Talk:Crab Nebula/Archive 1

FA discussion
Wow, this "FA" was produced without a single word of discussion on the article talk page. Well, let's see if we can get some discussion going on here after the fact. In the first place, Worldtraveller, could you please address my last question on the FAC: "All right, so you're telling me this says everything there is to be said about the Crab Nebula that is even remotely notable (by my definition)? What about these "Crab studies" you referred to?" Now, the point I was raising was that this article is fairly short by general FA standards, including stuff that there isn't a whole lot to say about in the first place; but when you take into account that this has "Crab studies" associated with it&mdash;and look at the refs!&mdash;you really get the overwhelming impression that what we have here is more like a Britannica summary piece than a comprehensive WP FA. Everyking 06:18, 22 May 2006 (UTC)


 * I don't know what your definition of 'even remotely notable' is. I have no idea what you think is missing.  I have a PhD in astronomy, as does ALoan who commented on the nominations page, and I have done my best to write a concise, interesting, detailed article about the object.  If I thought it wasn't comprehensive, I wouldn't have nominated it.  Just tell me what you think needs to be included, please!  Then I could either include it, or tell you why I think it wouldn't be worth including.


 * As for Britannica, they have 249 words. The Encyclopedia of Astrobiology, Astronomy & Spaceflight has 634.  This article has 2,689.  Worldtraveller 09:33, 22 May 2006 (UTC)

See, "concise". That's the scary word. When I'm hoping for a longer article, that isn't a good sign.

I'll go paragraph by paragraph; since this is FA now I guess it's important enough to be worth the trouble. I'll skip the intro because it looks good to me.

"First observed in 1731 by John Bevis, the nebula was independently rediscovered by Charles Messier in 1758, while he was observing a bright comet visible in that year. Messier catalogued it as the first entry in his catalogue of comet-like objects. The Earl of Rosse observed the nebula with his 36- and 72-inch metal reflecting telescopes at Birr Castle in the 1840s, and referred to the object as the Crab Nebula because a drawing he made of it looked like a crab."

A) Where is this information from? There are no citations in the paragraph.

B) Is there anything else to say about this information? Don't worry about whether it's notable, just throw out any info you know that's pertinent to this paragraph that isn't presently included, and then we'll discuss it. Everyking 11:44, 22 May 2006 (UTC)


 * 'Concise' means saying everything that needs to be said in as few words as possible. There is not anything further to say about the first paragraph, no.  Why are you hoping for a longer article?  For heaven's sake just tell me what else you want to see.  I'm not going to conduct a second FA discussion here, or fill the article with random shit, just to satisfy your illogical craving for a higher word count.  I'll add a citation for that paragraph though.  Worldtraveller 11:56, 22 May 2006 (UTC)
 * OK, I'll break it down into smaller fragments. Surely there is something more to say about Bevis's observation? What I "want to see", I guess, is a little more detail added to this 1731 discovery, for starters. Everyking 12:09, 22 May 2006 (UTC)
 * You clearly don't have a clue whether there actually is more to say or not, so your demands for more are not meaningful. I believe this article is comprehensive, I worked hard to make it so, and if you don't agree then the onus is on you to do the research and make it so.  Worldtraveller 12:19, 22 May 2006 (UTC)
 * All right, I'm considering this. It needs to be done with all your FAs, actually. But if I go to the trouble to research this and expand the article, you're not going to revert me, are you? Everyking 03:45, 23 May 2006 (UTC)

OK, a minute or two of looking and I found this: Bevis "added it to his sky atlas, Uranographia Britannica..." All right, so I present that to the expert and now he can tell me his opinion on the inclusion of it. Everyking 09:26, 23 May 2006 (UTC)


 * Experts have already reviewed this article and not found it wanting. I am not going to conduct a sentence-by-sentence analysis of it - I'm working on bringing other articles up to featured standards.  Worldtraveller 10:33, 23 May 2006 (UTC)


 * This is a wiki: add it in, if you want to. For my money, it is a pretty boring fact - he discovered the Crab in 1731, and getting on for 20 years later, he included his discovery in his sky atlas (it is not as if he would leave it out, is it).


