Green in comets is not CN, cyanogen
Posted 06 February 2013 - 04:44 AM
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*.
- Glen A W likes this
Posted 06 February 2013 - 05:35 AM
Posted 06 February 2013 - 07:43 AM
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.
Posted 06 February 2013 - 09:27 AM
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.
Posted 06 February 2013 - 09:50 PM
Posted 07 February 2013 - 04:20 AM
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.
Posted 08 February 2013 - 06:24 AM
Another misconception perpetuated by astronomers including those posting here !
More here on its (simple) chemistry - http://en.wikipedia.org/wiki/Cyanogen
Posted 08 February 2013 - 07:55 AM
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.
Posted 08 February 2013 - 09:28 PM
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.
Posted 08 February 2013 - 10:09 PM
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
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
Posted 09 February 2013 - 01:01 AM
Posted 09 February 2013 - 04:09 AM
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.
Posted 10 October 2013 - 01:33 PM
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!
Posted 10 October 2013 - 02:27 PM
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.
Posted 10 October 2013 - 02:41 PM
Posted 10 October 2013 - 03:05 PM
Posted 10 October 2013 - 03:23 PM
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.
Posted 10 October 2013 - 04:59 PM
Their webmaster email bounces a mail-box full error!
So yes its almost impossible to contact them!
Posted 10 October 2013 - 05:38 PM
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.
Posted 10 October 2013 - 06:32 PM
Posted 10 October 2013 - 06:41 PM
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.
Posted 10 October 2013 - 06:54 PM
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.
Posted 10 October 2013 - 08:57 PM
Posted 11 October 2013 - 01:49 AM
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.
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.