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Green in comets is not CN, cyanogen

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#1 freestar8n

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Posted 06 February 2013 - 04:44 AM

Hi-

This appears to be where comets are discussed most, so I thought I would post here one of my cometary pet peeves. Ever since, I think, around 2000, when comets come around and pictures show their green color, web sites of all sorts, including NASA, APOD, Spaceweather, and Bad Astronomy, point out that the green color is due to cyanogen, CN, in the comet. In fact, CN has several emission lines but mostly in the UV, and I don't know any in the green. I have not found a single reference pointing to a spectroscopic analysis of a bright green CN line in a comet. Instead they mainly appear to be related to molecular C2.

Sky and Telescope had a letter last summer pointing this out - thankfully - and I have contacted the other sources above about it, but they have not posted a correction and I expect they will continue broadcasting this misconception. I tried to contact Spaceweather recently, but their mailbox is always full.

So - points to Sky and Telescope for helping clarify this issue. I don't know how it started, but I guess a prominent web site somehow got it mixed up - and now everyone is just repeating it. Always good to know a primary source when you repeat something like that.

Comets have been known to carry "cyanogen" for over a hundred years, and comets have also been known to be green. But only recently did people start saying that the green is actually *due* to cyanogen. And this just does not appear to be true, and I haven't seen any source making this claim also cite a primary source for the info - though they may cite APOD or "NASA" for example.

So - spread the word - as these big (I hope) comets approach. I expect you will be hearing about the green cyanogen *a lot*.

Frank
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#2 David Knisely

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Posted 06 February 2013 - 05:35 AM

With all the comets I have seen, none have been as greenish as some of the brighter planetary nebulae (and they tend to be more of a bluish-green color). At most, comets are a sort of aqua color (bluish with hints of a bluish-green) visually rather than distinctly green. In any case, any bluish-green is indeed from the so-called "Swan Bands" of Carbon (C2). Cyanogen does have a broad strong violet emission line at around 3880 angstroms and a much weaker one at around 4200 angstroms, so that would be in the violet part of the spectrum. Nice to see the "Cyanogen" error cleared up a little. Clear skies to you.

#3 azure1961p

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Posted 06 February 2013 - 07:43 AM

Frank,

You questioned authority - and you were ignored! But you persevered and at least Sky responded. That seems like something they'd catch and correct and its to their on going credit they did. Bravo to you and the Sky fellas.

Pete

#4 freestar8n

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Posted 06 February 2013 - 09:27 AM

Thanks, Pete and David, for your comments. As I said - please spread the word.

I hadn't really set out to question authority originally - I was just curious to learn more about the corresponding spectral line to see if I could image with it. When I started looking for it - I quickly realized something was amiss.

Once I learned the main line was in the UV I worked with an astro-associate to try to image with that line using a Venus filter. I posted that attempt many years ago here with a description of the CN issue - but it didn't really catch on. The Venus filter wasn't ideal for the attempt - and I think the CN line is often faint anyway.

Ironically, another source that makes the CN error is a certain filter maker, who offers a "cyanogen" filter that is green. A cyanogen blocker, more like.

Although Sky and Tel were the only ones to take action and research it a bit then post the letter, some of the other people I contacted at least acknowledged the mistake - but didn't post anything to correct it. I don't know if any of the sites has since posted a repeat of the error, but I will be watching for the next green comet image on APOD or the bad astro blog. It's there right now on spaceweather - but I could never reach the author of that web site.

Thanks,
Frank

#5 azure1961p

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Posted 06 February 2013 - 09:50 PM

Well the name cyanogen is probably going to stick with some adhesion since the color of the gas does infact look l like cyan! I can see how they - or it - got started. I think its uphill to correct though.

Pete

#6 freestar8n

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Posted 07 February 2013 - 04:20 AM

Well amazingly enough - today's apod shows a big green comet - and reading the caption... They say it's C2. So I guess he remembered...

Just one apod saying it is cyanogen is very effective at spreading the misconception - and there have been several over the years. Maybe this will start undoing it.

People were genuinely concerned about CN in comets because I think Earth was going to intercept some of Halley's tail in the 1800's - so there was some press about it. Comets are evil enough already - then they contain poisonous gases and we fly through them. I guess there were no color pictures of them, though, so people didn't appreciate how green they can be - and never made a false link to cyanogen until around 2000.