 * Wouldn't it be more useful to expand the short article on John Bevis or indeed write an article on Uranographia Britannica - it is very rare, you know - only 16 copies are known. -- ALoan (Talk) 11:12, 23 May 2006 (UTC)

Just wanted to say: great article. I especially like how there is no superfluous nonsense in it. Everything is interesting and to the point! --24.82.175.172 01:21, 14 June 2006 (UTC)

Um. I see this: "all of which was written here is totally wrong, the crab nebula is actually earth, this is all just a large and extravagant hoax." at the top of the article; however, I can't find it in the text to remove it. Bizarrely, it appears to be in all past revisions, even though I know that it wasn't there before. I'm stumped. Everyking 04:12, 14 June 2006 (UTC)


 * It's gone now. No idea. Everyking 04:16, 14 June 2006 (UTC)

I saw the same. Now it's gone again. No trace of it in the history either. I wonder how it was done and how it was reverted.Lukas 04:17, 14 June 2006 (UTC)

Origins
The article states that "Tracing the expansion back revealed that the nebula must have formed about 900 years ago", but earlier in the article it says the nebula is "Located at a distance of about 6,300 light years (ly) (1.93 kpc) from Earth". Does this mean that the nebula actually formed 7,200 years ago as the expansion was observed from earth 900 years ago and the light would have taken 6,300 year to reach those observers on earth.WhizzBang 09:22, 14 June 2006 (UTC)
 * The text should be changed to something like "Tracing the expansion back revealed that the nebula, as seen from Earth, must have formed about 900 years ago;". However, that can wait until the article vanishes from the front page. Friendly Neighbour 19:55, 14 June 2006 (UTC)
 * I changed it back because stating "as observed from Earth" is superflous; it's like seeing a supernova now and saying 'OH! Look, a supernova just went off X years ago!'.  Its simply not done in the astronomical community.  All times in "ago"s are given with the implicit knowledge that its referring to our observation point.  Stating Earth as our reference point is borderline redundancy. Tigerhawkvok 06:12, 16 June 2006 (UTC)
 * I believe you're right that it's superfluous in papers or course books on astronomy. I'm not so sure about a popular science text like an encyclopedia article. The question above is for me a sign that the text you reverted was needed. Friendly Neighbour 07:38, 16 June 2006 (UTC)

Shock Fallous
Well that surprised me....I didn't know the Carb Nebula looked like that... I think perhaps I'll remove that! --Silex 15:39, 14 June 2006 (UTC)

Shockus Fallousus
Why oh why, wasn't the featured article locked for a day or two, for the wan^H^H^Hidiots? —The preceding unsigned comment was added by 165.146.128.240 (talk • contribs).

Diameter
The article states the diameter as 6 ly and 11 ly in two different locations. Anyone know the accepted value? Beefcalf 20:32, 14 June 2006 (UTC)


 * I noticed that the article says that the diameter is 11 and the radius is 3. I don't know what is correct but that needs to be changed.Opal56 00:56, 26 December 2006 (UTC)

1054 vs. 1731
Wording in the introduction paragraph: It says that it was observed in 1731 by Bevis but then it says that it was recorded by Chinese and Arab astronomers in 1054, which is earlier than that. I think it means that it was recorded as a phenomenon by Chinese and Arab scholars, but that Bevis was the one who identified it as a supernova. Is this the case? As it's written it's kind of confusing. --ColourBurst 03:21, 14 June 2006 (UTC)


 * I think it's correct as written. The 1054 observation was of light from the supernova explosion itself.  That phenomenon is very bright, but short-lived.  The 1731 observation is of the nebula, i.e. the supernova 'remnant' that is faint, diffuse, and long-lived.  They are two different phenomena with a common root cause (the supernova explosion).  -- Coneslayer 20:54, 14 June 2006 (UTC)

I'm tired of hitting blocks triggered by this denial of service idiot
can someone PLEASE sprotect this article?--152.163.100.200 02:16, 15 June 2006 (UTC)

