APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

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APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by APOD Robot » Sun May 17, 2015 4:06 am

Image NGC 2440: Pearl of a New White Dwarf

Explanation: Like a pearl, a white dwarf star shines best after being freed from its shell. In this analogy, however, the Sun would be a mollusk and its discarded hull would shine prettiest of all! In the above shell of gas and dust, the planetary nebula designated NGC 2440, contains one of the hottest white dwarf stars known. The glowing stellar pearl can be seen as the bright dot near the image center. The portion of NGC 2440 shown spans about one light year. The center of our Sun will eventually become a white dwarf, but not for another five billion years. The above false color image was captured by the Hubble Space Telescope in 1995. NGC 2440 lies about 4,000 light years distant toward the southern constellation Puppis.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sun May 17, 2015 4:46 am

It is a pity that I should be the first one to comment, since I have a complaint to make. Because I'm passionately interested in colors, it seems totally wrong to me that one of the hottest white dwarfs known, with a surface temperature of more than 200,000K, should be shown as orange. The color seems even more absurd in view of the fact that the surrounding nebula looks white or slightly purplish-white. I see no reason to choose a false color "cool" color for the hot star and a false color "much hotter color" for the nebula.

There could be reasons for the absurd color of the white dwarf. It could be that the young planetary nebula surrounding the dead star is very dusty, so that the white dwarf itself is heavily dust-reddened. Another interesting possibility could be that the star is so hot that its energy peak lies so far away from the visible part of the spectrum that its optical spectrum is fairly flat, so that it doesn't actually emit that much more blue than red light. Of course, that still shouldn't make it look orange, but dust-reddening could make sure of that.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Sun May 17, 2015 5:26 am

This is actually one of the few planetary nebulas in Hubble's archive that has wideband Red, Green, and Blue filtered data available. Of course, no one processes it with just those three because the narrowband imagery is just so much nicer to look at since it brings out the nebula much more clearly, but here is what it looks like with just the wideband RGB data. I tried to balance it against the few stars available.
NGC2440_WidebandRGB.jpg
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sun May 17, 2015 5:31 am

Thanks, Geck. The star actually looks very slightly bluish in the picture you posted.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Sun May 17, 2015 5:33 am

Ann wrote:It is a pity that I should be the first one to comment, since I have a complaint to make. Because I'm passionately interested in colors, it seems totally wrong to me that one of the hottest white dwarfs known, with a surface temperature of more than 200,000K, should be shown as orange. The color seems even more absurd in view of the fact that the surrounding nebula looks white or slightly purplish-white. I see no reason to choose a false color "cool" color for the hot star and a false color "much hotter color" for the nebula.
You need to get over your color prejudices.

False color is just that. The colors are not chosen for their aesthetics. Indeed, they are frequently chosen with the deliberate intent of avoiding anything close to realistic colors (to the extent that would even be possible with the filters involved). By choosing colors that don't reflect our visual expectations, we are able to see things that we might otherwise miss. Blues are frequently avoided, because our visual acuity is lowest at short wavelengths, and our ability to distinguish small differences in color is also poor in the blue part of the spectrum.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sun May 17, 2015 6:32 am

Chris Peterson wrote:
Ann wrote:It is a pity that I should be the first one to comment, since I have a complaint to make. Because I'm passionately interested in colors, it seems totally wrong to me that one of the hottest white dwarfs known, with a surface temperature of more than 200,000K, should be shown as orange. The color seems even more absurd in view of the fact that the surrounding nebula looks white or slightly purplish-white. I see no reason to choose a false color "cool" color for the hot star and a false color "much hotter color" for the nebula.
You need to get over your color prejudices.

False color is just that. The colors are not chosen for their aesthetics. Indeed, they are frequently chosen with the deliberate intent of avoiding anything close to realistic colors (to the extent that would even be possible with the filters involved). By choosing colors that don't reflect our visual expectations, we are able to see things that we might otherwise miss. Blues are frequently avoided, because our visual acuity is lowest at short wavelengths, and our ability to distinguish small differences in color is also poor in the blue part of the spectrum.
Thanks for your advice regarding my own color prejudices (I call them color preferences), which I'm not likely to follow, and thanks for your input about the colors chosen for false color images. I see your point, although I'm not at all convinced that the reasons you put forth offer a full explanation. Perhaps "tradition" and "generally accepted false color palette and use of that palette" are better reasons? That would explain, for example, why closeups of the Sun are usually shown as orange.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Sun May 17, 2015 7:22 am

