Starship Asterisk*

In the Center of the Cat's Eye Nebula

Explanation: Three thousand light-years away, a dying star throws off shells of glowing gas. This image from the Hubble Space Telescope reveals the Cat's Eye Nebula (NGC 6543), to be one of the most complex planetary nebulae known. Spanning half a light-year, the features seen in the Cat's Eye are so complex that astronomers suspect the bright central object may actually be a binary star system. The term planetary nebula, used to describe this general class of objects, is misleading. Although these objects may appear round and planet-like in small telescopes, high resolution images with large telescopes reveal them to be stars surrounded by cocoons of gas blown off in the late stages of stellar evolution. Gazing into this Cat's Eye, astronomers may well be seeing more than detailed structure, they may be seeing the fate of our Sun, destined to enter its own planetary nebula phase of evolution ... in about 5 billion years.

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Today's APOD is based on Hubble data, and it is beautifully detailed indeed. The other image, the one by Łukasz Sujka, has been produced with Łukasz Sujka's own equipment, including RGB filters. You can read about his image here.

Two things set the two images apart. The amateur equipment of Łukasz Sujka can't compete with the resolution of Hubble. The other difference is that the colors seem to be "inverted": While the inner part of the Cat's Eye Nebula is red in the Hubble/Villaverde image, this part of the nebula is cyan-colored in Sujka's picture. But the two "tails" sticking out from the central oval is red in Sujka's image, but blue in the Hubble/Villaverde one.

Which one is correct? Is any of them correct? I'm going to stick my neck out and say that it is the Łukasz Sujka image that got its colors right.

Check out this page, which is a (relatively old) collection of planetary nebula images from the Kitt Peak Advanced Observing Program. You can tell at a glance that two colors dominate in planetary nebulas: Green to cyan-colored, and red. Some planetary images from the KPAOP are both green and red, like the Ring Nebula. I'm not allowed to copy the old KNAOP images, so I'll show you a Hubble picture of the Ring Nebula instead, and also a picture by amateur Stefan Heutz:


As for the Stefan Heutz image, I don't know if I'm allowed to keep it. If it disappears, go to this page to see it.

Regarding the Hubble Heritage image of the Ring Nebula, this is part of the caption for it from Wikimedia Commons (and I apologize that there is a bit of Swedish on that page):

The colors are approximately true colors. The color image was assembled from three black-and-white photos taken through different color filters with the Hubble telescope's Wide Field Planetary Camera 2. Blue isolates emission from very hot helium, which is located primarily close to the hot central star. Green represents ionized oxygen, which is located farther from the star. Red shows ionized nitrogen, which is radiated from the coolest gas, located farthest from the star. The gradations of color illustrate how the gas glows because it is bathed in ultraviolet radiation from the remnant central star, whose surface temperature is a white-hot 120,000 degrees Celsius (216,000 degrees Fahrenheit).

As you can see from the caption, blue and green represent hotter gas closer to the central star, while red represents cooler gas farther from central star.


Most planetary nebula images nowadays have probably been produced using an OIII filter, which detects 501 nm light from ionized oxygen close to the central star, and a hydrogen alpha filter, which detects 656 nm light emitted by ionized hydrogen farther from the central star.

Some of the planetary nebulas from the Kitt Peak Observing Program are "all green". The way I understand it, that's because these planetaries have mostly lost their hydrogen, perhaps during their transition from red giants to white dwarfs, when the stars "cast off" their "hydrogen envelope".


Like a butterfly breaking out of and losing its cocoon, some central white dwarf stars may have lost all of their hydrogen during their transition to a white dwarf star. After all, a white dwarf is nothing but the exposed core of a former red giant star. And the colorful translucent draperies of gas surrounding the central star of a planetary nebula is its former then-opaque gaseous envelope, which it has now cast off and ionized!


The temperature of white dwarf Sirius B is "only" 25,000K, which is not a lot for a white dwarf. It has cooled down a lot since it lost its gaseous envelope. Its planetary nebula is long gone.

