I am constantly amazed and awed by the sheer beauty of planetary nebulae – the gorgeous structures created as stars die.
Among the most astonishing of them is NGC 7026, a youngish nebula about 6000 light years away in the constellation Cygnus, the Swan. Here’s a stunningly beautiful picture of it from the Hubble Space Telescope:
[Click to enlepidopterate.]
Planetary nebulae (or PNe for short) like this are sometimes called "butterfly nebulae" because of their shape. It’s easy to see why; there are two big lobes that are roughly shaped like butterfly wings. The history of those lobes is complex.
You can see the central star, right in the middle. That used to be a star much like the Sun, though more massive and hotter. As it ran out of fuel in its core, it swelled up to become a red giant. It started to blow a slow, dense wind of gas, like the solar wind but much thicker. This expanded into space around it. Eventually, the star blew off so much of its outer layers that the hotter lower layers were exposed. This causes the wind to speed up substantially and get much thinner. The fast wind catches up with and collides into the slower, older wind, carving all manners of weird shapes.
The overall shape of the nebula depends a lot on two things: material outside the star into which the winds are flowing, and what shape the winds themselves are.
You might think the winds would expand in a sphere, but there are forces that can change that. If the central star is a binary, for example, centrifugal force can cause the slow wind to be flattened, like someone sitting on a beach ball. It’s thicker along the equator as it expands. When the fast wind slams into it, it get slowed a lot by this thicker waist, but the thinner gas along the poles means the fast wind can plow on through. What finally happens is the formation of a double-lobed structure like a bowling pin with a ring around the middle… just like NGC 7026! The ring around the middle isn’t obvious in this picture, but you can see how the pinched waist is brighter, which is a tell-tale sign of a ring (similar to how some nebulae look like smoke rings).
That explains the gross structure. But look at all the detail! Those fingers of stuff pointing inward toward the middle, the complex lobe structure? What gives there?
The clue comes from the location: Cygnus the Swan is a part of the sky where we’re looking into the plane of our flat Milky Way galaxy’s disk. That means there’s more gas and dust there than usual, and NGC 7026 is in the thick of it. The lobes of gas from the star are slamming into all that junk, creating these weird patterns. Those fingers are very common when hot, fast gas flows past denser, cooler gas (it’s called a Rayleigh-Taylor instability, if you want details).
I strongly suspect that explains the butterfly-shaped structure as well. The junk surrounding the nebula is not smooth, and is denser in some places in others. Where there’s less material, the winds from the star can poke through more easily and expand. If you’ve ever blown up a balloon with a weak spot in it you get the same thing. It’s essentially a hernia!
I read a couple of research papers about NGC 7026, and this idea does seem to fit with what’s observed. The lobes are also filled with very hot gas that emits X-rays, and that also makes sense if the lobes are still plowing into surrounding material; the hot gas hasn’t been able to escape because the lobes are closed.
Which means one more thing: eventually all that material may blow out of that cloud, popping it. When it does – in what’s called a "blowout" – the gas will escape and it’ll probably form long, weird, filaments like a shredded balloon. And that means, if it’s even possible, this object will become even more interesting, and even more beautiful.
Image credit: ESA/Hubble & NASA
In a funny coincidence, my friend Travis Rector, an astronomer at the University of Alaska who takes amazing and incredibly beautiful astrophotographs, posted an image that may look familiar. It’s of NGC 6751, a planetary nebula about 5000 – 7000 light years away:
[Click to embiggen, and note I rotated the image 90° to make it fit better on the blog.]
It’s very pretty, but also very complex. Planetary nebulae are actually gaseous structures created when stars a bit more massive than the Sun die. When they turn into red giants they blow a dense, slow wind of gas into space — that can be seen in NGC 6751 as the big blue halo surrounding the whole thing. The edge is a bit brighter as it slams into gas floating in between the stars and piles up a bit. The halo is probably something like 1.5 light years across.
The interior part is actually the most interesting, You can see the star in the center. About halfway out to the big halo is a ring of material that looks like it’s broken up into individual knots like pearls on a necklace. Inside that ring is something that looks like a disk seen face-on, with faint filaments stretching from the central star out to the ring.
Studies of this object indicate this is in fact the case. Although it’s not clear, I suspect the central star may have enveloped and swallowed a companion star or giant planet, spinning it up to a much faster rotation speed. Otherwise, it’s hard to explain why it has a ring and disk of material; a solitary star could never spin fast enough to eject such a structure. This would also explain some of the more complex features of the nebula as well. If this is true, we’re nearly roughly down on the pole of the star (or maybe off by 20° or so), since the disk is very close to being a circle; if the system were tilted by much more the ring would be more elliptical, like looking at the rim of a glass from an angle.
