Actually, if you're a comet, it *is* easy being green

By Phil Plait | June 9, 2010 10:00 am

Yesterday, I wrote about the comet 2009 R1 McNaught which is currently in the extreme northern sky in the early morning. By coincidence, just hours after posting it, I got an email from the amateur astronomer Anthony Ayiomamitis (the same guy who took the very cool picture of the ISS and Jupiter in the daytime), who sent me this picture of the comet he took in Greece at just around the same time that post went live:


Wow, very pretty! The solid part of the comet, called the nucleus, is far smaller than a single pixel in this image, since the comet was more than 175 million km (110 million miles) away when he took this shot. The nucleus of even a huge comet is only a few dozen km across, so at that great distance is just a tiny dot. Anthony has details on his observations on his McNaught page.

The comet looks huge — and the fuzzy part can be bigger than planets! — because what you’re seeing is gas expanding away from the nucleus. Far from the Sun that gas is frozen, and the comet is solid. But heat it up, and that ice turns into a gas, creating the comet’s coma (Latin for hair). In that gas methane, water, ammonia, and lots of other things, many of which are pretty nasty.

But why is it green?

Ah, that’s a good question (I’m glad I asked it!) and takes just a little bit of background.

When the gas suffusing out from the nucleus gets hit by ultraviolet light it becomes ionized; one or more electrons get stripped off the atoms. That’s important because the Sun is blowing a wind of subatomic particles called the solar wind, and as it moves out from the Sun it carries a magnetic field with it. This field interacts with the comet’s ions in the coma, shearing them away (this process is pretty complicated, and not completely understood). The solar wind is moving far, far faster than the comet (many hundreds of km/sec, versus maybe just a few dozen), so the ion tail points straight away from the Sun. As far as the solar wind cares, the comet is just standing still.

And that brings us to the comet’s verdant glow. That green color is real, and not just from the way the picture was made! And it’s the same reason a neon sign glows. When you have an ionized atom or molecule (or just an excited one, with an electron bumped into a higher energy state so that it can fall back down), the electron can recombine with its parent. When it does, it gives off light. The color of the light depends very strongly on the type of atom or molecule. Excited hydrogen glows red, for example, which is why so many gas clouds in deep space glow that color.

In a comet, the molecule cyanogen (CN)2 and diatomic carbon (C2) both glow characteristically green, which is why some comets, like McNaught, are green. And I wouldn’t blame you if you thought that these comets must be mostly made of those two molecules since the comet is so green. But, like everything in science, there’s more going on…

Some atoms and molecules emit more strongly than others. Under the same circumstances, a kilo of cyanogen would glow much more fiercely than a kilo of, say, hydrogen. It depends on some relatively complicated quantum physics — forgive me if I leave off the details — but you can think of it as one person who can yell louder than a bunch of other people combined. That one person dominates the emitted sound, even though there are lots of people in the room. It’s the same in the comet: (CN)2 and C2 are strong emitters, so their presence dominates the color we see. That’s not the case for every comet (some may be deficient in those compounds), but it’s certainly true for McNaught; lots of observers are reporting its strongly blue-green color.

I’ve seen quite a few green comets in my time, and while it’s a little odd to see something glowing a ghostly hue like that in the sky, it’s always lovely. This comet promises to be a good one, so if you get a chance, go out and hunt it down.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (25)

  1. Messier Tidy Upper

    Great picture. :-)

    In a comet, the molecule cyanogen (CN)2 and diatomic carbon (C2) both glow characteristically green, which is why some comets, like McNaught, are green.

    My first thought was actually the presence of ionised Oxgen – OIII – which is responsible for the colouration of planetary nebulae isn’t it?

    The comet looks huge — and the fuzzy part can be bigger than planets! — because what you’re seeing is gas expanding away from the nucleus.

    For a great graphic showing the relative sizes of comets and planetary orbits see :

    Which compares & contrasts the lengths of the Great Comets of 1811 and 1843, and that of Halley’s Comet in 1910 compared with the inner solar system. Worth a look as far as considering how large comets really are. :-)


    PS. What no link to your two “no green stars” posts :


    then BA? Those were two of your best articles ever, IMHON! 😉

  2. Fritriac

    Watch out for the brackets!

    (CN)2 != CN2


  3. Ah, yes, missed that one. Thanks, I fixed it.

  4. I thought it was made of green cheese….

  5. Patrick

    I remember reading about a comet that approached earth in the late 1800s/early 1900s after we had discovered spectral analysis, and it was determined that cyanogen and cyanide were present, and there was a panic that everyone on earth would die if we passed through the tail.

    Good times.

  6. @Patrick

    I believe that would have been Haley’s comet, and it was mentioned in Carl Sagan’s The Demon-Haunted World, IIRC.

  7. Messier Tidy Upper

    @4. FreeSpeaker Says:

    I thought it was made of green cheese….

