APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

Post a reply


This question is a means of preventing automated form submissions by spambots.
Smilies
:D :) :ssmile: :( :o :shock: :? 8-) :lol2: :x :P :oops: :cry: :evil: :roll: :wink: :!: :?: :idea: :arrow: :| :mrgreen:
View more smilies
BBCode is ON
[img] is ON
[url] is ON
Smilies are ON
Topic review
   

Expand view Topic review: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Thu Mar 30, 2017 2:08 pm

Ann wrote:
Chris Peterson wrote:That said, consider the M57 spectrum. The 501 nm line will not appear as green or blue at all, because that line is not in isolation. We can simplify the spectral output to a 501 nm line and a 656 nm line. Visually, individually, these would appear as cyan and red. Present these two lines to the human eye, however, and we'll perceive yellow. In reality, we see something pushed down towards green because our eyes are more sensitive to the shorter wavelength line.
I have a quibble with that. Visually, hardly anyone appears to be able to see 656 nm line when it is emitted by objects in space. We are much more sensitive to green than red light. If we were to see an object in space that emitted only two wavelengths, at 501 and 656 nm, in more or less equal amounts, it seems certain that this object wouldn't appear yellow at us. If we were to see color at all in it, it would be green to our eyes.
Sorry if that was confusing. When I said "visual" I wasn't implying this is how we'd see the object directly. I just meant that if we mixed those two wavelengths of pure light, bright enough for us to see the color, we wouldn't see either color individually. And in the case under discussion here, we're talking about the photographic appearance given RGB images, which approximate what our eyes would see if they had more sensitivity.

Through the telescope, the only emission line that typically stimulates our color vision is oxygen, triggering all three of our color sensors. Ha only triggers red and green, but is usually not bright enough to do so significantly. So to the extent we see color through the eyepiece, it's nominally cyan, but pushed visually towards green by "white" light, which includes Ha and some continuum sources, or towards blue by scattered blue starlight. That's why when you read about the visual appearance of planetaries, they vary from blue to green.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Wed Mar 29, 2017 10:31 pm

Chris Peterson wrote:
Ann wrote:Yes, but now look at the two spectra you posted in a previous post.
Note, however, that this spectrum is completely useless for understanding the color of the Owl Nebula. M57 has less reflected light compared with emitted light (although you can still see some continuum light). A spectrum of the Owl Nebula would not look like a flat zero line with spikes at the emission sources, but like a low hot blackbody curve with spikes sitting on top of it. You're going to have a very difficult time assessing the color that someone would perceive when you look at a spectrum like that.
Good point. It is obvious from the appearances of these nebulas that there is more reflective dust behind the hot blue central star of the Owl Nebula than behind the central star of the Ring Nebula, so there must be more continuum light in the Owl than in the Ring.
That said, consider the M57 spectrum. The 501 nm line will not appear as green or blue at all, because that line is not in isolation. We can simplify the spectral output to a 501 nm line and a 656 nm line. Visually, individually, these would appear as cyan and red. Present these two lines to the human eye, however, and we'll perceive yellow. In reality, we see something pushed down towards green because our eyes are more sensitive to the shorter wavelength line.
I have a quibble with that. Visually, hardly anyone appears to be able to see 656 nm line when it is emitted by objects in space. We are much more sensitive to green than red light. If we were to see an object in space that emitted only two wavelengths, at 501 and 656 nm, in more or less equal amounts, it seems certain that this object wouldn't appear yellow at us. If we were to see color at all in it, it would be green to our eyes.

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Wed Mar 29, 2017 2:34 pm

Ann wrote:Yes, but now look at the two spectra you posted in a previous post.
Note, however, that this spectrum is completely useless for understanding the color of the Owl Nebula. M57 has less reflected light compared with emitted light (although you can still see some continuum light). A spectrum of the Owl Nebula would not look like a flat zero line with spikes at the emission sources, but like a low hot blackbody curve with spikes sitting on top of it. You're going to have a very difficult time assessing the color that someone would perceive when you look at a spectrum like that.

That said, consider the M57 spectrum. The 501 nm line will not appear as green or blue at all, because that line is not in isolation. We can simplify the spectral output to a 501 nm line and a 656 nm line. Visually, individually, these would appear as cyan and red. Present these two lines to the human eye, however, and we'll perceive yellow. In reality, we see something pushed down towards green because our eyes are more sensitive to the shorter wavelength line. The important point, however, is that we'll see a color substantially different than what we'd see with just the 501 nm line in isolation.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Wed Mar 29, 2017 2:08 pm

neufer wrote:
Ann wrote:
And note the streak of faint, pale, washed-out green from the comet. It is a different sort of green than the green color of the nebula. There is no hint of blue at all in the comet, but there is a hint of at least aqua in the nebula.
502nm OIII is on the blue side of cyan whereas the cometary 510nm C2 band is on the green side:

Cyan is greenish-blue color evoked by light with a predominant wavelength of between 490–520 nm.
I think you are right, Art.