Ayway. While some planetaries are "all green", perhaps because they lack hydrogen, others are "all red". My guess is that when the planetaries are all red, not only is there a lot of hydrogen present, but the central stars have cooled down so much that they don't do a good job of ionizing much oxygen any more.

But we may put it like this: In planetary nebulas, green represents hotter gas and red cooler gas. Therefore, if green gas is present, we expect to find it close to the central star. And if red gas is present, we expect to find it farther away from the central star. Unless the nebula is "all green" or "all red", of course!

Ann

Ann wrote: ↑Sun Jul 10, 2022 6:19 am But we may put it like this: In planetary nebulas, green represents hotter gas and red cooler gas. Therefore, if green gas is present, we expect to find it close to the central star. And if red gas is present, we expect to find it farther away from the central star. Unless the nebula is "all green" or "all red", of course!
Ann

I wish some expert come out and say if and where the thermal radiation from a hot stellar remnant translates to fluorescent colors of H, N, O.
I understand that red alpha H spectral line can be seen only under far UV stimulation; it's from electron's transition between high orbits.

Today's APOD! I like the whole Nebula better! Cat's Eye!

Ann wrote: ↑Sun Jul 10, 2022 6:19 am Which one is correct? Is any of them correct? I'm going to stick my neck out and say that it is the Łukasz Sujka image that got its colors right.

Don't stick it out so far your head gets cut off. There's not such thing as getting the colors "right".

Brilliant image…
“ The corpse, already greatly decayed and clotted with gore, stood erect before the eyes of the spectators. Upon its head, with red extended mouth and solitary eye of fire, sat the hideous beast whose craft had seduced me into murder, and whose informing voice had consigned me to the hangman. I had walled the monster up within the tomb!”

Edgar Allen Poes The Black Cat.

I know the colors are somewhat arbitrary but this APOD reminds me vividly of Poe’s horrid filled story that it seems to me a perfect (or purrfect?) crossover of art and science!

Eric

Chris Peterson wrote: ↑Sun Jul 10, 2022 1:42 pm

Ann wrote: ↑Sun Jul 10, 2022 6:19 am Which one is correct? Is any of them correct? I'm going to stick my neck out and say that it is the Łukasz Sujka image that got its colors right.

Don't stick it out so far your head gets cut off. There's not such thing as getting the colors "right".

Agreed, Chris. There is no such thing as "absolute color", but there are such things as "absolute wavelengths".

Therefore, it is indeed possible to say that the ionized gas closer to the central star in a planetary nebula tends to emit "shorter wavelengths" of light, and the ionized gas farther from the central star emits longer wavelengths of light. And because blue and green represent shorter wavelengths than red, it makes sense to show planetary nebulas as usually blue or green near the central star and red farther from the center.

Ann

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 1:42 pm

Ann wrote: ↑Sun Jul 10, 2022 6:19 am Which one is correct? Is any of them correct? I'm going to stick my neck out and say that it is the Łukasz Sujka image that got its colors right.

Don't stick it out so far your head gets cut off. There's not such thing as getting the colors "right".

Agreed, Chris. There is no such thing as "absolute color", but there are such things as "absolute wavelengths".

Therefore, it is indeed possible to say that the ionized gas closer to the central star in a planetary nebula tends to emit "shorter wavelengths" of light, and the ionized gas farther from the central star emits longer wavelengths of light. And because blue and green represent shorter wavelengths than red, it makes sense to show planetary nebulas as usually blue or green near the central star and red farther from the center.

"Makes sense"? I disagree. We create false color palettes to emphasize specific structures and details, and the best choice of colors for that purpose frequently places the wavelengths out of correspondence with the source. All that matters is that we know the details of the color mapping. Who cares if short wavelengths are represented by red and long ones by blue?

Chris Peterson wrote: ↑Sun Jul 10, 2022 3:29 pm

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm There is no such thing as "absolute color", but there are such things as "absolute wavelengths".
Therefore, it is indeed possible to say that the ionized gas closer to the central star in a planetary nebula tends to emit "shorter wavelengths" of light, and the ionized gas farther from the central star emits longer wavelengths of light. And because blue and green represent shorter wavelengths than red, it makes sense to show planetary nebulas as usually blue or green near the central star and red farther from the center.