Ann, you definitely have a blindingly strong preference for having colors be a certain way. You're probably pretty tired of hearing it because you don't agree, but I don't see how else you can understand unless people external from you give you their observations. And, you know, a prejudice is just the negative side of a preference. Preferring to have something one way means that you probably don't like having it another way, and your dislike of having it any way other than this one certain way is seen by others as a prejudice even though you only see it as your preference. (It could be argued that both the preference for and the preference against one thing are prejudices. This is getting a little too much into semantics for good taste.)
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by hoohaw » Sun May 17, 2015 10:01 am

An interesting and useful discussion! Reminds me that once a visitor on an underground tour of a cave in Virginia complained, when the guide shone UV on the rocks to show the fluorescence, that it was not the real appearance of the rock. The guide said, "I'll be happy to show you the REAL appearance," and turned out •all• the lights.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by owlice » Sun May 17, 2015 11:03 am

Chris Peterson wrote: You need to get over your color prejudices.
This, a thousand times.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sun May 17, 2015 1:36 pm

I would like to discuss the "information content" of today's APOD (or rather, the information content inherent in the exposures used for today's APOD) as well as the information content of the RGB image that Geck posted.

I think the central star stands out more in the RGB image because of its bluish color. Others might disagree. In today's APOD, the central star, even though it has an orange halo, is still whiter in color than the other stars, again suggesting that it is different from them.

In the RGB image, much of the nebula is green. I would guess, although I don't know, that the green color corresponds to OIII emission. Other parts of the nebula are reddish-brown, which I interpret as dust.

In today's APOD, the innermost part of the nebula (actually the central "hole" of it) is faintly orange, while the outer parts of it are slightly blue. The parts in between are generally purplish-white. I wouldn't know how to interpret the colors, although I must say that the structure of the bright inner part of the nebula is much better revealed in today's APOD than in the RGB image. Why is that?
Geck wrote:
This is actually one of the few planetary nebulas in Hubble's archive that has wideband Red, Green, and Blue filtered data available. Of course, no one processes it with just those three because the narrowband imagery is just so much nicer to look at since it brings out the nebula much more clearly
I think you are saying that today's APOD was made from narrowband images. But is that true? I followed the above false color image link of the caption and came to the Fast Facts page of Hubble Heritage. The way I interpret the information there, NGC 2440 was imaged through just two filters, F555W and F814W. To the best of my understanding, F555W would detect but not single out OIII emission, and neither filter would detect Ha. Both filters are broadband filters.

So the way I understand it, today's APOD has not been made from narrowband exposures. Why is it, then, that it reveals a lot more detail than the RGB image? I would have guessed that today's APOD was made from fairly recent Hubble exposures, but that is apparently not true. The exposures appear to be from 1995. Perhaps the exposure time was longer. I have no idea.

In any case, it seems to me that today's APOD was made from images taken through two broadband filters. The colors chosen for the final image appear to contradict the established practice to use blue (or cyan) color for the shortest wavelength exposure and red (or orange) color for the longest wavelength exposure. I expect, for example, that the all-but-empty central "hole" of a planetary nebula will be green from OIII emission. I don't expect to see a lot of dust in that "hole" at all. If today's APOD had mapped the exposure through the F555W filter as cyan and the exposure through the F814W filter as orange, then the innermost part of the nebula should have been cyan in color. There should probably be almost no trace of orange in there due to a lack of dust.

Interestingly, in the RGB image part of the "hole" of the nebula is bluish in color. Another part of the "hole" was greenish. In today's APOD, the part of the "hole" that is blue in the RGB image is orange here, and the part of the hole that is greenish in RGB is slightly bluish here.

Could it be that Forrest Hamilton has mapped the image taken through the infrared filter as blue and the one taken through the visible filter as orange? If so, there is most certainly no law forbidding anyone to map the longest wavelength exposure as blue and the shortest wavelength exposure as orange. But I think it would be slightly unexpected, and it would have been helpful if this kind of mapping had been spelled out clearly in the caption of today's APOD.