Wording changes and content questions
Thanks to everyone who's worked on this article, and congrats for getting it up to FA quality. JesseW, the juggling janitor 18:05, 15 June 2006 (UTC)
 * "Tracing the expansion back revealed that the nebula must have formed about 900 years ago as observed from Earth (giving it a true age of 7,200 years)."
 * Why would the true age be different than the age as observed on Earth? It would be good to explain this further, or link to an appropriate article. JesseW, the juggling janitor 18:05, 15 June 2006 (UTC)
 * The distance to the nebula of 6,300 light years means that light from it takes that long to get here, so if we saw the explosion 900 years ago then if you were near the nebula you'd have seen it 7,200 years ago. Personally I'd prefer to drop everything after '900 years ago', because it just gets confusing.  The extra stuff was added while it was on the main page.  Worldtraveller 18:41, 15 June 2006 (UTC)
 * I did revert it back, as I explain below. Its simple redundancy and borderline inaccurate when taken in context with standard astrophysical phraseology.  Imagine how confusing everything would be if we used two "ago"s for everything!  Just one "ago" is sufficient, and you provide distance.  It is understood that the "ago" is from our only feasible reference frame: Earth.  Tigerhawkvok 06:17, 16 June 2006 (UTC)
 * Sorry for not responding earlier; it's taken me a while to try and formulate a full response here. As for this point, it seems to be resolved, Tigerhawkvok has removed the problematic bit. JesseW, the juggling janitor 06:18, 21 June 2006 (UTC)
 * The Crab Nebula is currently expanding outwards at about 1,500 km/s. Images taken several years apart reveal the slow expansion of the nebula, and by comparing this angular expansion with its spectroscopically-determined expansion velocity, the nebula's distance can be estimated. Modern observations give a distance to the nebula of about 6,300 ly, meaning that it is about 11 ly in length.
 * This paragraph, is, at least to me, unclear. It begins by stating a velocity (1,500 km/s), without explaining how it was arrived at.  Then it (partly) explains how the velocity given above was found (I'm assuming that "Images taken several years apart reveal the slow expansion of the nebula" means that - "When scientists compared the size of the nebula shown in photographs taken over a number of years, the rate of expansion was 1,500 km/s." If this is wrong, the article should be clarified. Even if my interpretation is correct, it would be better to say this explicitly.)  Next, the paragraph brings up another velocity measure, again without fully explaining how it is derived (although it does contain a link, which is good).  I don't understand, and the paragraph doesn't explain, how comparing two velocity measures can produce a distence from earth estimate.  This should be explained, or an explanation should be linked to.  Then a number for the length of (I presume) the nebula is given - with no explanation of how it was derivied (I presume) from the distence value given just before it.  I don't even know exactly what "length" would mean in relation to a nebula.  I attempted to rephrase the paragraph to clarify this, but apparently didn't do it correctly (as it was reverted by Worldtraveller); I look forward to working with  the other editors on this page to fix up this paragraph so it can't be so misunderstood again. JesseW, the juggling janitor 18:05, 15 June 2006 (UTC)
 * The 1500km/s is measured spectroscopically, and is along the line of sight; what photographs reveal is an angular expansion, perpendicular to the line of sight. Assume the nebula is expanding uniformly, and you can work out the distance from simple trigonometry.  Then, once you know the distance you automatically know the actual size from the angular size, again from simple trigonometry.  To me the paragraph seems to explain this quite well, but I would say that because I wrote it.  The only reason I changed your wording was because it seemed to say much the same as mine, but used a past tense ('when images were examined...') which I didn't like as images were not examined at any one particular time in the past.  Worldtraveller 18:41, 15 June 2006 (UTC)
 * Generally, the rule of thumb is to think "How can I use the pythagorean theorem to figure this out?" There's not much clarification or phraseology change can do if you don't know that, I think....
 * I'm still confused by this paragraph (although that's hardly anyone's problem but mine), but one thing that certainly would help would be to specify the type of spectrographic measurement that is referred to in the article. According to (my reading of) the redshift article, there are a number of different types of measurements.  We should specify which one (or ones) are being cited here. JesseW, the juggling janitor 07:08, 21 June 2006 (UTC)
 * At the centre of the Crab Nebula are two faint stars, one of which was identified as the star responsible for the nebula as long ago as 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual.
 * This phrasing leaves open the question of wheather the star identified in 1942 as the progenitor star (I presume) still is identified as that. The most important point that this sentence should make is what the is thought to be the case now, not what "was" the case at some period in the past.  I proposed rephrasing the sentence to be: "At the centre of the Crab Nebula are two faint stars, one of which is the star responsible for existance of the nebula. It has been identified as such since 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual.", which makes the strong, clear statement of what the scientific consensus is, now (I presume), rather than the much weaker claim of what it was in 1942. JesseW, the juggling janitor 18:05, 15 June 2006 (UTC)
 * Although I don't see the ambiguity, myself, I don't object to your proposed wording, except I would want to replace 'has been...since' with 'was...in' - and exist'a'nce is spelt existence :) Worldtraveller 18:41, 15 June 2006 (UTC)
 * OK, I'll apply my proposed wording, with your amendments. JesseW, the juggling janitor 06:18, 21 June 2006 (UTC)
 * The extremely high level of the pulsar's energy output creates a dynamic region at the centre of the Crab Nebula.
 * I proposed changing "dynamic region" to "region of remarkably quick change". Dynamic is not an exiciting word; it is a utilitarian word, and in some areas, a buzzword - it is no kind of signal - "This is interesting!"  Readers (at least, in my case) are likely to skip over it, and miss the undoubtably interesting and unique facts in the rest of the paragraph: "While most astronomical objects evolve so slowly that changes are visible only over timescales of many years, the inner parts of the Crab show changes over timescales of only a few days."  - Changes over a few days, rather than "many years"?  This is a major, unusual, exciting aspect of the Crab Nebula - it shouldn't be hidden behind a topic sentence that mis-descirbes it as merely "dynamic".  Pardon the vehamence, but it just seems a mistake. JesseW, the juggling janitor 18:05, 15 June 2006 (UTC)
 * Well, to me 'dynamic' sounds pretty exciting! 'Remarkably' sounds somewhat like an opinion to me.  How about 'highly dynamic'?  Maybe 'The pulsar's extreme energy output creates a highly dynamic region...' - a bit more concise, maybe a bit more lively.
 * Dynamic also has some technical implications. "Dynamic" is used as in, not "static".  Its not a matter of WC. Tigerhawkvok 06:05, 16 June 2006 (UTC)
 * How'bout "The pulsar's extreme energy output creates a unusually dynamic region..." - I think the unusual quality is what I want to hightlight, so that solves the problem. I'll put this in, feel free to revert if you want to discuss it further. JesseW, the juggling janitor 06:18, 21 June 2006 (UTC)
 * Thanks for your input! Worldtraveller 18:41, 15 June 2006 (UTC)

