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Planetary Nebulae Morphology II, Gary Imm

Planetary Nebulae Morphology II

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Planetary Nebulae Morphology II, Gary Imm

Planetary Nebulae Morphology II

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Planetary nebulae are complex and challenging to understand – thousands of papers have been written on these objects over the past 50 years.  Each year, scientists learn more about them and pass this knowledge on to us.  This poster displays some of what I have learned over the past few years, but I still have more questions than answers.   

This is my second attempt at using a poster to better understand planetary nebulae morphology.  My first attempt from 3 years ago is here.  These are fascinating objects and I am always eager to learn more about how they develop. 

A planetary nebula (PN, plural PNe) is often simply described as a shell of ionized gas ejected from a red giant star late in life.  But the actual mechanics are so much more complex than that.  PNe come in a seemingly infinite array of shapes, sizes and colors.  It is believed that this variety is primarily determined by the nature of the progenitor. 

Many different PN morphology categorizations have been promoted for use.  I favor a simple 3 category view – spherical, bipolar, and elliptical, as described below:

Spherical – About 10% of PNe are believed to be spherical.  Because of the symmetry, the progenitor in these cases is believed to be a single star (no companion).  Although this subject is still being debated, it is believed that single stars do not have the required rotational speeds late in their life to form the bipolar and elliptical PNe described further below. 

Spherical PNe have no internal structure, a faint (if any) rim, and a circular projection.  The inner region can either be clear or somewhat occluded.  The right side of the poster shows 10 PNe which are believed to be spherical. 

Bipolar - About 20% of PNe are believed to be bipolar.  These PNe have the most interesting shapes.  To create such an axisymmetric and complex system, it is believed that the progenitor is likely a binary star system.  One of the stars, late in its life during its AGB phase, grows so large that its outer envelope forms a swirling equatorial disk (torus) around the companion.  The disk constrains the companion’s bi-polar flow, forming two polar lobes which expand (and sometimes break through) over time.   

It is important to note here that while the 2 companions are interacting, they avoid the common envelope phase for most of the interaction time (unlike the elliptical PN development described below).  The "common envelope phase" is when both companion stars of the binary system share the same ejected gas envelope of the late life AGB star.

The left side of the poster shows 5 different bipolar views:

- The 1st column shows an end-on view, where the torus appears as a circle.
- The 2nd column shows an inclined view, where the torus appears as an ellipse. 
- The 3rd column shows a side-on view, where the torus appears as a rectangle.  
   In this view, it is easy to see the hourglass shape of the lobes. 
- The 4th column also shows a side-on view, but this time with lobe breakout. 
- The 5th column shows multipolar examples, where multiple bipolar flows are occurring.
   It is believed that this is due to changes in the orientation of the binary star system over time.

Elliptical – The majority of PNe are believed to be elliptical.  In this case, like the bipolar case above, the progenitor is believed to be a binary star system.  For this case, however, the companion star orbits closer to the progenitor star so that it lies within its envelope for at least a portion of the formation time.  The resulting nebula then takes on more of an elliptical shape. 

I have included 2 types of elliptical PNE in the poster – ring type which have a thick circumferential ring around the rim, and disk type where the disk has a thinner rim and the inner region is occluded.  These types are simply my observation - I have not seen an explanation or discussion of why ellipticals take on these two types.


The complexities of the PN formation process also lead to many unusual PNe which seem to be a hybrid of the above 3 categories.  These complexities may include binary star separation over time, the destruction of one of the binary stars over time, the possibility that one of the binary stars is substellar, or the presence and influence of large planets.  

In addition to the above complexities, there are other reasons why PNe are confusing and difficult to interpret.  Here are 10 which come to mind:

1.    Viewing Perspective: Our viewing perspective is never definitively known and always results in only a 2-D view.   For PNe which are not perfectly symmetric (believed to be 90% of them), our perspective only leads to a partial understanding of the geometry.   In particular, for a bipolar PN, our view of the lobes and especially the torus will look different depending upon our viewing angle. 

2.    Overall Brightness:  The torus is much brighter than the lobes.  Often, especially in early visual observations and film photographs, all we see is the torus, which creates a misleading picture of the PN.  Most PNe have faint lobes but they have only been captured in recent years as our imaging technology has improved.

3.    Torus Color and Brightness:  Sometimes the HII and OIII signals in the torus region are both strong, leading to a thick white torus.  Other times one of the signals dominates, leading to a dimmer but more colorful torus.

4.    Age:  Younger PN are believed to have more detailed and intricate inner region patterns, which spread out and become more diffuse with age.

5.    Distance:  PNe distance from us can vary by a factor of 10 or more. We only see the brighter portions, such as the torus and rim regions, of PNe which are far away.

6.    Lobe Breakthrough: The ends of the lobes may break out as they expand over time.  The hourglass shape of the side view than looks completely different.

7.    Multi-polar evolution:  As stated above, often the binary star systems axis alignment changes over time, resulting in multiple directions of outflow and a more confusing (but more beautiful) multi-polar shape.

8.    Binary star configuration:  The orbital separation, speed, and mass of the progenitor pair determines the resulting PN shape.  Over time, these variables can change, leading to different PN shapes over time.

9.    Artifacts:  These tiny, bright objects are difficult to process.  Often, artifacts are introduced when we become too eager to bring out the color/detail and to suppress the noise.

10.   Classification:  Because of all of the above, PNe have been misclassified more often than any other type of DSO.  Many PNe start out (or end up) as galaxies, emission nebula, and HII regions, until our imaging technology gives us a better view to understand the true nature of the object.


For the above reasons, planetary nebulae are, in my opinion, the most difficult and complex of the DSOs to understand.  And also the most interesting and beautiful to examine.   I eagerly anticipate more scientific discoveries in this area in the coming years to improve our understanding of these wonderful objects.

If you would like to access all my DSO compilation posters, please click here.

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Planetary Nebulae Morphology II, Gary Imm