Anyway, I said this nebula may look familiar; that’s because I recently posted a picture of the star U Cam, which is much like the Sun but a lot older. Like the central star of NGC 6751, it too is dying, but in the case of U Cam it’s blasting out shells of material on a very short timescale. The ring we see around it is only about 700 years old, and was created in an event that lasted mere decades. The shells and rings in NGC 6751 took thousands of years to make, and the halo is probably more than 50,000 years old!
And while both huge structures are the results of dying stars, and both have an overall similar appearance, they’re very different in actual shape and origin. The halo around U Cam is like a soap bubble: a thin shell that has a sharp edge but is probably mostly empty on the inside. Astronomers call this a "detached shell". NGC 6751, on the other hand, is more like a balloon filled with gas. And down near the star NGC 6751 has that complex disk/ring affair going on, while U Cam is probably mostly empty space all the way down to the surface of the star. The gas making up the shell of U Cam has about the same mass as the Earth, while the nebula in NGC 6751 is tens of thousands times more massive!
There are lots of other differences between them as well, but I find it remarkable (literally, since I’ve been sitting here remarking on it!) that two objects with such similar appearance – and both caused by the efforts of dying stars – can actually be so different. Nature is filled with such things, of course. Spiral galaxies look similar to swirls of cream in a coffee cup, though the physics is entirely different. Stars and drops of water are spherical, but for two different reasons (gravity for the stars, surface tension for the droplets).
Sometimes moons look like Death Stars, but that may be pushing this a bit too far.
I think that’s cool. Shapes of objects can be revealing of their nature, but you have to be careful when making judgement based on shape alone. You have to look deeper to reveal the true nature.
If there’s a life lesson in that, feel free to find it on your own.
Image credit: D. Tran (PAL College), T.A. Rector (University of Alaska Anchorage), T. Bridges (Queen’s U.) and the Australian Gemini Office
I don’t get a chance very often to combine two previous posts, but I was thinking recently about planetary nebulae — winds of gas blown off by dying stars — and remembered my very favorite one in the whole sky, Abell 39:
[Click to ennebulenate.]
Isn’t that awesome? It’s like it’s right out of Star Trek. I’ve written about the giant haloes surrounding some planetary nebulae before, and also about why some objects look like smoke rings. In a (nut)shell, as a star like the Sun begins its long, slow path to dying, it expands into a red giant and blows off a thick wind of matter. This material expands spherically in most cases, streaming off in all directions into space and forming what’s called a giant outer halo.
In most planetaries (like the famous Cat’s Eye nebula, and the less famous but also cool NGC 6826) the outer halo slams into material floating in interstellar space, causing it to get all clumpy or form a bright rim as the surrounding matter gets plowed up. But the rim around Abell 39 isn’t like that; while it’s bright, it’s actually an illusion! Near the edge, we are seeing through more material than we are through the center, and that would be true no matter from what direction we see the nebula. That makes the outer edge look brighter than the inner parts, giving the nebula the appearance of a vast, eerie smoke ring.
It’s pretty rare to get such a near-perfect circle of gas from a planetary nebula, and to be honest Abell 39 is one of only a very few I’m aware of. One reason it’s so perfect is that it’s located well above the plane of the galaxy. Down here, in that plane, there’s copious gas and dust. But Abell 39 is well away from all that, so its expanding red giant wind can retain its almost exactly spherical shape.
It’s also huge: 5 – 6 light years across, which is twice as large or more than most other planetaries, implying it’s old. Given the expansion velocity and size, it must be 20,000 years old or so. It’s also 7000 light years way, so it’s fairly faint, making it a rare target for amateur astronomers… though not unknown.
Readers with a keen eye might have noticed the star in the middle is a bit off-center. No one knows why. I’d love to see a telescope like Hubble or Gemini North get a deep view of this smoke ring. Why is the star misplaced? Why is the limb on the lower left brighter than on the right? Plus, the hint of detail in this image would be greatly enhanced in a deeper exposure, and that would really be incredible to see.
Image credit: WIYN/NOAO/NSF
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. Read More
Every now and again I’ll see an image of an astronomical object and think, what the heck?
CRL 618 is definitely one such object!
This Hubble image threw me for a sec: it looks like a planetary nebula, but where’s the central star? What are those long fingers of matter? So I started going through the scientific literature and found some good explanations. And I learned something!
CRL 618 is a star announcing it’s on its way to becoming a planetary nebula.
Use the thumbnails and arrows to browse the images, and click on the images themselves to go through to blog posts with more details and descriptions.