The knotty halo of the Cat's Eye

By Phil Plait | May 12, 2011 7:00 am

One of my favorite types of object in the sky are planetary nebulae. These are typically compact, fantastically-shaped baubles caused the winds blown from stars as they die. One of the most complex and interesting is the famous Cat’s Eye nebula (NGC 6543, seen here). It’s easy to spot in a small telescope, and with large telescope an incredible amount of detail can be seen.

What most people don’t know is that there’s more to PNe (as we in the know call them) than the bright inner region. Many have giant outer halos, too! And the one surrounding the Cat’s Eye is fantastically complex and a treat for the eye and brain:

[Click to felinopticenate.]

That stunning image is from the 2.5 meter Isaac Newton Telescope on the island of La Palma in the Canaries. The halo around the bright inner region is actually huge expanding shell of material centered on the nebula’s central star. And I do mean huge; that halo is nearly 6 light years across — 60 trillion km, or almost 40 trillion miles!

When the star first started to die, it expanded into a red giant, much larger and cooler than our Sun. The star blew a thick, slow wind of material from its surface, much denser and slower than the Sun’s solar wind. This material expanded and slammed into the material (gas and dust) between the stars. As it did so, the long delicate fingers and smaller, denser knots of material formed.

This is common in space when expanding material interacts with stationary material. It’s called Rayleigh-Taylor instability, and can create incredibly beautiful and intricate-looking structures. In the Cat’s Eye halo, ultraviolet light from the central star causes the outer halo to glow like a fluorescent light. Different gases glow in different colors; here red is hydrogen, blue is sulfur, and green is oxygen.

Not that I’m knocking the inner region of the Cat’s Eye; it’s pretty cool too. The small picture at the top of this post is from Hubble, and shows the core of the nebula. It formed after the outer halo; the star was a red giant when it blew the wind that formed the halo, but as it aged and lost mass it heated up, blowing a faster and thinner wind. This wind caught up with stuff blown out earlier, forming the amazing shapes in the inner part.

Deeper images of the core show even more things going on, too. The picture here is also from Hubble, but shows different detail. Those concentric rings are shells of gas expelled as the star undergoes nuclear paroxysms in its core; oscillations in pressure squeeze and relax the core, making it fuse atomic nuclei faster or slower — think of it as a cosmic CPR. When this happens the star’s surface responds by blasting out material which expands and glows. In the Cat’s Eye these events happen every 1500 years or so, making the star look like it’s sitting in the center of a series of Russian matryoshka nesting dolls. It’s common in other planetary nebulae as well.

Many other nebulae have such outer haloes; for my Master’s degree I studied NGC 6826 which has an ethereal, almost perfectly circular halo, seen here (the image is a negative which makes it easier to see faint detail; click it to get a bigger version). Unlike the Cat’s Eye, NGC 6826’s halo has a rim around it, probably due to matter piling up as the expanding halo snowplows the surrounding material. You can see dips and kinks in the outer rim, where the halo has rammed slightly denser material. When compressed that material glows more brightly, which you can see in the upper left part of the halo.

I still remember those long nights at the University of Virginia’s Fan Mountain observatory, using the brand new digital camera (in 1989!), laboriously changing the filters and taking one half-hour exposure after another of NGC 6826. Initially I was going after the inner region to examine its structure, but when my advisor and I saw that halo (which at the time wasn’t well-known) we changed direction in mid-course and I worked on the outer material instead. That was fun, and resulted in a short paper in the Astronomical Journal.

But it also started a life-long love of these beautiful objects. Every time I see a new one, or an old one in a new way, I get a little thrill. I feel a connection to them because of the years I spent studying them, and because of the deeper knowledge I have of them. They can be stunningly beautiful, and for me that beauty is amplified by understanding.