Not an SNR?
To the anon who says that Crab isn't an SNR: care to give us a link to your review paper? These abstracts on ADS seem to indicate that Pulsar Wind Nebulae are considered a TYPE of SNR, albeit one distinctly different than a shell SNR. It sounds like you are just arguing semantics.    -- Etacar11   13:02, 19 September 2006 (UTC)

Not a SNR indeed
I'm happy to use this forum to explain further my concerns regarding the classification of the Crab Nebula as a supernova remnant. A supernova remnant is the result of a supernova explosion, of course. However, what you are seeing in the Crab is NOT the result of a supernova explosion, but the result of injection of particles and magnetic fields by a central pulsar, forming a pulsar wind nebula. If the supernova of 1054 had not formed an active pulsar, there would be nothing to see here. Conversely, if the pulsar had formed through some other mechanism besides core-collapse, the Crab Nebula would still be there, largely the same, except for the swept-up filaments on the outer edges.

Since the nebula is powered by the pulsar and does not owe its properties to the supernova explosion, it clearly is not a supernova remnant. The supernova remnant formed by the explosion of 1054 is invisible and somewhere further out, beyond the boundary of the Crab Nebula. See Seward et al., 2006, ApJ, 636, 873 for a good discussion of this.

It is certainly true that most literature (especially on the WWW rather than in the refereed literature) refers to the Crab as a supernova remnant. However, this is a historical accident, in that it was identified as an expanding optical nebula coincident with a supernova explosion, along with other apparently similar objects. It was only many decades later, with the advent of multi-wavelength astronomy and the discovery of pulsars, that it was realized that the Crab is an object with fundamentally different physical properties.

The bottom line is that a supernova remnant's energy and emissions come from a supernova explosion, while the Crab's considerable energy output entirely results from the rotation of its central pulsar. These are distinct processes - the latter is a "plerion" or "pulsar wind nebula", not a supernova remnant. Many people still refer to the "Crab supernova remnant", but that reflects historical usage, not scientific accuracy.

Hope this detailed explanation explains my argument. —The preceding unsigned comment was added by 211.31.86.254 (talk • contribs).


 * Not to me. To quote from Seward et al:
 * the Crab Nebula remains unique or, more accurately, peculiar when compared with other supernova remnants - so there in the first sentence they are saying it's a supernova remnant.
 * The bulk of the emission comes from the surrounding pulsar-wind nebula (PWN or synchrotron nebula) which is � 2′ in diameter...The PWN is surrounded by a 5′ ×7′ optical nebula comprising an array of He-rich filaments moving outwards with velocities of 1000-1500 km s−1 - aha! Wouldn't that be the remnants of the star destroyed in the explosion?  Doesn't the article already say all this?  Isn't it a little odd to insist that the nebula is not an SNR, when all available sources including one which you claimed says the opposite, all agree that it is an SNR?  81.179.243.112 00:30, 20 September 2006 (UTC)

As I said above, the issue at hand is one of scientific accuracy rather than historical usage.; even if most people still refer to the "Crab supernova remnant", this doesn't make it correct. However you choose to define a supernova remnant, the Crab Nebula doesn't pass this definition. To quote Wikipedia's entry for supernova remnant:
 * A supernova remnant (SNR) is the structure resulting from the gigantic explosion of a star in a supernova.

Since the Crab Nebula is not the result of a supernova explosion, but rather results from the injection of energy from a central pulsar, how can it be a supernova remnant? Either the Crab Nebula is not a SNR as I have argued above, or the definition of supernova remnant in Wikipedia is wrong and needs to be substantially amended.

As you quote from Seward et al, there are helium-rich filaments moving outwards. Indeed this is material ejected by the original supernova explosion. However, there are two problems with calling this a "supernova remnant". First, if one continues with the definition of a supernova remnant on Wikipedia, it says that a SNR ...
 * consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.