Or perhaps, indeed, the coloring of today's APOD is even more creative. Perhaps Forrest Hamilton has indeed colored his processed image according to his own sense of aesthetics. Again there is nothing wrong with that, but again, it would have been helpful to be told about it.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Sun May 17, 2015 3:49 pm

Ann wrote:I think the central star stands out more in the RGB image because of its bluish color.
Perhaps. But the star is of minimal interest in this image. Stars are usually of low interest, since their characteristics are specified by just a few parameters. This image is about a nebula, not a star.
I wouldn't know how to interpret the colors, although I must say that the structure of the bright inner part of the nebula is much better revealed in today's APOD than in the RGB image. Why is that?
As a rule, you interpret the colors by looking at the color mapping:

R = 814W (broad near-IR with an upper cut below H-alpha)
G = 814W + 555W
B = 555W (broad visible with a lower cut above H-alpha)

One reason for the image clarity may be the decision to block most of the hydrogen emission. Another is that the near-IR component shows some structure that is blocked by dust in the visible component.
In any case, it seems to me that today's APOD was made from images taken through two broadband filters. The colors chosen for the final image appear to contradict the established practice to use blue (or cyan) color for the shortest wavelength exposure and red (or orange) color for the longest wavelength exposure.
The short wavelength data is mapped to a blue-biased cyan. The long wavelength data is mapped to a red-biased yellow. That is, the shorter wavelengths in the data are mapped to the shorter wavelengths on the display, and likewise for the longer wavelengths.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sun May 17, 2015 4:30 pm

Chris wrote:
The short wavelength data is mapped to a blue-biased cyan. The long wavelength data is mapped to a red-biased yellow. That is, the shorter wavelengths in the data are mapped to the shorter wavelengths on the display, and likewise for the longer wavelengths.
It doesn't look that way to me.

But I found some information on this page:
Filters: Red: F675W (R), Green: F656N (H-alpha), Blue: F673N ()


So we have a general broadband red filter which is mapped as red, a narrowband H-alpha filter which is also covered by the red filter and which is mapped as green, and a narrowband SII filter which is redder than the green filter and which is mapped as blue.

All the filters are basically red, and the filter mapped as blue detects a redder wavelength than the filter that is mapped as green.

I'm beginning to understand my own color confusion in response to this image.

Excuse me, Forrest Hamilton, it really wasn't your fault!

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Sun May 17, 2015 4:51 pm

Ann wrote:
Chris wrote:
The short wavelength data is mapped to a blue-biased cyan. The long wavelength data is mapped to a red-biased yellow. That is, the shorter wavelengths in the data are mapped to the shorter wavelengths on the display, and likewise for the longer wavelengths.
It doesn't look that way to me.
You're the one that provided the link to the data page for this image (or one that used very similar processing and the same data). It's pretty clear about the mapping.
But I found some information on this page...
That's for a completely different image (Herbig-Haro 32). Confusing because it's what you get when you follow the link from a release that contained four nebulas in a single composite image.

I believe the mapping is as I described above, and as the image fact sheet provides.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Sun May 17, 2015 5:11 pm

Today's APOD is certainly mapped to Orange / Cyan with F814W / F555W because that's the only data available for this particular view of it. It was captured in wideband RGB as well as three narrowband filters with a different dataset. This one was taken with WFPC2's Planetary Camera. All other Hubble images of this nebula are taken with the Wide Field Camera. http://en.wikipedia.org/wiki/Wide_Field ... y_Camera_2

Astronomical imagery with scientific purpose will continue to defy your expectations, Ann. It's always going to be hard for you to interpret scientific imagery as long as you continue to expect things to follow the standards of astrophotographers who primarily create their images as works of art.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Sun May 17, 2015 5:59 pm

FWIW. I took the original 555W and 814W FITS files from the MAST site, used FITS Liberator to do a log conversion, brought the two images into Photoshop and combined them into a single image weighted as

Red channel = 120% 814W
Green channel = 50% 814W + 50% 555W
Blue channel = 102% 555W

Rotated and cropped to match the APOD version. That's it. I certainly could have done more to match the colors, but all I was interested in was a sanity check that the data description matches the image. Which it clearly does.
hst_06119_10_clp.jpg
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Boomer12k » Sun May 17, 2015 10:38 pm

So...it is the Red "eye" of My Pretty Pony????

I like Geckzilla's RGB photo...I can recognize the "lobes" now...the other is too TWO dimensional looking to me...

