Talk:Planetary nebulae
Some of my remarks have not been dealt with, but since the 2nd reviewer deleted all my comments, this may the cause of that. I just repeat them here. The original report can be found in the history section.
Objects in this transition period, known as “proto-planetary nebulae”....
I tried to add a sentence regarding the possible confusion with proto-planetary disks surrounding young stars and could not find a place to insert without interrupting the logical flow of the paragraph. Since there is no allowance for footnotes, I prefer just to omit this reference.
We need to warn the reader about the possible confusion with planet forming disks around young stars.
Computer modeling (Steffen & Schoenberner 2006) has demonstrated that these multiple shell structures are the dynamical consequence of interacting winds (see previous section).
This is partly true. Some of the shells are entirely due to photo-ionization dynamic effects, at least that is what comes out of the models (done well before 2006 by Marten & Schönberner and myself). To avoid the term photo-ionization, one could say that some are caused by the gradual heating due to stellar radiation.
Sentence added.
How these molecules are made and what effect they have (e.g., on the solar system) from being distributed throughout the Galaxy are topics of high current interest.
It is still not mentioned that PNe contribute to the spread of heavy elements. The current text suggest that molecules from PNe were involved in the formation of our solar system. There is some ndication that dust from AGB/PN ended up in the mix that made our solar system, but I do not think that this claim can be made for individual molecules.
Sentence added at the beginning of paragraph and some wording change in the paragraph.
I am not entirely clear on the intended audience of this article, but I will assume that it is for general use. This being the case I have tried to examine the article and see where more clarification of the basics would be a good thing, so that a general reader who comes across this article on the internet does not get wrong impressions. I suspect that I am not going to catch all of these by any means.
I would like the first two sentences to be revised a bit to something like the following:
Planetary Nebulae are astronomical objects made up of primarily gaseous material. Many of these can be observed through telescopes where they are seen as a fuzzy, extended nebula (Latin for "cloud"), often with a star visible in the center. When the nebulae were discovered it was soon recognized that the star was associated with the nebula, and we now know that the nebula is glowing gas which is illuminated by radiation from the star.
The point is to make it clear that the star is producing the nebula but that we do not always see the star in the images. The first part is stated later in the article, but it is so fundamental that I think it should be up near the top. This is a matter of taste of course. Trying to get a bit of the fundamentals in early is in my view a good thing.
In the second paragraph it is stated that the name planetary nebula was coined because they look similar to the greenish disks of planets Uranus and Neptune. But all the pictures shown are the usual highly colourful ones that have little or no relation to the actual colour of the nebula in those cases where one can actually see any colour at all. It would seem helpful to either (a) try to show an image of a planetary nebula in the correct colour or (b) make it clear that the colours in Figures 1, 3, and 4 are highly artificial, so people do not get confused. As nice as the HST pictures are, I think that we need to make it clear that planetary nebulae are not actually blazing red and green or the like. Sometimes the colour schemes used are quite strange, chosen to highlight some aspects of the structure. Unless this is made very clear people just assume that the colours they see are the real ones of the nebula.
When the high temperatures of some of these stars is discussed in the "Physical and spectral properties of planetary nebulae" section it would be useful to mention that when the star is hot it is optically faint and so for these planetary nebulae no star may be visible even though we infer that the star is there in the center. The picture of NGC 2346 in Figure 3 shows no star, for example.
Also in that section in the next paragraph it is remarked that the spectra of planetary nebulae are distinctly different than those of stars. Why not show examples? I have uploaded two figures that may be useful here, the first a simulated solar spectrum and the second a simulated PN prism spectrum.
(Note that both of these images were created by me and are therefore mine to donate). Feel free to use these if you like. No credit to me is needed.
In this paragraph near the end it says "atomic lines that the generally forbidden under terrestrial conditions are possible in the nebulae". While this is probably intended to more or less echo the term "forbidden lines" it may be more clear for the public at large to say something about the lines observed in planetary nebulae being suppressed under high density conditions as in the laboratory on earth but which can be produced in the low density conditions of space. Otherwise people may think that the physics is different in space as opposed to on Earth since the lines are forbidden here but allowed out in space. Then the term "forbidden lines" could be in quotes in the last sentence to indicate it as terminology.
In the last paragraph of this section a "million-degree bubble" is mentioned. I would add "of extremely low density gas" here.
Where the morphology is being discussed there is mention of a "fast, collimated stellar wind". It might be better to say "high speed" or "high velocity" in place of "fast". Fast might mean "of brief duration in time". Similarly where the "transition to bipolar form" is mentioned it is implicitly being assumed that the reader knows that the circumstellar shell starts out close to spherical and then this bipolar structure develops. This needs to be stated explicitly.
I would hope that the term "asymptotic giant branch" is cross-referenced to some article where this is defined. If not it is better to avoid using this terminology. Similarly when the reference to the work by Paczynski is given and it is stated that the stars generate energy by hydrogen shell burning in a shell above the core some link needs to be made to an article on stellar evolution or stellar nucleosynthesis...and I would suggest explicitly saying that the core is He from the previous phase of nuclear reactions.
I would rotate the image in Figure 5 to the normal orientation of N up and E at left. Some image like this comes up for this object in google sky (I believe) in the correct orientation so it should be done properly here too. Aside from Figure 1 I do not know off hand whether the other images are in the correct orientation, but if not I think they should be, People will be using Google Sky or desktop planetarium programs and may see images of these objects there. I think we should orient them properly. It does not take too much work to do this and match with a black background to make them square again.
In Figure 5 I would like to see the caption state that the dark stuff in the image is dust in and around the nebula silhouetted against the gas. Why not be explicit?
I think the article generally reads well and is at a good level. There seem to be a few small issues of grammar here and there in the text, mostly inconsistent usage of singular and plural forms in a few places.
I tried to add a sentence regarding the possible confusion with proto-planetary disks surrounding young stars and could not find a place to insert without interrupting the logical flow of the paragraph. Since there is no allowance for footnotes, I prefer just to omit this reference.