So if the Crab Nebula is a SNR, where is the interstellar material, as is seen in Cassiopeia A, Tycho, Kepler, and every other SNR? The answer is that this material has not interacted with any interstellar material, because it is still inside a larger invisible supernova remnant. Second, the filaments are a negligible part of the energy and luminosity of the Crab Nebula - the reason why the Crab is the most studied object in astronomy is because of its extreme synchrotron luminosity across the entire electromagnetic spectrum, resulting from the energy injection of the central pulsar. It is well established that the total mass in the filaments is minuscule, and cannot possibly correspond to the bulk of ejecta from the explosion (which must be farther out).

To put things another way, if we eventually discover the faint outer shell at much larger distances from the central pulsar, what will we call it? It will certainly meet the definition of a supernova remnant. But if that outer shell is the supernova remnant, where does that leave the Crab? In your proposed definition, the supernova of 1054 would then have left two supernova remnants: the Crab Nebula, and the surrounding outer shell. This doesn't make much sense to me. However, if the Crab Nebula is a pulsar wind nebula (which every paper in the modern literature agrees with), then it all makes sense.

Let me provide one further example. As stated above, the Crab Nebula is a bubble of energetic particles powered by a central pulsar. Historical usage calls it a supernova remnant mainly because it is coincident with the supernova of 1054. However, there are many other objects that look extremely similar to the Crab Nebula, but which are produced by pulsars which are billions of years old, long after any original explosion has completely dissipated. Are these supernova remnants? Of course not; they are pulsar wind nebulae (sometimes called "plerions"). But by the current definition, they must be supernova remnants also, which doesn't make sense.

The bottom line is that to resolve this issue, one must carefully look at physics and definitions, not just at what WWW pages and journal papers say. I reiterate that the reason why most papers talk about the "Crab supernova remnant" is historical usage, but this is factually incorrect. I suggest that the best possible entry for this article should say that many people say that the Crab is a SNR, but explain why this is not actually right. I await your thoughts on this. —The preceding unsigned comment was added by 211.31.86.254 (talk • contribs).


 * But we are not here to carry out original research. All the references in this article support the description of the Crab as a supernova remnant.  Your suggestion that we ignore what journal papers say and take your word for it instead is just not realistic.  128.40.1.175 16:58, 25 September 2006 (UTC)

Here is a detailed quote from the literature which I hope provides support for the above arguments. From Gaensler & Slane, "The Evolution and Structure of Pulsar Wind Nebulae", published in September 2006 (see http://arxiv.org/abs/astro-ph/0601081):


 * The Crab Nebula is almost certainly associated with a supernova (SN) explosion observed in 1054 CE. However, this source differs substantially from what is now seen at the sites of other recent SNe, in that the Crab Nebula is centrally filled at all wavelengths, while sources such as Tycho's and Kepler's supernova remnants (SNRs) show a shell morphology. This and other simple observations show that the Crab Nebula is anomalous, its energetics dominated by continuous injectionof magnetic fields and relativistic particles from a central source.




 * Observations over the last several decades have identified 40 to 50 further sources, in both our own Galaxy and in the Magellanic Clouds, with properties similar to those of the Crab Nebula - these sources are known as "pulsar wind nebulae" (PWNe). Sometimes a PWN is surrounded by a shell-like SNR, and the system is termed "composite". In other cases, best typified by the Crab itself, no surrounding shell is seen.

It thus seems pretty clear that the Crab Nebula is the prototype of a separate class of objects, pulsar wind nebulae, and is not a supernova remnant. —The preceding unsigned comment was added by 211.31.86.254 (talk • contribs) and expanded by 130.155.193.54 (talk • contribs).

Please sign your edits on Talk pages with four tildes so we know who says what and which edit starts and stops where. It is really rude towards the reader not to sign them. And users 130.155.193.54 and 211.31.86.254, if you really are one person, please consider setting up an account so that we know you are not vandalizing each other's edits. It will take you less time than I waste on finding and marking who said what. Friendly Neighbour 07:16, 21 September 2006 (UTC)


 * Might I respectfully ask 128.40.1.175 what your credentials are for asserting your knowledge of the Crab Nebula? You are definitive in your rebuttals of my (previously anonymous) comments, but it would be helpful if you can provide some background to bolster your claims. I am happy to do the same if the arguments above are considered not sufficient. Tubbs334 17:02, 25 September 2006 (UTC)


 * I don't really think my credentials are the point. The sources cited by this article simply don't support your claims.  You haven't cited peer-reviewed articles which say 'the crab nebula is not an SNR' - in fact, the source you quoted earlier (Seward et al) clearly states that the crab is an SNR.  Our credentials are immaterial, as all we are doing is documenting the existing body of knowledge on the Crab in an encyclopaedic fashion, rather than carrying out any original research.  128.40.1.175 17:37, 25 September 2006 (UTC)


 * Striving for a consistent definition is distinct from carrying out original research. As discussed above, the Crab Nebula does not have properties covered by the definition of supernova remnant. So either the definition of supernova remnant on Wikipedia needs to be substantially revised, or the Crab Nebula is not a supernova remnant. Please rebut this point if you disagree with it.

