light. The dynamics of scattering is highly determined by the size of the particles with respect to the wavelength of the light.
Chris

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

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Sat Jan 14, 2023 3:04 pm

Chris Peterson wrote: Sat Jan 14, 2023 2:49 pm
VictorBorun wrote: Sat Jan 14, 2023 12:48 pm
Chris Peterson wrote: Mon Jan 09, 2023 8:33 pm
I think then you'd need to know something about the nature of the dust- composition and particle size- in order to decide how reflection and scatter balance out.
I wonder what do *reflect* and *scatter* mean here.
Is *reflect* what the dust particle far behind the star does; like make a photon go right back but a small angle we get in the picture as the angular distance between the star and a point in the cloud?
Is *scatter* what the dust particle in front of the faraway star does; like make a photon go astray by the same small angle?
But what do we call another scenario, like make a photon turn by 90° — this is what a dust particle would do were it at the same distance as the star?
A scatterer.png
They are different processes. In reflection, a photon bounces off a particle. In scattering, a photon is absorbed by a particle and a new one is emitted. Reflection is best understood by viewing photons as particles, scattering is best understood by considering the wave nature of light. The dynamics of scattering is highly determined by the size of the particles with respect to the wavelength of the light.
In general, are all types and sizes dust particles - no matter where they are located with respect to a star - capable of both reflecting and scattering photons of the appropriate wavelength?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by Chris Peterson » Sat Jan 14, 2023 3:22 pm

johnnydeep wrote: Sat Jan 14, 2023 3:04 pm
Chris Peterson wrote: Sat Jan 14, 2023 2:49 pm
VictorBorun wrote: Sat Jan 14, 2023 12:48 pm

I wonder what do *reflect* and *scatter* mean here.
Is *reflect* what the dust particle far behind the star does; like make a photon go right back but a small angle we get in the picture as the angular distance between the star and a point in the cloud?
Is *scatter* what the dust particle in front of the faraway star does; like make a photon go astray by the same small angle?
But what do we call another scenario, like make a photon turn by 90° — this is what a dust particle would do were it at the same distance as the star?
A scatterer.png
They are different processes. In reflection, a photon bounces off a particle. In scattering, a photon is absorbed by a particle and a new one is emitted. Reflection is best understood by viewing photons as particles, scattering is best understood by considering the wave nature of light. The dynamics of scattering is highly determined by the size of the particles with respect to the wavelength of the light.
In general, are all types and sizes dust particles - no matter where they are located with respect to a star - capable of both reflecting and scattering photons of the appropriate wavelength?
Any particle can scatter light. Any particle can reflect light unless it is a perfect blackbody... which doesn't exist in nature. So yes.
Chris

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

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Messier 45: The Daughters of Atlas... (2023 Jan 05)

Post by johnnydeep » Sat Jan 14, 2023 3:33 pm

Chris Peterson wrote: Sat Jan 14, 2023 3:22 pm
johnnydeep wrote: Sat Jan 14, 2023 3:04 pm
Chris Peterson wrote: Sat Jan 14, 2023 2:49 pm

They are different processes. In reflection, a photon bounces off a particle. In scattering, a photon is absorbed by a particle and a new one is emitted. Reflection is best understood by viewing photons as particles, scattering is best understood by considering the wave nature of light. The dynamics of scattering is highly determined by the size of the particles with respect to the wavelength of the light.
In general, are all types and sizes dust particles - no matter where they are located with respect to a star - capable of both reflecting and scattering photons of the appropriate wavelength?
Any particle can scatter light. Any particle can reflect light unless it is a perfect blackbody... which doesn't exist in nature. So yes.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."