by neufer » Sun Jan 08, 2017 4:07 pm
Chris Peterson wrote:NCTom wrote:
Why a tube instead of a sphere? is this related to the magnetic poles of the star?
Or the physical spin axis. Or both in some combination.
- Binary companion stars, planets, accretion disks and polar jets may also play a role.
https://en.wikipedia.org/wiki/Planetary_nebula#Physical_characteristics wrote:
<<Only about 20% of planetary nebulae are spherically symmetric. A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar, and other types, although the majority of them belong to just three types: spherical, elliptical and bipolar.
Bipolar nebulae are concentrated in the galactic plane, likely produced by relatively young massive progenitor stars; and bipolars in the galactic bulge appear to prefer orienting their orbital axes parallel to the galactic plane. On the other hand, spherical nebulae are likely produced by the old stars similar to the Sun.
The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different. Nevertheless,
the reason for the huge variety of physical shapes is not fully understood. Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes.>>
https://en.wikipedia.org/wiki/Cat%27s_Eye_Nebula#Kinematics_and_morphology wrote:
<<The Cat's Eye Nebula is structurally a very complex nebula, and the mechanism or mechanisms that have given rise to its complicated morphology are not well understood. The central bright part of the nebular consists of the inner elongated bubble (inner ellipse) filled with hot gas. It in turn is nested into a pair of larger spherical bubbles conjoined together along their waist. The waist is observed as the second larger ellipse lying perpendicular to the bubble with hot gas.
The structure of the bright portion of the nebula is primarily caused by the interaction of a fast stellar wind being emitted by the central PNN with the visible material ejected during the formation of the nebula. This interaction causes the emission of X-rays discussed above. The stellar wind, blowing with the velocity as high as 1900 km/s, has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends.
It is also suspected that the central WR:+O7 spectral class PNN star, HD 1064963 / BD +66 1066 / PPM 20679 of the nebula
may be generated by a binary star. The existence of an accretion disk caused by mass transfer between the two components of the system may give rise to polar jets, which would interact with previously ejected material. Over time, the direction of the polar jets would vary due to precession.>>
[quote="Chris Peterson"][quote="NCTom"]
Why a tube instead of a sphere? is this related to the magnetic poles of the star?[/quote]
Or the physical spin axis. Or both in some combination.[/quote]
[list]Binary companion stars, planets, accretion disks and polar jets may also play a role.[/list]
[quote=" https://en.wikipedia.org/wiki/Planetary_nebula#Physical_characteristics"]
<<Only about 20% of planetary nebulae are spherically symmetric. A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar, and other types, although the majority of them belong to just three types: spherical, elliptical and bipolar. [b][color=#0000FF]Bipolar nebulae are concentrated in the galactic plane, likely produced by relatively young massive progenitor stars; and bipolars in the galactic bulge appear to prefer orienting their orbital axes parallel to the galactic plane. On the other hand, spherical nebulae are likely produced by the old stars similar to the Sun.[/color][/b]
The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different. Nevertheless, [b][u][color=#FF0000]the reason for the huge variety of physical shapes is not fully understood[/color][/u][/b]. Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes.>>[/quote][quote=" https://en.wikipedia.org/wiki/Cat%27s_Eye_Nebula#Kinematics_and_morphology"]
[float=right][img3="[b][color=#0000FF][size=150]The Cat's Eye Nebula using optical images from the HST
and X-ray data from the Chandra X-ray Observatory[/size][/color][/b]"]https://upload.wikimedia.org/wikipedia/commons/5/5a/NGC6543.jpg[/img3][/float]<<The Cat's Eye Nebula is structurally a very complex nebula, and the mechanism or mechanisms that have given rise to its complicated morphology are not well understood. The central bright part of the nebular consists of the inner elongated bubble (inner ellipse) filled with hot gas. It in turn is nested into a pair of larger spherical bubbles conjoined together along their waist. The waist is observed as the second larger ellipse lying perpendicular to the bubble with hot gas.
The structure of the bright portion of the nebula is primarily caused by the interaction of a fast stellar wind being emitted by the central PNN with the visible material ejected during the formation of the nebula. This interaction causes the emission of X-rays discussed above. The stellar wind, blowing with the velocity as high as 1900 km/s, has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends.
It is also suspected that the central WR:+O7 spectral class PNN star, HD 1064963 / BD +66 1066 / PPM 20679 of the nebula [b][color=#0000FF]may be generated by a binary star. The existence of an accretion disk caused by mass transfer between the two components of the system may give rise to polar jets, which would interact with previously ejected material. Over time, the direction of the polar jets would vary due to precession[/color][/b].>>[/quote]