As mentioned (several times) earlier, most papers say "the Crab is a SNR" because this is historical usage, reflecting a time when very few SNRs were known, and it was not yet realized that the Crab was different. If you ask any person working in the field (and over the last few days I have taken the liberty of polling several experts on the Crab Nebula), you will find close to uniform agreement that the Crab Nebula is a pulsar wind nebula, not a supernova remnant. If the people who work in this field all agree on this point, it doesn't seem tenable to me to retain incorrect information on the Wikipedia entry.

Experts are encouraged to contribute their knowledge to this project. Having worked intensely on this and other objects like it for many years, I would like to think I know this topic and can contribute accordingly. If you too are an active researcher on pulsar wind nebulae and supernova remnants, we should take this off-line and try to iterate on these points. But if you are rather repeating what you have read, then I don't really know what to say further, expect that what you have read is unfortunately incorrect. Tubbs334 20:05, 25 September 2006 (UTC)


 * I warned you both on your talk pages that you are one revert from breaking WP:3RR. Further warfare has to wait some 16 hours if you do not want to get blocked.
 * Anyway, I believe we should make a compromise version. I propose something closer to the original version but with a sentence added about "more precise description of the phenomenon is...". What do you think? Friendly Neighbour 20:25, 25 September 2006 (UTC)

OK. I am mostly responsible for this article. I no longer write articles here but I feel I must defend my work on this article. I researched it thoroughly, and nowhere in any paper I read did I find any suggestion that the Crab is not a supernova remnant. Here are some example quotes from recent papers about the Crab:
 * The AD 1054 supernova remnant, Crab Nebula... - Beiger & Haensel, 2003
 * The presence of hydrogen in its filaments technically classifies the Crab Nebula as a Type II supernova remnant - Fesen, Shull & Hurford, (1997)
 * The very hard (flat) radio spectra in plerions (i.e., filled-center supernova remnants) like the Crab - Temim et al 2006
 * the Crab Nebula remains unique or, more accurately, peculiar when compared with other supernova remnants. - Seward, Gorenstein and Smith, 2005
 * The Crab nebula (=G184·6−5·8), the remnant of the SN of AD 1054 (e.g. Stephenson & Green 2002), shows a centrally brightened morphology, and it is the best known of the class of ‘filled-centre’ supernova remnants - Green, Tuffs and Popescu, 2004

And here's some websites:
 * http://imagine.gsfc.nasa.gov/docs/science/know_l2/supernova_remnants.html
 * http://www.seds.org/messier/snr.html
 * http://chandra.harvard.edu/xray_sources/supernovas2.html
 * http://www.mrao.cam.ac.uk/surveys/snrs/snrs.data.html
 * http://www.astro.psu.edu/xray/snr/snr.html

Many of these point out what is already mentioned in this article; that the energy which powers the synchotron emission comes from the pulsar. All of them confirm that the Crab is indeed a supernova remnant. I do not see where there is any room for confusion. The Crab is a supernova remnant, much of the emission from which is powered by the pulsar. These two statements are not contradictory, and the article already describes the situation. Worldtraveller 22:38, 25 September 2006 (UTC)


 * The definition of a supernova remnant on Wikipedia (which to me is quite correct) says:


 * A supernova remnant (SNR) is the structure resulting from the gigantic explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.

This does not describe the Crab Nebula. First, the structure does not result from an explosion; even if there hadn't been a supernova in 1054, but the pulsar was formed through some more exotic mechanism, the Crab Nebula would still look much the same as it does now. Second, the Crab Nebula does not consist of ejected material; it has a very small ejecta component, but most of it is synchrotron emission unrelated to the supernova. Finally, there is no interstellar material swept up and shocked along the way.

If the Crab does not meet any of these criteria, how can it be a SNR?

Worldtraveller's point is that many WWW sites and articles talk about the Crab supernova remnant. However, as I have pointed out multiple times above, this is historical usage, but is factually incorrect. It is fine by me, as Friendly Neighbour suggests, to note this confusion on the WWW site, but the true situation needs also to be noted.

If many many sites say that the Crab is a SNR, there are two possibilities. Either the Crab is a SNR, or it is a common but incorrect misconception that the Crab is a SNR. No amount of WWW surfing or reading can distinguish between these two possibilities. To break this deadlock needs the attention of an expert. Without tooting my horn, I am an active and prolific researcher on this topic, and have discussed this in the last few days with several of my peers. The consensus is clear - common usage that we all sometimes slip into is to refer to the "Crab SNR". But when asked to justify this, we will all readily admit that the Crab is not really a SNR, but rather is a pulsar wind nebula.

