A butterfly in the Swan

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Sep 21, 2012
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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

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