"Makes sense"? I disagree. We create false color palettes to emphasize specific structures and details, and the best choice of colors for that purpose frequently places the wavelengths out of correspondence with the source. All that matters is that we know the details of the color mapping. Who cares if short wavelengths are represented by red and long ones by blue?

So we have 3 ways to build color scheme for an exotic scene outside of natural optic colors like Sun (thermal spectrum), skyshine (Rayleigh scattering) or deserts, greens and animals (pigments and inks).

1) follow the ordering of the wavelengths. It makes a color legend easier to memorize; besides sometimes, e.g. in an IR/UF/X-ray overlay, it makes colder look redder and hotter look bluer

2) use colors to show more details. Human vision is more detailed in green than in red, and is poor in blue. So show the most detailed layer in green and the least detailed in blue

3) for luminescent colors of H N O near several hot stars or hot stellar remnants of different temperatures study the effect of an UF source's temperature on the visible luminescence and use colors to highlight the difference of the temperatures of those several hot stars or hot stellar remnants

XgeoX wrote: ↑Sun Jul 10, 2022 2:56 pm Brilliant image…
“ The corpse, already greatly decayed and clotted with gore, stood erect before the eyes of the spectators. Upon its head, with red extended mouth and solitary eye of fire, sat the hideous beast whose craft had seduced me into murder, and whose informing voice had consigned me to the hangman. I had walled the monster up within the tomb!”

Edgar Allen Poes The Black Cat.

I know the colors are somewhat arbitrary but this APOD reminds me vividly of Poe’s horrid filled story that it seems to me a perfect (or purrfect?) crossover of art and science!

Eric

Nice reference. There are a plethora of nice images found by searching for "black cat red eye". Here's one:

VictorBorun wrote: ↑Sun Jul 10, 2022 6:02 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 3:29 pm

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm There is no such thing as "absolute color", but there are such things as "absolute wavelengths".
Therefore, it is indeed possible to say that the ionized gas closer to the central star in a planetary nebula tends to emit "shorter wavelengths" of light, and the ionized gas farther from the central star emits longer wavelengths of light. And because blue and green represent shorter wavelengths than red, it makes sense to show planetary nebulas as usually blue or green near the central star and red farther from the center.

"Makes sense"? I disagree. We create false color palettes to emphasize specific structures and details, and the best choice of colors for that purpose frequently places the wavelengths out of correspondence with the source. All that matters is that we know the details of the color mapping. Who cares if short wavelengths are represented by red and long ones by blue?

So we have 3 ways to build color scheme for an exotic scene outside of natural optic colors like Sun (thermal spectrum), skyshine (Rayleigh scattering) or deserts, greens and animals (pigments and inks).

1) follow the ordering of the wavelengths. It makes a color legend easier to memorize; besides sometimes, e.g. in an IR/UF/X-ray overlay, it makes colder look redder and hotter look bluer

2) use colors to show more details. Human vision is more detailed in green than in red, and is poor in blue. So show the most detailed layer in green and the least detailed in blue

3) for luminescent colors of H N O near several hot stars or hot stellar remnants of different temperatures study the effect of an UF source's temperature on the visible luminescence and use colors to highlight the difference of the temperatures of those several hot stars or hot stellar remnants

Probably more ways than just those three. The important point being that none are "right" or "wrong". Each has reasonable justification given the intent of the imager (or analyzer).

Chris Peterson wrote: ↑Sun Jul 10, 2022 3:29 pm

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 1:42 pm

Don't stick it out so far your head gets cut off. There's not such thing as getting the colors "right".

Agreed, Chris. There is no such thing as "absolute color", but there are such things as "absolute wavelengths".

Therefore, it is indeed possible to say that the ionized gas closer to the central star in a planetary nebula tends to emit "shorter wavelengths" of light, and the ionized gas farther from the central star emits longer wavelengths of light. And because blue and green represent shorter wavelengths than red, it makes sense to show planetary nebulas as usually blue or green near the central star and red farther from the center.