Or to put things another way, if the Crab is a SNR, then what is the entry in Wikipedia for pulsar wind nebulae all about? The Crab is certainly a puslar wind nebula. So shouldn't this article just redirect to "supernova remnant"? Or is the Crab two different distinct objects at once? None of these possibilites is tenable, so the only remaining possibility is that the Crab is not a SNR. Tubbs334 02:18, 26 September 2006 (UTC)
 * Tubbs; first of all, thanks for taking the time to post here. You have stated that the current consensus of expert opinion on this is that the Crab is a PWN, not a SNR, and that this has changed recently. Could you provide some examples of papers that state that?  If you provided two papers that made this claim, and they were more recent than the papers claiming otherwise, that would seem to strongly support your position.  While a poll of your peers is certainly useful, it's not possible for others to verify this; citations of papers are.  If this is not feasible for some reason, please let us know why. Thanks again. JesseW, the juggling janitor 04:18, 26 September 2006 (UTC)
 * I'm really mystified by this. The Crab does result from a supernova explosion.  Who seriously denies this?  A link to any paper or any website that says that the Crab is not an SNR is essential if your edits are to be taken seriously.  In the face of mountains of peer-reviewed journals which say that the Crab is an SNR should we a) describe it as an SNR or b) believe that all the researchers who've written recent papers on the Crab don't actually know what it is, because someone we don't know anything about says so?  Very few people consider that adopting option B is a good way to write a reliable encyclopaedia.  You can argue the semantics here for as long as you like but this provides no evidence that you're right.  Verifiable sources are what we need and if you can't provide them then sorry, there's no way we can change an article so it flies in the face of all the references it actually cites.  Worldtraveller 09:38, 26 September 2006 (UTC)


 * OK, I can see your point, and will try and build consensus. Three recent papers are as follows:

"Ground-based gamma-ray observations of pulsars and their nebulae" (2005) by de Jager & Venter, http://arxiv.org/abs/astro-ph?papernum=0511098 , starts off in Section 1 by defining a pulsar wind nebula. On p2, they say:


 * "The Crab Nebula is considered to be the most important prototype pulsar wind nebula"

A second paper is "Pulsar wind nebula in EGRET error boxes" by Roberts et al (none of the authors are in common with the previous paper), published in 2004, and available without a site license from http://arxiv.org/abs/astro-ph/0409104. On the second page, this has a table of "pulsar wind nebulae coincident with EGRET sources", of which the Crab Nebula is listed as the second entry.

A third paper is "Near-Infrared, Kilosecond Variability of the Wisps and Jet in the Crab Pulsar Wind Nebula" by Melatos et al, 2005 (license-free version at http://arxiv.org/abs/astro-ph/0509914). Apart from the title which implies that the Crab is a PWN and not a SNR (the term "Crab PWN" is repeated throughout the abstract and main text), they begin by saying:


 * "Many young pulsars in supernova remnants are embedded in synchrotron nebulae known as pulsar wind nebulae."

implying that a supernova remnant and a pulsar wind nebula are distinct objects.

Finally for now, "The Crab Nebula's Wisps in Radio and Optical" by Bietenholz et al, 2004, see http://arxiv.org/abs/astro-ph/0408061, begins by saying:


 * "The Crab Nebula is the most easily accessible pulsar [wind] nebula"

So there are four examples, all in the recent literature, all clearly indicating that the Crab Nebula is a pulsar wind nebula rather than a supernova remnant.

How does that sit with the community?

Sincerely, Tubbs334 04:36, 29 September 2006 (UTC) q


 * Our article makes clear that energy from the pulsar powers the synchrotron emission from the nebula and I would not object in the slightest if you were to explicitly mention pulsar wind nebulae at this section. But nowhere in any of the links you've given does it say that the crab is not an SNR.  PWNs and SNRs are not mutually exclusive, as demonstrated clearly by the line you quote from Melatos et al.  Both Bietenholz et al (2004) and Melatos et al (2005) give 'supernova remnants' among their keywords after the abstract, and the Crab is surely best defined as an SNR that contains a PWN.  Worldtraveller 06:47, 29 September 2006 (UTC)

It seems this discussion has long ended, and I am late to bed, but I wanted to comment in support of Worldtraveller.

I have to agree with Worldtraveller completely here. I do not see that a PWN is not a SNR. In fact, I would argue there is no reasonably plausible hypothesis that pulsars are not themselves "structures resulting from a supernova explosion", and therefore also meet the definition of a SNR. The neutron star (pulsar), material around it (whether directly from the former supergiant star or not) which is radiating from any of various mechanisms (all of which have to do with the fact that a supernova occurred there), may all be considered a SNR.