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Sun May 17, 2015 10:49 pm

Boomer12k wrote:I like Geckzilla's RGB photo...I can recognize the "lobes" now...the other is too TWO dimensional looking to me...
Of course, the RGB image covers a much wider field of view. The lobes are largely out of the field of view of today's APOD, which may be why you see it as flatter (although I don't- the central "cave" looks very 3D to my eyes).
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Mon May 18, 2015 1:11 am

Chris wrote:
You're the one that provided the link to the data page for this image (or one that used very similar processing and the same data). It's pretty clear about the mapping.
All right, Chris. But it has happened to me before that I have followed a link to Hubble Heritage, checked out the Fast Facts page, and been told here that I got my facts wrong, and I should have checked out another Hubble page instead.

So this really is a F555W and F814W image. The way I understand it, F814W basically traces dust. Would it be correct, then, to say that the orange-looking parts in today's APOD are either dusty or otherwise very cool (like some of the stars), while the non-orange parts are dominated by visual light?

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Mon May 18, 2015 1:15 am

Geck wrote:
Astronomical imagery with scientific purpose will continue to defy your expectations, Ann. It's always going to be hard for you to interpret scientific imagery as long as you continue to expect things to follow the standards of astrophotographers who primarily create their images as works of art.
If I am told what filters are used and what the colors mean (for example, the F814W filter is mapped as orange and traces dust) it should not be too hard for me to understand the images.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Mon May 18, 2015 1:22 am

In my experience, F814W sees through a lot of dust. It won't get through the thickest of dust, but in meager amounts the light at that wavelength just goes right through it. You can think of it like red light that has some extra dust-penetrating ability. It doesn't mean dust is or isn't there. It's not that kind of infrared.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Mon May 18, 2015 1:45 am

geckzilla wrote:In my experience, F814W sees through a lot of dust. It won't get through the thickest of dust, but in meager amounts the light at that wavelength just goes right through it. You can think of it like red light that has some extra dust-penetrating ability. It doesn't mean dust is or isn't there. It's not that kind of infrared.
Okay. Thanks.

What made today's APOD so difficult for me to read was the fact that it was made from exposures through an unusual filter combination. There are several "conventional" filter combinations, which I can quickly learn to read. Apart from RGB, there is the Hubble palette of OIII, Ha, SII, the Spitzer palette of two or three different wavelengths of infrared light, the GALEX palette of far ultraviolet and near ultraviolet, and the extremely common Hubble two-filter palette of F606W and F814W. It's easy to learn what these filters measure and what their mapped colors typically look like.

The Hubble filter set of F606W and F814W is similar to F555W and F815W. But the filter set of F606W and F814W is typically used for imagings of gas-poor and dust-poor objects like globular clusters and galaxies. I wouldn't expect it to be used for a nebula.

When I see a picture made from an unusual filter combination and a non-obvious color mapping, used on an unexpected object, and I get no explanation, I definitely get confused.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Mon May 18, 2015 2:26 am

It's not always obvious what the observational goals were just from looking at one of the images produced. I find that it often helps a lot to read the abstracts for any given Hubble observation.
http://archive.stsci.edu/proposal_searc ... st&id=6119

Sometimes the choices still aren't obvious. I have often been perplexed by seemingly nonsensical observations only to later realize exactly why something was done. Sometimes I ask but I like to figure it out on my own.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by DavidLeodis » Mon May 18, 2015 1:19 pm

The image has often been used as an APOD (I've no problem with that) and I'm amused how part of the explanation has changed. In the explanation to its first use in the APOD of December 3 1996 it states "Like a butterfly, a white dwarf star begins its life by casting off a cocoon that enclosed its former self. In this analogy, however, the Sun would be a caterpillar and the ejected shell of gas would become the prettiest of all!" whereas in the explanation to at least its current use (I've not checked other explanations) it states "Like a pearl, a white dwarf star shines best after being freed from its shell. In this analogy, however, the Sun would be a mollusk and its discarded hull would shine prettiest of all!". :)

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Mon May 18, 2015 3:43 pm

Well, butterflies do tend to get a lot more respect than mollusks. Mollusks are pretty great.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Mon May 18, 2015 4:10 pm

geckzilla wrote:Mollusks are pretty great.
Especially with a shot of Tabasco or horseradish.
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