"Makes sense"? I disagree. We create false color palettes to emphasize specific structures and details, and the best choice of colors for that purpose frequently places the wavelengths out of correspondence with the source. All that matters is that we know the details of the color mapping. Who cares if short wavelengths are represented by red and long ones by blue?

Who cares, Chris? As in, what person cares? Name that person?

Are you kidding me?

Ann

Ann wrote: ↑Sun Jul 10, 2022 8:20 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 3:29 pm

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm

Agreed, Chris. There is no such thing as "absolute color", but there are such things as "absolute wavelengths".

Therefore, it is indeed possible to say that the ionized gas closer to the central star in a planetary nebula tends to emit "shorter wavelengths" of light, and the ionized gas farther from the central star emits longer wavelengths of light. And because blue and green represent shorter wavelengths than red, it makes sense to show planetary nebulas as usually blue or green near the central star and red farther from the center.

"Makes sense"? I disagree. We create false color palettes to emphasize specific structures and details, and the best choice of colors for that purpose frequently places the wavelengths out of correspondence with the source. All that matters is that we know the details of the color mapping. Who cares if short wavelengths are represented by red and long ones by blue?

Who cares, Chris? As in, what person cares? Name that person?

Are you kidding me? :shock: :roll: :lol2:

Ann

I mean exactly that. What difference does it make what colors are used to represent different wavelengths if the intent isn't to approximated actual colors?

Chris Peterson wrote: ↑Sun Jul 10, 2022 8:52 pm

Ann wrote: ↑Sun Jul 10, 2022 8:20 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 3:29 pm

"Makes sense"? I disagree. We create false color palettes to emphasize specific structures and details, and the best choice of colors for that purpose frequently places the wavelengths out of correspondence with the source. All that matters is that we know the details of the color mapping. Who cares if short wavelengths are represented by red and long ones by blue?

Who cares, Chris? As in, what person cares? Name that person?

Are you kidding me?

Ann

I mean exactly that. What difference does it make what colors are used to represent different wavelengths if the intent isn't to approximated actual colors?

I have no dog in this fight, but even I can agree that it is natural (for a human) to map longer wavelengths to visible spectrum red tints and shorter wavelengths to visible spectrum blue tints because that's how we (humans) naturally see visible redder and bluer tints respectively. But I agree that ultimately it's a matter of preference.

johnnydeep wrote: ↑Sun Jul 10, 2022 9:02 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 8:52 pm

Ann wrote: ↑Sun Jul 10, 2022 8:20 pm

Who cares, Chris? As in, what person cares? Name that person?

Are you kidding me? :shock: :roll: :lol2:

Ann

I mean exactly that. What difference does it make what colors are used to represent different wavelengths if the intent isn't to approximated actual colors?

I have no dog in this fight, but even I can agree that it is natural (for a human) to map longer wavelengths to visible spectrum red tints and shorter wavelengths to visible spectrum blue tints because that's how we (humans) naturally see visible redder and bluer tints respectively. But I agree that ultimately it's a matter of preference.

There's nothing very natural about it. 99% of the people who look at pictures like this have no clue about what colors correspond to what wavelength ranges! And for narrowband images, the mapping doesn't matter because we don't care that oxygen is blue or that hydrogen is green, we only care that we can distinguish the two.

Well, as long as I'm not kicked out of this forum, I'm going to keep commenting on the color of APODs. Sorry, Chris.

And you are going to set me straight. That's the two of us, I guess.

Ann

Ann wrote: ↑Sun Jul 10, 2022 9:14 pm Well, as long as I'm not kicked out of this forum, I'm going to keep commenting on the color of APODs. Sorry, Chris.

And you are going to set me straight. That's the two of us, I guess.

Ann

You're more than welcome to comment on the colors! But please don't call them "right" or "wrong" just because you might prefer different aesthetics. Because doing so makes you factually wrong.

I have never liked planetary nebulas much. The reason is precisely because I wasn't able, for the longest time, to figure out what color they really are, or why. (Now I think I know - they glow at 501 nm and 656 nm.)

The wild and strange colors in Hubble images of planetaries made me go off planetaries entirely for a while. I couldn't muster any interest in them at all.