Image credits: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA); D. López and R. Barrena (IAC)


Related posts:

Night FLIERs
Frankenstein nebula
Warm dusty rings glow around a weird binary star
Down the throat of a dying star

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (29)

  1. kevbo

    I tried to read your paper, but when it became obvious that the earth wasn’t in immediate danger, I lost interest…

  2. Okay, now I want to try to see some with my wimpy little telescope that my wife bought me for Saturnalia. Any recommendations?

  3. astronomynut

    @ Larian,
    Try looking for M57 the ring nebula in Lyra. It’s small but really bright and shows in even the most wimpy scope. The bright star Vega should be rising in the NE an hour or so after sunset. Star charts will show M57 almost exactly halfway between the two outer stars of the parallelogram of Lyra.

  4. M57 it is! :D Any more? And just for reference, here are the specs on my scope: http://www.celestron.com/c3/product.php?CatID=63&ProdID=431

    I want to get out there with the family some night, and even invite neighborhood kids and adults. Make a star party out of it as it were. :)

  5. @Larian –
    Nothing wimpy about that scope. 120mm is a formidibly-sized amateur refractor and and excellent starting point.

    You should easily be able to find M57 with a 120mm refractor, but be aware that it’s TINY. Under low power, it will look more like an out of focus star than a ring. And while Vega is rising around sunset, M-57 probably isn’t placed very well for observing until close to midnight.

    A couple other good planeteries are M-97 (The Owl Nebula) in Ursa Major, which is probably better placed for viewing this time of year, and M27 (The Dumbbell Nebula) in Vulpecula, which is one of my favorites. However, it’s more of a summer object.

    I think M-97 would be your best bet right now.

  6. M@!

    It looks like I cannot reach the full version of the image of NGC 6826:

    “You don’t have permission to access /~rcorradi/HALOES/n6826w.jpg on this server. “

  7. Thanks drksky! I will be checking them all out. Since my scope is all manual tracking, and I put the Barlow in with my 6mm eyepiece, it should be a nice challenge for my dexterity. ;)

  8. Chris A.

    @Larian:
    For M-97, you’re going to want the lowest power you’ve got–it’s a large, low surface brightness target.
    I’d also recommend NGC 6210, the Turtle. It’s not very big, but it has a stunning turquoise color. Also, the Ghost of Jupiter, NGC 3242, which looks just like its name suggests. And, if you’re willing to wait a few months, the Blue Snowball in Andromeda (NGC 7662) is a treat.

  9. Thank you Chris A. I suppose I could just do some google searches, but I like hearing from you folks a whole lot more. :)

  10. Pete Jackson

    @1kevbo: Earth is not in danger? The shell goes out to 6 light years from the central star; imagine if alpha Centauri, only 4.3 light years away went planetary! Of course, we’d still have 100,000 years to plan our funeral.

  11. Steve
  12. Wayne

    Is it possible to see the structure of these nebula evolving?

    If these stars explode and fling matterial out at near light speed and the centre has a diameter of 6 light years, then I would expect that there would be visible changes over the course of a year. If this is true, has anyone ever made an annimation showing the change over time?

    If there is, I would be very interested in seeing it.

    Please reply,
    Wayne

  13. Messier Tidy Upper

    @ 9. Pete Jackson : Well the shell of a planetary nebula is pretty tenous. Plus might the solar wind / solar heliosheath bubble protect us? I’m not sure at all just how bad it would be for us really. Not that I want to see it happen! :-o

    Mind you Planetarie Nebulae are exceedingly rare and short-lived stages of stellar lives so theodds of this happening are remote indeed. Getting close to a supernovae or even novae is probably more of a concern!

