APOD: The Cat's Eye Nebula (2016 Jul 03)

[APOD Robot]

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 to be one of the most complex planetary nebulae known. In fact, 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 reveal them to be stars surrounded by cocoons of gas blown off in the late stages of stellar evolution.

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[Ann]

Today's APOD is from 1995.

For a long time, HST images were not "standardized" when it comes to how mapped color was shown. Today, OIII emission is usually shown as blue, Hα as green and SII (or NII) as red. This consistent treatment of color in mapped color images makes it so much easier to understand what we are seeing in them.

In today's APOD, according to this page, red represents Hα (obviously), blue represents neutral oxygen at 6300 angstroms (what????) and green represents ionized nitrogen at 6584 angstroms. This choice of color leads to an extremely red portrait of NGC 6543, with a pair of odd extremely green "wings" and an intensely blue central star.

Click to view full size image

NGC 40. Photo: CRCiencia.

Click to view full size image

NGC 6543, the Cat's Eye Nebula. Photo: Adam Block/NOAO/AURA/NSF

The fact that the Cat's Eye Nebula is made to look so red in today's APOD is very unfortunate, due to the fact that some planetary nebulas really are very red. One example is NGC 40, which seems to lack OIII emission altogether. Most planetary nebulas are not like that, however, and NGC 6543 certainly isn't. Adam Block's old but chromatically very accurate image from 2003 shows that the Cat's Eye is dominated by green OIII emission, with some Hα in the mix. The "wings" that look so oddly green in today's APOD are clearly red in Block's image.

We learn as we go, and nowadays the Hubble images are colorized in a way that allows most of the pictures to be "read" at a glance. That was not the case in the early days of HST. To appreciate how fast and loose HST image processors have played with color particularly when producing planetary nebula images, take a look at this 1.4 MB image. (Yes, I know that some objects there aren't planetaries.)

So the best thing I can say about today's APOD is that, yes, you've come a long way, Hubble-baby.

Ann

[Chris Peterson]

Today's APOD is from 1995.

For a long time, HST images were not "standardized" when it comes to how mapped color was shown. Today, OIII emission is usually shown as blue, Hα as green and SII (or NII) as red. This consistent treatment of color in mapped color images makes it so much easier to understand what we are seeing in them.

You're way, way off base here. You seem to be talking about the Hubble palette (which has been in use for a long time). It standardizes a color scheme for a specific, commonly useful species triplet: Ha, SII, and [OIII]. It could not possibly be applied to today's image.

In today's APOD, according to this page, red represents Hα (obviously), blue represents neutral oxygen at 6300 angstroms (what????) and green represents ionized nitrogen at 6584 angstroms. This choice of color leads to an extremely red portrait of NGC 6543, with a pair of odd extremely green "wings" and an intensely blue central star.

As you note, this image is looking at a completely different set of elements. One that has no standardized palette. All three species- Ha, [OI], and [NII] emit red lines, quite close to each other. Even visually, if we were to look in just these three bands the nebula would appear red. The fact that it appears red in this treatment simply reflects the fact that Ha is the dominant emission source. There's nothing "odd" about the wings; we're simply seeing a region of nitrogen emission.

The fact that the Cat's Eye Nebula is made to look so red in today's APOD is very unfortunate, due to the fact that some planetary nebulas really are very red.

Why that is "unfortunate" is very unclear. This image shows, quite precisely, exactly what it is intended to show.

[Boomer12k]

Mine came out just a blurry patch.... this is much better. Going to have to get better focus for my shot next time, but it is so small, I think atmospheric disturbance affects it too.

:---[===] *

[Ann]

Today's APOD is from 1995.

For a long time, HST images were not "standardized" when it comes to how mapped color was shown. Today, OIII emission is usually shown as blue, Hα as green and SII (or NII) as red. This consistent treatment of color in mapped color images makes it so much easier to understand what we are seeing in them.

You're way, way off base here. You seem to be talking about the Hubble palette (which has been in use for a long time). It standardizes a color scheme for a specific, commonly useful species triplet: Ha, SII, and [OIII]. It could not possibly be applied to today's image.

In today's APOD, according to this page, red represents Hα (obviously), blue represents neutral oxygen at 6300 angstroms (what????) and green represents ionized nitrogen at 6584 angstroms. This choice of color leads to an extremely red portrait of NGC 6543, with a pair of odd extremely green "wings" and an intensely blue central star.