Sometimes people who specialize in something may find their terminology becomes more fine-grained to further differentiate classes of things. I think you may be using "SNR" to describe a specific kind of object, the whole class of which we (and I believe most astronomers!) are calling a "SNR". I suspect, Tubbs334, that you may have a personal interest in the crab nebula -not- being a SNR because you specialize in PWN and recognize the crab as the definitive, unique, object of the PWN class. However, your highly specific use of the term SNR is not, in my experience, the usual use of the term by professional astronomers.

We are not arguing about the nature of the object itself, I think, but only semantics, the terms used to describe it. I have no objection to the crab also being called a PWN, but in my mind a PWN is also a SNR. In the context of an encyclopedia for the masses, we should not change the definition of a reasonably well-understood term to meet one person's (perhaps highly specialized, and perhaps not entirely objective) understanding.

Out of curiosity, I wonder if there is a broader term Tubbs334 would use for "any detectable (directly or indirectly) matter and/or radiation indicating a past supernova explosion"? Myrrhlin 07:27, 15 February 2007 (UTC)

When was Crab Nebula recognized as Supernova
I added a sentence about CN being the first astronomical object recognized as supernova connected. The 1939 paper which made the connection its title is luckily available on-line (Mayall N.U. (1939), The Crab Nebula, a Probable Supernova, Astronomical Society of the Pacific Leaflets, v. 3, p.145). User:24.61.13.238 has recently claimed that the it was recognized in already in 1921 giving the following reference as evidence: Lundmark K. (1921), Suspected New Stars Recorded in Old Chronicles and Among Recent Meridian Observations, Publications of the Astronomical Society of the Pacific, v. 33, p.225. The paper is also available on-line. It does not use the word "supernova" (a category which was created by Walter Baade and Fritz Zwicky in their 1934 paper) and, more importantly, it does not identify the 1054 nova with any modern astronomical object. The paper lists the event in the table on pages 10-11 among many other historical novae and assigns it a "probability of the reality of the object" mark of 2 (suggesting the author was not convinced the nova existence was even certain). This is obviously not enough to claim that Lundmark recognized the event as connected to Crab Nebula (which is not even mentinoed in the article). User:24.61.13.238, could you at least research your edits instead of leaving the tedious task to others? Friendly Neighbour 07:40, 30 September 2006 (UTC)


 * Look again at the Lundmark paper. At the bottom of page 11, the footnote for the event of 1054 says "Near NGC 1952". So I would posit that the first tentative connection between SN 1054 and the Crab Nebula was made by Lundmark. Obviously Mayall spelt things out much more definitively, so including both references, as you have done, is fine by me. 24.61.13.238 15:38, 30 September 2006 (UTC)


 * Point for you but that's definitely not enough to say that he identified the nebula with the year 1054 "nova" (to say that he could have identified it with a "supernova" is anachroinic as no one knew at the time that such a separate category is even needed). Anyway, the actual identification of the nebula with any kind of stellar explosion could only be done after the large double (positive and negative) Doppler shift has been recorded. And that has been done in the late 1930s. Mayall paper is exactly about this discovery. Friendly Neighbour 16:00, 30 September 2006 (UTC)


 * I agree that Lundmark did not (and could not have) put this all together in 1921. But credit should be given (as Mayall indeed does) to the first person to make the possible connection between the event and the object, whether Lundmark fully understood what was going on or not.


 * Lucky guesses should still be recognized. Indeed Baade & Zwicky are rightly credited as proposing neutron stars as the aftermath of supernovae in one of their 1934 papers, even though they got some of the details wrong and couldn't possibly have known enough to properly predict this. And yet, their chutzpah is rightly recognized as the first place in which this idea appeared. Same goes here for our friend Knut.


 * Anyway, I think we have consensus on this; both papers should be cited.

24.61.13.238 16:45, 30 September 2006 (UTC)

small change in the section regarding Progenitor Star
It previously stated that Type Ia supernovas are a result of white dwarf stars exceeding the Chandrasekhar Limit, causing an explosion. This is incorrect. It is currently accepted that it does not EXCEED the limit. Instead, as it nears the Limit, it begins burning in a deflagration (which may or may not progress into a detonation). This is exactly the explanation you see if you follow the link for Type Ia supernovas. Bigchiefbc 01:37, 25 October 2006 (UTC)

Double Entry in Messier Objects Category
Do we really want this article listed twice in the Messier Objects category? Once under 'Crad Nebula' and once under 'M1'? My thought is no, just once under M1. WilliamKF 23:46, 15 November 2006 (UTC)

Lord Rosse's crab drawing
Can someone scan and upload Lord Rosse's "crab" drawing? That would be a nice addition to this article. &mdash; coe l acan t a lk  &mdash; 03:22, 25 December 2006 (UTC)