Frank

#7 Tonk

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Posted 08 February 2013 - 06:24 AM

A moot point - but my pet peeve as a chemist is everyone gets the formula for cyanogen wrong too. Its NOT "CN", it is NCCN, triple bonds between the nitrogen and carbon and a single bond between the two carbons. If you want you can write (CN)2 but not just CN - grrrrr

Another misconception perpetuated by astronomers including those posting here ! ;)

More here on its (simple) chemistry - http://en.wikipedia.org/wiki/Cyanogen

#8 freestar8n

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Posted 08 February 2013 - 07:55 AM

I have had this discussion with chemists several times - and I simply point them to journals such as Nature and Science - not to mention astronomical and physical chemistry journals - where the term is used in this way to refer to CN, the "cyanogen radical" - as cyanogen.

So - for me it's neither a misconception nor an error. It's just recognizing an established convention used the same way in many different domains.

I have no problem with the astronomical use of the term "metal" in stars either.

Your own wikipedia link says in the first paragraph:

"Certain derivatives of cyanogen are also called "cyanogen" even though they contain only one CN group."

And a single CN by itself is consistent with that usage.

But if you want to use "cyanogen" only to refer to NCCN - that's ok with me.

Frank

#9 Tonk

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Posted 08 February 2013 - 09:28 PM

From Wikipedia

Certain derivatives of cyanogen are also called "cyanogen" even though they contain only one CN group. For example cyanogen bromide has the formula NCBr.


The wikipedia wording isn't good here and doesn't help understand what "derivative" means. Note that "cyanogen" can appear as a prefix in names of certain cyanogen derivatives (substances made from cyanogen). They give an example "cyanogen bromide", so its "cyanogen something". It does not mean "Cyanogen" by itself means either CN or (CN)2

And a single CN by itself is consistent with that usage.


Well now that I've explained it - its not!

But if you want to use "cyanogen" only to refer to NCCN - that's ok with me.


yeah what do chemists actually know about their own subject - well "cyanogen" by itself refers to just one substance (CN)2

It's just recognizing an established convention used the same way in many different domains.


I can only say that for a chemist this is an ignorant "convention" if indeed it is. Would a physicist accept E=mc as a "convention" because non-physicists can't be bothered with the square term. That is how silly and incorrect it looks

My peeve stands - its the misuse of CN to mean cyanogen. CN is the nitrile radical or cyano radical

All I say is if you are peeved with the mistake of astronomers attributing green spectral lines to cyanogen, at least have the grace to accept that using CN to represent cyanogen is exercising exactly the same sort of mistake - one that should be corrected.


#10 Tonk

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Posted 08 February 2013 - 10:09 PM

This is amusing stuff from Bad Astronomy - at least they sorted out the correct formula for cyanogen even if origin of green spectral lines is incorrect

http://blogs.discove...y-if-youre-a...

Here is a Harvard paper on the fluorescence spectra of the CN radical - strong emissions in the red and violet portions of the visible spectrum

http://articles.adsa...?db_key=AST&...

From a chemistry point of view the paper incorrectly refers to cyanogen in the title but the body of the paper refers correctly to the CN radical (i.e. the nitrile or cyano radical).

Having read the paper it appears when anyone in the physics/astronomy world refers to "cyanogen" they are in fact referring to the nitrile (or cyano) radical. From the chemistry stand point these are very very different species (different masses for one, spectra being the other!!!!!).

So the misuse isn't the formula CN but actually the name "cyanogen". Bit of an eye opener re sloppy misappropriation of names!!!!!!!!!

Now that we have established that "green is due to cyanogen" is a doubly compounded error - here is the real importance of the nitrile/cyano radical to astronomy

http://en.wikipedia....i/Cyano_radical

#11 David Knisely

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Posted 09 February 2013 - 01:01 AM

Again, in Astronomy, there are some unusual conventions that may ruffle a few feathers of some of those in other fields, but are still standard. "White dwarfs" aren't all white, Brown Dwarfs aren't brown, elements heavier than lithium are often referred to as "metals", and CN is the common radical which exists in space, produces some notable emission lines, and is sometimes referred to by astronomers as "cyanogen" or "the cyanogen radical". However, right here on this forum, "CN" is most often used as a abbreviation for "Cloudynights" :). It's as simple as that. Clear skies to you.

#12 freestar8n

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Posted 09 February 2013 - 04:09 AM

Yes - that Phil Plait article is the one that prompted me to send him a note. As I recall he acknowledged in a reply that CN was not green, but he never posted a correction. Since he does post corrections to other items he gets wrong and has literally made a career out of correcting astronomy misconceptions propogated by the media - the dude lost a ton of points with me. Even today people can search on green comets and find his article - and of course it must be true.