I sort of agree with Chris that wavelength is not the end-all and be-all of color perception. The background light plays a huge part in determining how we perceive the color of the wavelengths that are visible to us. Still, I believe that if we can more or less recreate the same conditions, our color perception should be more or less the same.

I think the spectrum of the comet versus the spectrum of the nebula is all we need to understand why one object is at least slightly blue, while the other is "all green". First of all, I agree with you that most people would definitely see 510 nm as green, whereas it is less obvious that they would see 501 nm as green. Actually, I believe that even 501 nm would look mostly greenish to most people.

















Yes, but now look at the two spectra you posted in a previous post. Note how there is a relatively tall spike of Hβ shortward of the OIII line in the planetary's spectrum, at 486 nm, which would definitely look bluish to most people. Together these two wavelenghts, 501 nm from OIII and 486 nm from Hβ may create a sense of blue, green or aqua in different people.

The comet is quite different. The nearest tall spike shortward of the tallest one at 510 nm is still firmly in the part of the spectrum which most people would clearly see as green, and the spikes at around blue 470 nm are not so tall. There is simply more green and less blue light being emitted by the comet than by the planetary.

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Wed Mar 29, 2017 1:48 pm

neufer wrote:
Ann wrote: And note the streak of faint, pale, washed-out green from the comet. It is a different sort of green than the green color of the nebula. There is no hint of blue at all in the comet, but there is a hint of at least aqua in the nebula.
502nm OIII is on the blue side of cyan whereas the cometary 510nm C2 band is on the green side:

Cyan is greenish-blue color evoked by light with a predominant wavelength of between 490–520 nm.
Except when we're talking about color. The color cyan may be evoked by the mix of many other wavelength combinations. There may be little connection between color and wavelength.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by neufer » Wed Mar 29, 2017 1:39 pm

Ann wrote:
And note the streak of faint, pale, washed-out green from the comet. It is a different sort of green than the green color of the nebula. There is no hint of blue at all in the comet, but there is a hint of at least aqua in the nebula.
502nm OIII is on the blue side of cyan whereas the cometary 510nm C2 band is on the green side:

Cyan is greenish-blue color evoked by light with a predominant wavelength of between 490–520 nm.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Wed Mar 29, 2017 3:38 am

I apologize for calling the Owl Nebula the Own Nebula. Stupid. :evil:

Anyway, there is a nice picture of the Owl Nebula, the comet and the galaxy in the Recent Submissions thread over at the Latest Sky Photography forum. I don't know how to show it here except as a link. Jean-Francois Gout holds the copyright to the picture.

Note the greenish color of the Owl Nebula, which of course is greatly affected by the color balance of the picture itself. Note the colorless appearance of the galaxy, M108, apart from a hint of yellow at the center. Note the barest hint of blue or blue-green in the star FI UMa, which I would rather call HD 97302. The star, interestingly, is of spectral class A4V and perhaps slightly blue for its class, with a B-V index of 0.1.

And note the streak of faint, pale, washed-out green from the comet. It is a different sort of green than the green color of the nebula. There is no hint of blue at all in the comet, but there is a hint of at least aqua in the nebula. Also note that the Owl Nebula is by far the most intensely colored object in this image.

I think that this is a good and revealing picture. It appears to be a single exposure, not a composite one, and it brings out the "relative appearances" of the objects very well.

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Mon Mar 27, 2017 11:35 pm

Chris Peterson wrote:
Ann wrote:
Chris Peterson wrote: Spectra tell you very little about color. The reason that the Owl Nebula looks quite blue in well exposed RGB images is because it isn't purely an emission nebula, but also a reflection nebula, with its dust reflecting and scattering the blue light from its very hot central star.
I doubt that reflection nebulosity affects the color of planetary nebulas.
You would be completely wrong in that assumption. In fact, reflection plays an important role in determining the color of many planetary nebulas. Just do an image search and you'll find that most RGB images of the Owl show a lot of blue; usually that's the dominant color. This is true for a number of planetary nebulas (the Blue Snowball is even named for it!) Do some research on the Owl Nebula and you'll find many references to its bluish color caused by reflection from its central star.
I googled pictures of the Own Nebula and found a confusion of colors.

