by Recycled Electrons » Tue Aug 02, 2005 9:05 pm
Kim wrote:Not sure as to why it is spherical, but a great many of these faint PN's exhibit a similar structure, they are faint because they are very large and spread out, hence it's fairly logical to assume they are at a very advanced stage (old!) maybe by this time in the PN's existence the expansion has evened out from it's original hourglass shape (which many brighter pn's seem to have) and thus exhibits the sphere shape. It could also be a function of it's position reklative to us, but a good many are then similarly oriented.
Planetary Nebulae are only visible for roughly 40,000 to 60,000 years. In the context of stellar evolution, this is practically a blink of an eye. Thus you can't really distinguish 'old' PNe from 'young' PNe, since if they are visible, they are all roughly the same age.
PNe ought to be spherical. The standard stellar evolution model predicts their formation from old red giants or asymptotic giants (depending on the mass of the star) puffing off their outer layers. The result is the core of the once-giant star contracting to become a white dwarf, which ionizes the expanding outer layers making them visible to us as a PN. According to the standard model, unless a star has an incredibly strong magnetic field (which is not observed), the resultant nebula should be spherical. Abell 39 is a classic example of what PNe should look like.
The question is not, "Why is Abell 39 spherical?" but rather "Why are so many others asymmetric, or axisymmetric?"
The most likely answer to this is simply an argument of binarity. Recent studies have shown that a surprisingly large number of PNe have binary star systems at their cores. This would quite easily explain the axisymmetric shapes of most PNe. The gravitational interactions between the companion stars would shape the expanding nebula.
Check out this article for more information.
http://www.noao.edu/outreach/press/pr04/pr0402.html It looks like someone from Michigan Tech, where RJN and several people on this board work, was involved on the project.
[quote="Kim"]Not sure as to why it is spherical, but a great many of these faint PN's exhibit a similar structure, they are faint because they are very large and spread out, hence it's fairly logical to assume they are at a very advanced stage (old!) maybe by this time in the PN's existence the expansion has evened out from it's original hourglass shape (which many brighter pn's seem to have) and thus exhibits the sphere shape. It could also be a function of it's position reklative to us, but a good many are then similarly oriented.[/quote]
Planetary Nebulae are only visible for roughly 40,000 to 60,000 years. In the context of stellar evolution, this is practically a blink of an eye. Thus you can't really distinguish 'old' PNe from 'young' PNe, since if they are visible, they are all roughly the same age.
PNe ought to be spherical. The standard stellar evolution model predicts their formation from old red giants or asymptotic giants (depending on the mass of the star) puffing off their outer layers. The result is the core of the once-giant star contracting to become a white dwarf, which ionizes the expanding outer layers making them visible to us as a PN. According to the standard model, unless a star has an incredibly strong magnetic field (which is not observed), the resultant nebula should be spherical. Abell 39 is a classic example of what PNe should look like.
The question is not, "Why is Abell 39 spherical?" but rather "Why are so many others asymmetric, or axisymmetric?"
The most likely answer to this is simply an argument of binarity. Recent studies have shown that a surprisingly large number of PNe have binary star systems at their cores. This would quite easily explain the axisymmetric shapes of most PNe. The gravitational interactions between the companion stars would shape the expanding nebula.
Check out this article for more information. [url]http://www.noao.edu/outreach/press/pr04/pr0402.html[/url] It looks like someone from Michigan Tech, where RJN and several people on this board work, was involved on the project.