Starship Asterisk*

<< "The Ring Nebula lies about 4,000 light years away, and is roughly 500 times the diameter of our Solar System. In this picture by the Hubble Space Telescope in 1998, dust filaments and globules are visible far from the central star. This helps indicate that the Ring Nebula is not spherical, but cylindrical." >>

http://apod.nasa.gov/apod/ap091115.html

Guess what Chris, "We are looking lengthwise 'down the barrel' (like rifling in a gunbarrel) . . "

kovil wrote:Guess what Chris, "We are looking lengthwise 'down the barrel' (like rifling in a gunbarrel) . . "

It's been known for quite a while that some planetary nebulas are cylindrical structures. However, there is nothing resembling rifling, which is a spiral pattern.

Why did you address this to me?

Because you have the reputation of being some sort of guru, Chris.

Whilst I have known for some time of the "cylindrical" concept regarding the nature of some planetary nebulae, and the current beautiful APOD seems to bear this out, I have difficulty in working out why (in the sense of solar dynamics.)

Any axis for such a cylinder would presumably be the star's spin axis, but I assume that a star becoming a nova is similar to that which will happen to our own Sun.

The rotational period of our own star, while not possible to define as the Sun does not rotate as if it were a solid body, is many days. Why does this necessitate the cylindrical (as opposed to something more spherical) nature of the nova phenomenon, especially considering the violence of the event?

jerbil wrote:Whilst I have known for some time of the "cylindrical" concept regarding the nature of some planetary nebulae, and the current beautiful APOD seems to bear this out, I have difficulty in working out why (in the sense of solar dynamics.)

You're not alone! The dynamics of what goes on during the formation and evolution of a planetary nebula are complex and poorly understood. Presumably the final morphology is dependent on some combination of the progenitor's spin and magnetic properties, as well as the possible presence of a binary companion.

The rotational period of our own star, while not possible to define as the Sun does not rotate as if it were a solid body, is many days. Why does this necessitate the cylindrical (as opposed to something more spherical) nature of the nova phenomenon, especially considering the violence of the event?

There's no certainty that the planetary nebula ultimately produced by the Sun will be cylindrical. Many are spherical. In fact, I don't think there are any truly cylindrical nebulas. M57, for instance, is bipolar, and if we could see it from the side would probably look something like a axially rotated figure-eight. We aren't looking down a cylinder, but down an oblate sphere (two of them, actually).

That said, it seems reasonable that an asymmetric planetary nebula would be born from some asymmetry in the system: the spin axis, magnetic axis, or ecliptic axis in a multiple system.

So all we have to do to get a really great 3D image is move to the side and take another photo... What could be simpler?

-Noel

I've pondered myself on the shapes of PlNs, particularly those of cylindrical perspective, brought on by, aptly enough, this very object (M57) - from 'way back when' I first learned of its barrel shape.

We 'Earthlingers' are assimilated into thinking of explosions as bubble-shaped, in seeing the shockwave effects that immediately follow. I believe this hemispherical form is brought by the key properties of uniform expansion against a basically uniform atmospheric pressure surrounding the event, or, a uniform 'resistance' to the uniform expansion. Watching ultra-slowed explosions of shaped charges seemingly support that theory.

My analysis of why the M57's of this world look like they do is, in addition to the magnetic and axial configurations as Chris suggests, might also be due to the lack of surrounding atmospheric 'backpressure' in the vacuum of space, and largely, after viewing videos of artist concepts of stellar novae, that the stars don't "blow up" evenly, but tend to blow out more in various places about their body. Whether due to molecular content within those areas, the non-uniform consistency of the plasmatic content, solar-wind directions & forces resulting from a 'shaped charge' effect, or simply higher magnetic linear forces, I don't know, but I think something or things in those elements help shape what we see as a result.

Speaking of molecular content - the fine filamentary structure that adds such exquisiteness to the pictures we see, is frequently mentioned with "scientists don't know why" captionry. I've long had the feeling we're seeing the still photo-excited and solar-windsworled trace paths of certain molecular elements as they 'blew away'.

Whatever the cause, they're some beautiful sights, aren't they!?!
Clear'ns!
~S*H

NoelC wrote:So all we have to do to get a really great 3D image is move to the side and take another photo... What could be simpler?

-Noel

What simpler?? Just wait until it spins to a new orientation & shoot that. Much less expensive too!
Which do ya think might happen first?

"Bring the mountain to Muhammed!"
Clear'ns!
~S*H

Star*Hopper wrote:What simpler?? Just wait until it spins to a new orientation & shoot that. Much less expensive too!
Which do ya think might happen first?

Nice thought, but that won't work. The nebula will have dissipated in just a few thousand years- long before it will move enough to present us with a significantly different view. That also poses a problem with sending a camera to get a different perspective: even at the speed of light, the travel times are roughly the same as the lifetime of the nebula.

Aaah. Then we'd better hurry!


~*