I made a quick search for "dust reflection in the Owl Nebula" and "dust reflection in planetary nebulae" and found nothing to suggest that reflection nebulosity makes planetary nebulas look bluer.

Do you have any sources that confirm, or at least suggest, that dust reflection can make planetary nebulas look bluer?

But if I try to hear what you are saying instead of just finding fault with your argument, then yes, I can see that from our point of view there might well be dust behind the central star of the Owl Nebula that may scatter its blue light our way.

It's just that I have never heard of a reflection nebula whose blue light can be seen visually. So once again, Chris, do you have any sources that say that reflection nebulosity can affect the visual appearance of the color of a deep-space object?

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Mon Mar 27, 2017 3:26 pm

Ann wrote:
Chris Peterson wrote: Spectra tell you very little about color. The reason that the Owl Nebula looks quite blue in well exposed RGB images is because it isn't purely an emission nebula, but also a reflection nebula, with its dust reflecting and scattering the blue light from its very hot central star.
I doubt that reflection nebulosity affects the color of planetary nebulas.
You would be completely wrong in that assumption. In fact, reflection plays an important role in determining the color of many planetary nebulas. Just do an image search and you'll find that most RGB images of the Owl show a lot of blue; usually that's the dominant color. This is true for a number of planetary nebulas (the Blue Snowball is even named for it!) Do some research on the Owl Nebula and you'll find many references to its bluish color caused by reflection from its central star.
Planetary nebulas are typically bluest near the hot central star. But there is usually not much dust near the central star, and not much reflection nebulosity there.
That depends upon the nebula. Some still have enough dust or molecular gases to reflect and/or scatter light.
In any case, even if there was reflection nebulosity in the "blue" part of the nebula, reflection nebulosity is typically so faint that it doesn't stimulate the color-sensitive rods in our retinas at all.
People do report that the Owl appears bluish. The same people who see some other bright, compact nebulas as green. I've seen both colors through the eyepiece.
Also bear in mind that there is probably a lot of dust in the coma of a comet. If dust makes the color bluer, the comet's coma should be bluer than it is.
You're misunderstanding me if you think I'm saying dust makes things look bluer.
One astrophotographer that I have the greatest respect for is Adam Block. Check out this page to see a sample of his (old) planetary nebula images. Note that with one obvious exception, NGC 6826, all his pictures of planetary nebulas show them to be either greenish or reddish. I once wrote to him and asked him why his planetary nebulas look so green, and he explained that the color of OIII falls right between the green and the blue filters he was using at that time. On balance, and with the filters he was using, the planetaries were more green than blue.
Which was exactly my point with respect to this APOD: without technical details, we have no good way to evaluate the color. Because small differences in the filters used can result in substantial differences in the color whenever we have sources that contain narrowband emission lines.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Mon Mar 27, 2017 5:11 am

Chris Peterson wrote: Spectra tell you very little about color. The reason that the Owl Nebula looks quite blue in well exposed RGB images is because it isn't purely an emission nebula, but also a reflection nebula, with its dust reflecting and scattering the blue light from its very hot central star.
Infrared portrait of the Helix Nebula, highlighting dust.
Photo: Spitzer Space Telescope.
"Visual" portrait of the Helix Nebula.
NASA, WIYN, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI),
& T. A. Rector (NRAO)



















I doubt that reflection nebulosity affects the color of planetary nebulas.

Planetary nebulas are typically bluest near the hot central star. But there is usually not much dust near the central star, and not much reflection nebulosity there. The infrared, dust-highlighting Spitzer Space Telescope portrait of the Helix Nebula at left shows a "hole" in the inner part of the nebula. The picture at right, which shows a "typical visual" appearance of the nebula, is bluest where there is little dust.

In any case, even if there was reflection nebulosity in the "blue" part of the nebula, reflection nebulosity is typically so faint that it doesn't stimulate the color-sensitive rods in our retinas at all.

Also bear in mind that there is probably a lot of dust in the coma of a comet. If dust makes the color bluer, the comet's coma should be bluer than it is.

One astrophotographer that I have the greatest respect for is Adam Block. Check out this page to see a sample of his (old) planetary nebula images. Note that with one obvious exception, NGC 6826, all his pictures of planetary nebulas show them to be either greenish or reddish. I once wrote to him and asked him why his planetary nebulas look so green, and he explained that the color of OIII falls right between the green and the blue filters he was using at that time. On balance, and with the filters he was using, the planetaries were more green than blue.
(BTW, it also has a bluish cast visually when viewed in a large aperture telescope.)
I remember reading Burnham's Celestial Handbook, where he wrote that some people see planetary nebulas as green, and others see them as blue. Personally I have only ever looked at one planetary nebula through a telescope, and that was the Ring Nebula. It looked like a gray smoke ring to me.