As you note, this image is looking at a completely different set of elements. One that has no standardized palette. All three species- Ha, [OI], and [NII] emit red lines, quite close to each other. Even visually, if we were to look in just these three bands the nebula would appear red. The fact that it appears red in this treatment simply reflects the fact that Ha is the dominant emission source. There's nothing "odd" about the wings; we're simply seeing a region of nitrogen emission.

Okay, Chris. But tell me, why would [OI] be so interesting? I thought nebula images usually looked at ionized elements. Do you know of any other famous Hubble nebula image where one filter has been reserved for an unionized element?

Ann

[geckzilla]

This particular set of observations was intended to characterize the x-ray emission discovered in planetary nebulas.
http://archive.stsci.edu/proposal_searc ... st&id=5403

[Chris Peterson]

You're way, way off base here. You seem to be talking about the Hubble palette (which has been in use for a long time). It standardizes a color scheme for a specific, commonly useful species triplet: Ha, SII, and [OIII]. It could not possibly be applied to today's image.

In today's APOD, according to this page, red represents Hα (obviously), blue represents neutral oxygen at 6300 angstroms (what????) and green represents ionized nitrogen at 6584 angstroms. This choice of color leads to an extremely red portrait of NGC 6543, with a pair of odd extremely green "wings" and an intensely blue central star.

As you note, this image is looking at a completely different set of elements. One that has no standardized palette. All three species- Ha, [OI], and [NII] emit red lines, quite close to each other. Even visually, if we were to look in just these three bands the nebula would appear red. The fact that it appears red in this treatment simply reflects the fact that Ha is the dominant emission source. There's nothing "odd" about the wings; we're simply seeing a region of nitrogen emission.

Okay, Chris. But tell me, why would [OI] be so interesting? I thought nebula images usually looked at ionized elements. Do you know of any other famous Hubble images where one filter has been reserved for an unionized element?

I don't know the research goals. But one of the narrowband filters on the WFPC2 is very close to the [OI] emission, which I assume is by design. Any of these lines may simply used as markers for the presence of some element; the actual ionization state isn't necessarily important.

That said, we can get some idea from the work of the image authors. From a poster some years back describing this work, they note that they "observed NGC 6543 with the HST WFPC2 camera in ten narrow-band filters covering a range of emission lines. These images were used to further look at the physical conditions in NGC 6543." In the same poster they report "The ionization state in the nebula is illustrated with a false-color map made by combining the [O III] 5007, [O II] 3727, and [O I] 6300 images. As expected, knots in the equatorial ring have neutral cores ([O I]) running through them which is surrounded by strong [O II] emission. The high-ionization gas ([O III]) is not confined to the elliptical bubble but extends out, disk-like, to the ring of knots."

The image this latter describes is seen on Harrington's personal webpage, with [O I] in red, [O II] in blue, and [O III] in green.


The data they collected appears to have generated several papers or posters addressing different aspects of planetary nebulas.

[Joe Stieber]

Visually in a modest amateur telescope without any filters (e.g., my 12.5-inch Newtonian), the Cat's Eye Nebula has a pale blue-green color, which is not unusual for planetary nebulae. It also shows the "blinking" effect when magnified, similar to the Blinking Planetary, NGC 6826 in Cygnus. Stare at the central star and the nebulosity fades away, but look a bit off center and the nebulosity pops back into view. This, of course, is a visual effect, not a behavior of the nebula itself.

[neufer]

Visually in a modest amateur telescope without any filters (e.g., my 12.5-inch Newtonian), the Cat's Eye Nebula has a pale blue-green color, which is not unusual for planetary nebulae. It also shows the "blinking" effect when magnified, similar to the Blinking Planetary, NGC 6826 in Cygnus.

blinking: A British epithet. Usually pronounced blinkin'.

JP: "I was trying to make you smile the other night and you wouldn't smile! I wanted to slap your face!

Me: (thinking) You just try to slap my face and see what happens! I will knock your blinking block off!

[starsurfer]

One example is NGC 40, which seems to lack OIII emission altogether.
Ann

Erm, NGC 40 has plenty of OIII emission! However nearly all of it overlaps with the Ha, so it isn't distinctly visible separately. The overlapping is also what produces its nice pink colour.

There are indeed some Ha only or mostly Ha planetary nebulae, a small list:

1. IsWe 2 by Don Goldman
2. KeWe 8 by Don Goldman
3. PFP 1 by Marco Lorenzi
4. WeBo 1 by Adam Block
5. YM 16 by Don Goldman
6. Du 1 by Capella Observatory