Ann

Ann wrote: ↑Sun Jul 10, 2022 9:37 pm I have never liked planetary nebulas much. The reason is precisely because I wasn't able, for the longest time, to figure out what color they really are, or why. (Now I think I know - they glow at 501 nm and 656 nm.)

The wild and strange colors in Hubble images of planetaries made me go off planetaries entirely for a while. I couldn't muster any interest in them at all.

Ann

Easy. They are all gray. ;-)

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm ... There is no such thing as "absolute color", but there are such things as "absolute wavelengths".

Ann, just in good fun -- or to be another thorn in your side --

Ahh, Mr. Doppler, that's an interesting phenomenon ...

MarkBour wrote: ↑Sun Jul 10, 2022 9:49 pm

Ann wrote: ↑Sun Jul 10, 2022 3:25 pm ... There is no such thing as "absolute color", but there are such things as "absolute wavelengths".

Ann, just in good fun -- or to be another thorn in your side --

Ahh, Mr. Doppler, that's an interesting phenomenon ...

Hey, Mister Doppler, no problem! I know what you are, and I can read you (mostly)!

Ann

Chris Peterson wrote: ↑Sun Jul 10, 2022 9:47 pm

Ann wrote: ↑Sun Jul 10, 2022 9:37 pm I have never liked planetary nebulas much. The reason is precisely because I wasn't able, for the longest time, to figure out what color they really are, or why. (Now I think I know - they glow at 501 nm and 656 nm.)

The wild and strange colors in Hubble images of planetaries made me go off planetaries entirely for a while. I couldn't muster any interest in them at all.

Ann

Easy. They are all gray.

Well, Chris, the day you have convinced me that all astronomical objects are gray, I'm outta here.

Seriously.

Ann

Ann wrote: ↑Sun Jul 10, 2022 10:03 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 9:47 pm

Ann wrote: ↑Sun Jul 10, 2022 9:37 pm I have never liked planetary nebulas much. The reason is precisely because I wasn't able, for the longest time, to figure out what color they really are, or why. (Now I think I know - they glow at 501 nm and 656 nm.)

The wild and strange colors in Hubble images of planetaries made me go off planetaries entirely for a while. I couldn't muster any interest in them at all.

Ann

Easy. They are all gray. ;-)

Well, Chris, the day you have convinced me that all astronomical objects are gray, I'm outta here.

Seriously.

Ann

Just ask anybody who has ever observed deep sky objects directly.

Chris Peterson wrote: ↑Sun Jul 10, 2022 9:47 pm

Ann wrote: ↑Sun Jul 10, 2022 9:37 pm I have never liked planetary nebulas much. The reason is precisely because I wasn't able, for the longest time, to figure out what color they really are, or why. (Now I think I know - they glow at 501 nm and 656 nm.)

The wild and strange colors in Hubble images of planetaries made me go off planetaries entirely for a while. I couldn't muster any interest in them at all.

Ann

Easy. They are all gray.

Some of the high surface brightness ones do show colour for sure. NGC 3918 looks blue at the eyepiece of a moderate aperture scope and NGC 3242 shows a blue-green cast.

BillT wrote: ↑Sun Jul 10, 2022 10:43 pm

Chris Peterson wrote: ↑Sun Jul 10, 2022 9:47 pm

Ann wrote: ↑Sun Jul 10, 2022 9:37 pm I have never liked planetary nebulas much. The reason is precisely because I wasn't able, for the longest time, to figure out what color they really are, or why. (Now I think I know - they glow at 501 nm and 656 nm.)

The wild and strange colors in Hubble images of planetaries made me go off planetaries entirely for a while. I couldn't muster any interest in them at all.

Ann

Easy. They are all gray. ;-)

Some of the high surface brightness ones do show colour for sure. NGC 3918 looks blue at the eyepiece of a moderate aperture scope and NGC 3242 shows a blue-green cast.

I've never seen color at the eyepiece, but a small number of experienced observers do seem to get a hint of green with a handful of objects. (Of course, those same objects typically have both shorter and longer wavelengths as well, which appear in images but cannot be seen visually.)