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Mon Mar 27, 2017 4:29 am

Ann wrote:
neufer wrote:
Chris Peterson wrote: All of the colors in this image seem like what we might call "natural". Certainly the comet is green and the nebula quite blue.
  • The Hubble palette Blue of doubly ionized oxygen (OIII) in the cores of planetary nebula
    should "naturally" be a 502nm cyan/green only slightly bluer than the 510nm C2 green of comets .
Yes, that's what I thought, which is why I asked.

Judging from the spectra you posted, there is very little difference between the comet and the planetary when it comes to their emission of green and blue light
Spectra tell you very little about color. The reason that the Owl Nebula looks quite blue in well exposed RGB images is because it isn't purely an emission nebula, but also a reflection nebula, with its dust reflecting and scattering the blue light from its very hot central star. (BTW, it also has a bluish cast visually when viewed in a large aperture telescope.)

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Mon Mar 27, 2017 3:51 am

neufer wrote:
Chris Peterson wrote:
Ann wrote:
To those in the know (Chris?), would you say that the Owl Nebula is indeed
a lot bluer than the comet, as it appears to be in this image?
All of the colors in this image seem like what we might call "natural". Certainly the comet is green and the nebula quite blue.
  • The Hubble palette Blue of doubly ionized oxygen (OIII) in the cores of planetary nebula
    should "naturally" be a 502nm cyan/green only slightly bluer than the 510nm C2 green of comets .
Yes, that's what I thought, which is why I asked.

Judging from the spectra you posted, there is very little difference between the comet and the planetary when it comes to their emission of green and blue light.

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Mon Mar 27, 2017 1:21 am

neufer wrote:
All of the colors in this image seem like what we might call "natural". Certainly the comet is green and the nebula quite blue.
  • The Hubble palette Blue of doubly ionized oxygen (OIII) in the cores of planetary nebula
    should "naturally" be a 502nm cyan/green only slightly bluer than the 510nm C2 green of comets .
Yes, but we're not seeing this in narrowband. In RGB, it is common for the colors of nebulas to look quite different from any of their individual spectral emissions, because we have a mix of emissions as well as some degree of broadband continuum.

Furthermore, lacking any information about the equipment used, it's hard to know what colors to even expect in the final image.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by neufer » Mon Mar 27, 2017 12:55 am

Chris Peterson wrote:
Ann wrote:
To those in the know (Chris?), would you say that the Owl Nebula is indeed
a lot bluer than the comet, as it appears to be in this image?
All of the colors in this image seem like what we might call "natural". Certainly the comet is green and the nebula quite blue.
  • The Hubble palette Blue of doubly ionized oxygen (OIII) in the cores of planetary nebula
    should "naturally" be a 502nm cyan/green only slightly bluer than the 510nm C2 green of comets .
https://messierthanacomet.com/objects-by-type/planetary-nebulae/ wrote: <<The radiation emitted by the ionized gas of a planetary nebula features a distinctive spectrum, characterized by two especially bright emission lines at 500.7 nm and 495.9 nm, both in the green part of the visual spectrum. First noticed in 1864, these were initially thought to be indicative of a new element—later referred to as “nebulium”—as the spectral lines did not match any known elements at the time. However, this was found not to be the case six decades later, when they were instead revealed to be the forbidden lines of doubly ionized oxygen (O-III), which can only materialize in conditions of extremely low density (incapable of simulation in laboratories on Earth).>>
http://newburyastro.org.uk/whatsup/news/20150111/comet-lovejoy.html wrote: <<Newbury Astro member, David Boyd, recorded this spectrum of Comet C/2014 Q2 Lovejoy on 6th January 2015. The spectrum has been artificially coloured, and above it is the spectral profile showing the intensity of light emitted at each wavelength across the spectrum.

The spectrum tells us about the chemical composition of the comet. Molecules sublimating from the nucleus of the comet as it approaches the Sun emit light at very specific wavelengths characteristic of the individual molecules. The resulting emission lines appear as sharp spikes in the spectrum and the strength of the spikes tell us how much of each kind of molecule is present in the comet’s nucleus. Some of these lines have been labelled with the chemical names of their parent molecules. The spikes at around 470nm and 510nm show the presence of diatomic carbon (C2) which, with along with the spike labeled CN (cyanogen), are responsible for the comet’s green colour.>>

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Sun Mar 26, 2017 2:55 pm

Ann wrote:To those in the know (Chris?), would you say that the Owl Nebula is indeed a lot bluer than the comet, as it appears to be in this image?
All of the colors in this image seem like what we might call "natural". Certainly the comet is green and the nebula quite blue. Unfortunately, we have no information about how this image was made, so it is scientifically useless. I would never have accepted it as an APOD.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Ann » Sun Mar 26, 2017 5:20 am

To those in the know (Chris?), would you say that the Owl Nebula is indeed a lot bluer than the comet, as it appears to be in this image?