    No, that’s the Moon .. Apollo moon “rock” tests secretly confirmed this but then NASA and the Moon Walkers (& there’s a good band name for some group!) were paid off by a conspiracy of French cheese makers to hide the truth and keep those fancy, smelly cheese prices high! 😉

    Meanwhile out in the depths of space, the alien super Mouse race armada approaches with our Luna firm its sights and little twitching noses! 😉

  8. Messier Tidy Upper

    @6. Todd W. & 5. Patrick :

    Around the same time (~ish) and with the traditional accompaniment to fancy cheeses in mind there’s the much more civilised idea of making Comet Wines – see :

  9. Michel

    clouds till monday here…

  10. Anthony has some darn fine photos on his website. That one of the moon rising behind Mt. Lykabettos in Athens is polú kalá!

  11. SMo

    One of the coolest things one first learns about comets is that their tails have almost nothing to do with the direction they’re moving. It’s one of the little things that surprises almost everyone I mention it to.

  12. Regner Trampedach

    Messier Tidy Upper @ 1: Well, it takes a lot more energy to kick three electrons out of an oxygen atom than one electron out of just about anything, including CN and C2. That energy is available around a 10,000K white dwarf, but much less so around a 5,777K Sun.
    Cheers, Regner

  13. George Martin

    Regner Trampedach @12

    Since this is an Astronomy post, wouldn’t you only have to remove two electrons to get OIII? If I remember correctly, the Astronomical and Chemical notations for ionization states differ by one. For instance HI (aitch one) is the neutral hydrogen atom. It’s ground state gives rise to the 21 cm line. You have to pay attention to context when someone says “aitch two”. Is it HII or H2?


  14. Ozonator

    With >5% chance, AGW goes poof the comet before “Boom goes London and boom Paree” (from Randy Newman’s song “Political Science lyrics”).

    “A). … 3). Regular qualitative predictions for catastrophic, violent ecosystems (~quakes to ~CMEs) … There may be a larger, secondary increase in exported global warming energy to discharge … as solar activity should be harmonically limited to visibly ripping out titanic spots on the farside of our star and a nasty Bz storm hitting us. Worse then last week (5/29 – 6/5/10) … Saturn or Jupiter may develop another energetic event from expanding gas looking like a new spot, hurricane, volcano, meteorite hit, band, stripe, skid mark, ring, lightning storm, or carbon footprint of Allah … Mercury, Mars, and/or Venus may show similar anomalies at the same time. It is quite possible that some comet traveling in the “empty” space on the backside of our star will suddenly expand in a flash of light and die” (GBRWE 6/6 – 12/10’s Extreme Planetary Warnings for Earthquakes, Volcanoes, and Solar/Terrestrial Flares from Human Activities; Robert Rhodes, Supplemental; GBRWE 6/6 – 12/10, 6/5/10).

  15. Ooooooh-kaaaaaayyyyyyyyyyyy.

    Oh, nurse! Nurse Ratchet?

  16. ggremlin

    We have found the SOURCE of cleaning bowl products!

    Now we need their technology to get up there. Who’s with me? :)

  17. MadScientist

    Morning? I’m in the northern Viking colonies at the moment; there’s about 4 hours of twilight but that’s the closest it comes to being dark – it’s still too bright to see a comet unless perhaps the comet is as bright as Jupiter or Venus.

  18. Andrew

    If I remember correctly, green auroral emission is also oxygen – a forbidden transition of neutral oxygen atoms – [OI] – at 557.7nm, somewhat similar to the forbidden [OIII] (O^{+2}) transitions referred to above, at 495.9 and 500.7 nm.

    I think cyanogen fluoresces at 511 and 514 nm. Presuambly it takes its name from Prussian blue, rather than the cometary glow.

  19. olderwithmoreinsurance

    @George Martin: you’re correct OIII means twice ionized, which I’m sure chemists find maddening.

  20. @#1

    OIII emissions are also what makes Hanny’s Voorwerp green, iirc. Hasn’t Hubble imaged it already? I wanna see!

  21. Ozonator

    Either the following exported AGW is gonna poof the comet or fluff Jupiter – or if it reads the Skeptic Tank polls, make a new Fox News show about ice dancing.

    A giant CME (coronal mass ejection) blasted off of the southwestern limb of the Sun ~ 2010/06/05 15:49 – with a partial ring observed (more toward the Earth or away) generally toward Jupiter, Mercury, Uranus, Neptune, and/or possibly a glancing blow to the Earth (“THE SUN NOW” – “LASCO C3”; (planets by, 2010-06-6). Heading toward the Jupiter quadrant of the solar system, “CRACKLING SUNSPOT: New sunspot 1078 is growing rapidly and crackling with low-level solar flares. Click on the image to view some of the action recorded on June 8th by NASA’s Solar Dynamics Observatory (SDO) … So far the relatively minor “crackles” shown in the SDO movie have had no effect on Earth” (“What’s up in Space”; Dr. Tony Phillips;, 6/9/10).

  22. Pi-needles

    @20. olderwithmoreinsurance Says:

    @George Martin: you’re correct OIII means twice ionized, which I’m sure chemists find maddening.

    Probably almost as maddening as astronomers referring to *all* elements beyond hydrogen & helium as “metals” .. ! 😉

    (Course, we do know enough to say that many elements consist in large part of a certain alcoholic beverage – eg. there’s oxygen which is a mix of air and gin & hydrogen which is a mixture of water & gin! Right folks? 😉 )


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