I think the argument against this use of "cyanogen" would have more merit if it were only found in astronomy specific journals - but as I said, its use in Nature and Science make it accepted as far as I'm concerned. Others who feel a particular fondness for its specific usage in their own field of study are free to disagree and use it as they please.

Cyanogen jets in comet Halley, A'Hearn et al., Nature, 1986

The Spectrum and Spatial Distribution of
Cyanogen in Comet Hale-Bopp (C/1995 O1) at
Large Heliocentric Distance, Wagner et al., Science, 2007.

Frank

#13 BadAstronomer

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Posted 10 October 2013 - 01:33 PM

Hi everyone-

freestar8n, I actually did followup with your query, but you're correct, I never wrote about it. I found some interesting info about the emission lines from comets, but never heard back from anyone I contacted about it (except Emily Lakdawalla, who steered me to some interesting info).

I found a decent labeled spectrum of comet LINEAR: http://www.astrosurf...2_1107010_s.gif

The strongest green line for that comet is from C2, at 5200 A. The other C2 line is bluer. There is a strong CN line at 3800 (on the UV/blue edge) and a weaker one at 4200, which is clearly blue.

I poked around and found contradictory data, but not from professional astronomers; Lumicon, for example, says their green filter is for CN (as is pointed out in this thread).

I never followed up because I didn't want to base a post on a single graph, and I didn't have much else to go on at the time... and then it essentially got buried in my emails, which happens with frustrating frequency. Too many things to keep track of!

As it happens, I am posting about ISON on Friday (Oct. 11) and I'll link to this thread. This is an interesting situation, and worth getting the word out about!

Thanks,

Phil

#14 freestar8n

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Posted 10 October 2013 - 02:27 PM

Hi-

Thanks. Yes I think word is getting out about it. I told the APOD people about a year ago or so and I think they have stopped saying it. I wanted them actually to point out that it is now a misconception - but I don't think they ever did. Perhaps because they may have contributed to it.

For me it was a natural thing to look into. If it's cyanogen, what are the wavelengths of the green emission lines. I couldn't find them in texts of journal articles anywhere, and eventually realized that it was a fairly recent myth that just exploded. It's particularly interesting because it is very plausible - plus it is compelling.

Anyway, spread the word, and thanks for commenting here. At first I didn't realize who you were.

But it is very definitely wrong to say the green is due to cyanogen.

From my research I think it started in the early 2000's but I'm not sure. I assume it did start from one source and then spread.

Good reminder to consult primary sources on things that are perfectly believable, but you don't really know for sure. In that situation, you are likely to repeat it without checking - which is what the person you heard it from did.

Oh - and welcome to cloudy nights.

Thanks,
Frank

#15 freestar8n

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Posted 10 October 2013 - 02:41 PM

I should add that when I contacted Phil originally, he replied promptly and with interest and sent a follow up note that he had looked into it and basically agreed. So I appreciated that took the time and showed interest then, and I think it's very cool for him to follow up on it here.

Thanks again,
Frank

#16 Tonk

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Posted 10 October 2013 - 03:05 PM

I notice that right now SpaceWeather.com is still quoting "Cyanogen" as the source of the green hue in ISON

#17 freestar8n

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Posted 10 October 2013 - 03:23 PM

Yes, I'm afraid so. I tried to contact them last year but I could never get through via the web site or email channels. They accepted pictures but not comments I guess. They talk about CN with every green comet - it does make an exciting caption.

I assume they get a lot of comments and feedback - some of which is even crazier than mine.

Well - I will try to send them a note now.

Frank

#18 Tonk

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Posted 10 October 2013 - 04:59 PM

I actually submitted a messages via the photo upload page about this but I guess they don't read the comments

Their webmaster email bounces a mail-box full error!

So yes its almost impossible to contact them!

#19 amicus sidera

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Posted 10 October 2013 - 05:38 PM

All this points up why a healthy skepticism vis-a-vis the proclamations of the purveyors of "Science" should be maintained.

For example, almost 80 years ago engineers at a large Midwestern radio company published specifications for RF coils that were and are incorrect; they dictate a coil longer than it is wide in the direction of EMF, whereas the polar opposite, e.g., slightly wider than long, will produce a coil with superior merit, or "Q". This error has been replicated countless times in numerous textbooks and reference materials since then, and for all I know continues to the present day.

Trust, but verify.