Ann

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Sat Mar 25, 2017 10:52 pm

deviod wrote:Surely this image is a composite, or there's some heavy-handed processing. How else can you explain the disappearance of the stars around the comet's head?
Comet images are almost always constructed from multiple shorter images combined in such a way that stars are removed, unless the intent is to show their motion against the background. However, in an image like this, you want both a frozen comet and a frozen background. It would be difficult to process such an image stack without introducing some kind of artifacts.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by deviod » Sat Mar 25, 2017 10:36 pm

Surely this image is a composite, or there's some heavy-handed processing. How else can you explain the disappearance of the stars around the comet's head?

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by DavidLeodis » Sat Mar 25, 2017 12:52 pm

smitty wrote:Thank you, Art Neuendorffer and Chris Peterson, for your helpful explanations! Mystery explained!
Thanks also from me as I had wondered the same thing. :)

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by smitty » Fri Mar 24, 2017 2:10 pm

Thank you, Art Neuendorffer and Chris Peterson, for your helpful explanations! Mystery explained!

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by neufer » Fri Mar 24, 2017 1:52 pm

Chris Peterson wrote:
smitty wrote:
I'm confused/puzzled by the discussion for this APOD; if the orbital period of the comet is 5.4 years, why has it been sighted so infrequently (1858, 1907, and 1951)?
Before the 1900s, many comets went unobserved. By the 20th century there were more eyes on the sky, but it was still easy to miss a dim comet, and the comet isn't always favorably placed in its orbit with respect to Earth (perihelion can occur when it's opposite the Earth with respect to the Sun). This comet is rarely visible without a telescope. Some earlier apparitions may have been casually observed, but not associated with this specific named comet.
Every 5.4 years Comet 41P/Tuttle-Giacobini-Kresak closes in on a perihelion of 1.045 AU (on April 12, 2017 this year). If the Earth just happens to be close by around this time (i.e., 0.142 AU away on April 1, 2017) it is easily visible in binoculars (magnitude ~8.75 on April 1, 2017). However, this happens with just ~0.15/π = 3% of those perihelions. (Comet 41P/Tuttle-Giacobini-Kresak spends most of its time at magnitude ~25+.)

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Chris Peterson » Fri Mar 24, 2017 1:33 pm

smitty wrote:Maybe I'm just being dense here, but I'm confused/puzzled by the discussion for this APOD; if the orbital period of the comet is 5.4 years, why has it been sighted so infrequently (1858, 1907, and 1951)?
Before the 1900s, many comets went unobserved. By the 20th century there were more eyes on the sky, but it was still easy to miss a dim comet, and the comet isn't always favorably placed in its orbit with respect to Earth (perihelion can occur when it's opposite the Earth with respect to the Sun). This comet is rarely visible without a telescope. Some earlier apparitions may have been casually observed, but not associated with this specific named comet.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by smitty » Fri Mar 24, 2017 1:13 pm

Maybe I'm just being dense here, but I'm confused/puzzled by the discussion for this APOD; if the orbital period of the comet is 5.4 years, why has it been sighted so infrequently (1858, 1907, and 1951)?

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Case » Fri Mar 24, 2017 11:07 am

APOD Robot wrote:... comet 41P orbits the Sun with a period of about 5.4 years.
The link is awesome in its background detail: The interactive orbit diagram (WebGL) allows one to line up the Earth and the comet, and see the Big Dipper in the background. Obviously it should work like that, but it’s quite cool that it works so well and so smoothly.

Re: APOD: The Comet, the Owl, and the Galaxy (2017 Mar 24)

by Case » Fri Mar 24, 2017 10:52 am

APOD Robot wrote:Sweeping through northern springtime skies just below the Big Dipper, the faint greenish comet was about 75 light-seconds from our fair planet.
The comet would have passed right in between M108 and M97, around 2017-03-22 12:30 UTC, well positioned for observation in e.g. Tokyo (21:30 JST) or Hawaii (02:30 HAST). Despite this moment, the triangle composition in todays APOD is likely more aesthetically pleasing. :D

Top