APOD: The Opposing Tails of Comet Garradd (2012 Feb 28)

[neufer]

I don't know how uncommon resonance fluorescence is.

Let's just say that it often blends in so well with normal scattering as to (generally) go unnoticed.

That's part of it. But the most common type of fluorescence involves the absorption of a short wavelength photon
and subsequent emission of one of longer wavelength.

Common, a. [OE. commun, comon, OF. comun, F. commun, fr. L. communis; com- + munis ready to be of service; cf. Skr. mi to make fast, set up, build, Coth. gamains common, G. gemein, and E. mean low, common.]

• 1. Belonging or relating equally, or similary, to more than one; as, you and I have a common interest in the property.
  
  2. Belonging to or shared by, affecting or serving, all the members of a class.
  
  3. Often met with; usual; frequent; customary.
  
  4. Not distinguished or exceptional; inconspicuous; ordinary; plebeian; -- often in a depreciatory sense.
  
  5. Profane; polluted.
  
  6. Given to habits of lewdness; prostitute.

I'm not sure that the explanation given in your quoted text is correct. My understanding is that the green color we see from cyanogen and diatomic carbon in comas is the product of resonant fluorescence, where a photon is absorbed, kicking an electron into a higher energy state, from which it subsequently relaxes, releasing a photon. This is different from an ionization process, as described. I don't think the green color of the coma requires energies high enough to ionize the gas.

• Indeed.

Nor does the green color of the coma really involve cyanogen (CN):

[size=150]The final[/size] [url=http://apod.nasa.gov/apod/ap090225.html][size=150]C/2007 N3 Lulin[/size][/url][size=150] spectrum. Because the binning zone include the nucleus region and a small part of coma, the plot is mixture of nucleus and coma spectra.[/size]

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[Chris Peterson]

How can we explain the color of comet Hyakutake, which had a bright blue-green coma and a bright blue ion tail?

I don't know any specifics... but how is this odd? The color of the coma will range from neutral to yellowish to green/blue depending on its composition (the relative amounts of largely colorless gases like water and CO₂, fluorescent or colored gases, and the amount and size of dust particles). The ion tail of all comets that I'm familiar with is blue, for the earlier stated reason.

Visually, I never saw any green in the coma of Hyakutake, just a neutral glowing head and a very long pale blue tail. Photographically, the comet looked very much like many comets, with a green coma and blue ion tail.

[Chris Peterson]

Nor does the green color of the coma really involve cyanogen (CN):

[size=150]The final[/size] [url=http://apod.nasa.gov/apod/ap090225.html][size=150]C/2007 N3 Lulin[/size][/url][size=150] spectrum. Because the binning zone include the nucleus region and a small part of coma, the plot is mixture of nucleus and coma spectra.[/size]

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CN is cyanide (which is also common in comets). Cyanogen is (CN)₂, and may not have been present in high concentrations in the coma of Lulin. It seems clear that the green coma in that case was primarily from diatomic carbon, which I think is often, but not always, the case.

[neufer]

Click to play embedded YouTube video.

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CN is cyanide (which is also common in comets). Cyanogen is (CN)₂, and may not have been present in high concentrations in the coma of Lulin. It seems clear that the green coma in that case was primarily from diatomic carbon, which I think is often, but not always, the case.