    Great pictures & write up there – thanks BA. :-)

    There’s more to the Cat’s Eye than the Human eye can see for sure! ;-)

  14. Off-topic, sorry, but think its worth reminding folks of the great planetary conjunction happening in the morning skies presently with Venus, Jupiter, Mercury and Mars – four out of the five “classical” unaided eye planets being very close together and “dancing” in the twilight for a few dawns. :-)

    Click on my name for a linked Aussie news item on this. Astronomer (& author & Anglo-Australian Observatory director) Fred Watson is great there – him being followed by an astrologer they also include in that article not-so-much. :roll:

    Still they do give Fred Watson the last word rebutting astrology so that’s something I guess! ;-)

  15. Joel

    With regards to the possible effects on the Earth of something like this happening nearby, a few hasty, back of envelope calculations (which are probably mistake-ridden) suggest that assuming the central star (estimated to have originally been approx. 5 solar masses) had lost all its mass into the six light year diameter nebula (which it hasn’t), then the average density of the nebula would be 1.04*10^-19 kg/km^3. For comparison, 1.0*10^-19 is roughly the mass of a virus. Average density of the solar wind at 1 AU is 7*10^-11 kg/km^3. I think what with that, and other things like the Earth’s magnetosphere, we’d probably be pretty safe.

  16. @ Larian:

    I remember when I first spied m57 with my little 2 inch refractor. It was tiny, but the ring was very clear and the surrounding sky quite dark so it just popped right out. Pushing the magnification way up with a barlow, I convinced myself I could see the central star, tho I think that was just wishful thinking.

    A really great target, tho, and one you’ll never forget. Warn the neighbors, however, as they may be hearing a loud “Whoop!” coming over the fence.

  17. @12. Joel : Thanks – good work there. :-)

  18. I just recently enjoyed the view of M57 with my Astromaster 114mm Reflector. Reminded me of a smoke ring in the sky. Viewed it with my 32mm ep and decided to see if I could see it with my 12.5mm… wow, what a site! I wrote about it on my blog and even took a few pictures with a small point and shoot camera. Not the best quality but those colors came out with only 15 second shutter speed. 1600ISO so it’s quite grainy, and F3.6 (or maybe it was 3.1, can’t remember)
    I’ve made note of all the planetary nebula mentioned in the comments here which are now my newest targets. I was going to attempt for the cats eye nebula tonight but after I let my telescope cool down outside and adjusted my eyes to the dark I stepped out, looked up and saw a sheet of clouds. So I have to hold off for now :(

  19. Joe R.

    This is a winner of a quote: “Beauty is amplified by understanding.” Right up there with some of the best science quotes I’ve heard.

  20. r0blar

    What’s the name of a star bottom left of NGC 6543 halo? (on the big picture)

    Looks massive as it energizes surrounding material quite well.

  21. The NGC 6826 link gives “You don’t have permission to access /~rcorradi/HALOES/n6826w.jpg on this server.” error to me.
    Have i missed something?

  22. Dave

    I think the easy way to describe the process of arriving at such spectacular structures is as follows:
    1. Take all the esoteric functions in calculus.
    2. Using various permutations (random or intelligent selection) create 3D surfaces, and vary the critical parameters to get a vast variety of shapes sizes and distortions.
    3. Use multiple such 3D surfaces with varying colours and colour gradients.

    Even with all such permutations, the universe will provide a better display of variety and surprise :-)
    This was one way I could figure to comprehend the stellar cloud shapes.
    However, an interesting observation is that similar surfaces and colours are found here on Earth too, to our human eyes, of course.
    I think what is remarkable is that stars have a much simpler toolkit (than say that available to a plant in a tropical canopy), but produce equally stunning pictures.
    And after all that we have Lord Time! ^_^

  23. Matt B.

    Once an Alpha Cen PNa got to be 6 c-yr wide, it would appear almost 90 degrees wide in the sky (ruining observations in 14% of the sky for a lot of other stuff).

  24. Paul A.

    There are spiky projections in the picture of the inner core on opposite ends at about one and seven o’clock. Are they caused by this thing spinning on its axis?

  25. Wzrd1

    At the 5:30 point, it appears to be a nice dust cloud being well energized by the gas. Possible stellar nursery?

  26. Joel

    Huh. Just got back from seeing Thor. Turns out this is all part of Yggdrasil.

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