#20 RobK

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Posted 10 October 2013 - 06:32 PM

I suppose a picture's worth a thousand words. Here's my spectrum of 'gassy' comet C/2012 F6 (Lemmon) taken on 28 Feb this year. You can see the bright green (and blue!) broad diatomic carbon emissions but the main CN emission is so far down the blue-violet end that it is outside my camera response range! This comet did have a strong CN line at the time, picked up by others with better range at the blue end.

Cheers -

Rob

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#21 freestar8n

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Posted 10 October 2013 - 06:41 PM

Thanks - nice spectrum. The green bands show very well.

I assume there is a decent UV CN emission in most of these comets - it just isn't green. Somehow the idea that "comets are green" and "comets contain CN" became "comets are green because of CN."

I started looking into this around 2006-7 originally and worked with a friend with a UV filter to try to make an image that showed the weak CN content.

The image is shown here. The top is a normal color view, but the next is a cutaway showing the intensities in the UV, green, and red. These weren't normalized so it isn't entirely meaningful, but it was intended to get the message across that CN is UV, and the UV content is small.

A UV "Venus" filter was used for a "CN" filter.

Frank

#22 Tonk

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Posted 10 October 2013 - 06:54 PM

Unfortunately one of the failings in peer reviewed publications is that once an error slips through then only a published correction by the authors or a follow up critique paper by others can put the record straight, But then less diligent researchers may only read the original error and so it goes

I've got a small stack of these papers ready for critique in my PhD thesis (not astronomy - but chemistry. Most are just counterexamples I've found that prove that some information algorithm is not complete etc)

There you go - nothing wrong with this - mistakes get made. Its is up to follow-on researchers to apply a critical mind and not quote verbatim earlier work without doing proper homework to find original sources and confirming work etc.

#23 RobK

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Posted 10 October 2013 - 08:57 PM

After my last post, I don't really feel comfortable playing Devil's advocate. But is there something that hasn't been addressed here? There's no doubt that the emissions from CN are not green, but could light passing through a cloud of CN gas transmit as green or blue? Or could sunlight reflecting off a cloud of CN gas be green or blue? I don't know the answer, maybe a chemist could reply. After all, it's commonly called "cyanogen", not "ultravioletogen".

Rob

#24 freestar8n

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Posted 11 October 2013 - 01:49 AM

The name "cyanogen" is from cyanide - not the color cyan. I believe the association of cyanide with "dark blue" is because it was first synthesized from a colored material. An early publicized comet/cyanide connection was Halley's comet, when spectroscopy revealed it did contain some of the associated molecules and radicals. Since Earth was going to pass through part of the tail - this caused a bit of a scare - but nothing to do with the color green. "Green cyanogen" happened over a century later.

There is nothing subtle about the cyanogen UV lines - and they are commonly studied in comets. I don't think it's understood what the parent species are and what reactions are taking place.

There are also CN lines in the red that are observable.

On the subject of "staying skeptical" - the scary thing about this is that although I am definitely on the skeptical side, my investigation of this issue wasn't prompted by it at all. I was just curious - mainly to see what filter would be best, and how it related to Oiii emission. I didn't have the slightest doubt that it was true.

That's scary because think of all the other things that make perfect sense and come from good sources - that haven't been checked against a primary source. There are plenty of obviously questionable claims thrown around that can be dismissed - but this one is completely plausible.

One thing that caused immediate alarm was that although I could find tons of sources on the web about this "fact" - not a single one cited a primary source. Many cited another web site as a source - and that site had no reference.

It's hard to say "there is no CN in the green" in an absolute sense, particularly if there are indirect mechanisms possible as you suggest. But it's easy to say that there is no backing for the green CN claim - and to deduce that at some point someone made a mistake, and such a compelling tidbit spread like wildfire.

I have seen more references, including an nbc news website, mentioning green cyanogen. So with ISON arriving, it will be interesting to see if this myth actually grows, or if it is suppressed.

The power of the web can work both ways.

Maybe if I made a video about the topic that included cats and twerking or something - the truth would spread more effectively.

Frank

#25 David Knisely

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Posted 11 October 2013 - 01:51 AM

I notice that right now SpaceWeather.com is still quoting "Cyanogen" as the source of the green hue in ISON


Yea, but SpaceWeather also keeps calling active regions "Sunspot 12345" instead of the more proper AR 12345 or NOAA 12345. I called them on it a few times since an active region usually contains more than one sunspot, but they keep on doing it. I guess Spaceweather.com also keeps forgetting about the blue-green emission from the carbon molecules in comets and just jumping on the "cyanogen" bandwagon (sigh). They must have near-UV sensitive eyes on that web site :). Clear skies to you.






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