Starship Asterisk*

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

http://www.astrosurf.com/buil/lulin/result.htm wrote:

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

Chris Peterson wrote:

neufer wrote:
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.

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Chris Peterson wrote:
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.

http://www.astrosurf.com/buil/lulin/result.htm wrote:

[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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neufer wrote: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.

Chris Peterson wrote:

Ann wrote:
One more question, though. Why is the comet coma green?

Only some cometary comas are green. Those are the ones that contain a substantial amount of cyanogen, (CN)₂ and/or diatomic carbon, C₂. Both of these actually fluoresce green when they absorb sunlight (a fairly uncommon sort of fluorescence called resonance fluorescence, where the emitted and absorbed radiation has the same wavelength).

http://blogs.discovermagazine.com/badastronomy/2010/06/09/actually-if-youre-a-comet-it-is-easy-being-green/ wrote:
Actually, if you’re a comet, it *is* easy being green

[size=150][color=#00BF00]comet 2009 R1 McNaught[/color][/size]

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<<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:

• ayiomamitis_mcnaught2009r1

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.>>

Ann wrote:One more question, though. Why is the comet coma green?

Chris Peterson wrote:

Ann wrote:APOD Robot wrote:

the ion tail shines slightly blue as the carbon monoxide ions reflect blue sunlight more efficiently

As a lover of all blue things, I have tried to understand why the ion tails of comets are blue. I haven't been able to find a really good explanation.

The explanation given in today's caption must be wrong, or at least that's what I think. Surely ions don't reflect light? Surely they emit light instead?

Ann-

Any light emitted by the ionized gas in the tail is too faint to detect visually, or to be above the noise floor in typical images. The caption is correct: CO⁺ ions are just the right size to preferentially scatter blue wavelengths (the same sort of Rayleigh scattering that makes the sky blue). So we see the ion tail as blue. The larger particles in the dust tail preferentially scatter longer wavelengths, so we typically see it as an unsaturated yellow.

saturn2 wrote:What is the speed of the Comet Garradd?
I think the speed has to do with the tail of dust and ice.

TNT wrote:I wonder where comet Garradd is currently located in its trip through the solar system.

TNT wrote:I wonder where comet Garradd is currently located in its trip through the solar system.

Miguel wrote:Why are there seemingly so many “pairs” of stars in this photo? Is that just coincidence?

Wolf Kotenberg wrote:Wonder if we can convince the big poobah of the skies to bring these things closer to our control tower in the sky ? or not such a good idea ?!

nstahl wrote:Ok, not in the caption but from the Bad Astronomy post at the "intermediate viewing angles" link we see:

But the dust and rock isn’t affected as much. As it moves off the comet, it tends to lag behind a bit, following the comet in its orbit.

Chris Peterson wrote:

nstahl wrote:Ok, it occurs to me people are accepting that the dust tail would naturally follow the comet in its orbit. And if the comet were flying through the air, that would be natural. But there's no atmosphere out there retarding those dust particles, which presumably get expelled forward as often as backward. So why aren't about half of those dust particles in the same orbit as the comet slowly getting ahead of it instead of all of them apparently slowly getting behind it? Is it interaction with the very few molecules out in the solar system retarding them?

Nothing in the caption or links suggests that the dust tail is following the nucleus in its orbit. In fact, both the ion tail and dust tail are pushed directly away from the nucleus by the solar wind and by radiation pressure (regardless of the direction they are actually ejected in). The ions, being very low mass, are pushed in a nearly straight line. The dust is massive enough that it follows the rules of orbital dynamics, so as it is pushed outwards, it orbits more slowly than the nucleus. This results in the dust tail sweeping backwards- but usually still substantially away from the Sun. There can actually be a wide variation in dust tail position with respect to the Sun and cometary orbit, depending on the size of the dust particles and the orbital velocity.

But the dust and rock isn’t affected as much. As it moves off the comet, it tends to lag behind a bit, following the comet in its orbit.

Chris Peterson wrote:There is also a third "tail", which is the cometary debris stream. This is too tenuous to be visible in images, but is what produces meteor showers in the case of comets which cross Earth's orbit. This material is large enough that it's only moderately affected by the solar wind and radiation pressure, so it does just what you suggest: disperses uniformly around the nucleus in all directions. Because if is ejected relatively slowly, it stays along nearly the same orbit as the nucleus. Material ejected inwards moves slightly faster, material ejected outwards moves slightly slower. This serves to fill the entire orbit with debris over a long time, with the greatest density near the nucleus (which is one reason why meteor showers are often more intense in the few years before and after a comet crosses Earth's orbit).

Ann wrote:APOD Robot wrote:

the ion tail shines slightly blue as the carbon monoxide ions reflect blue sunlight more efficiently

As a lover of all blue things, I have tried to understand why the ion tails of comets are blue. I haven't been able to find a really good explanation.

The explanation given in today's caption must be wrong, or at least that's what I think. Surely ions don't reflect light? Surely they emit light instead?

nstahl wrote:Ok, it occurs to me people are accepting that the dust tail would naturally follow the comet in its orbit. And if the comet were flying through the air, that would be natural. But there's no atmosphere out there retarding those dust particles, which presumably get expelled forward as often as backward. So why aren't about half of those dust particles in the same orbit as the comet slowly getting ahead of it instead of all of them apparently slowly getting behind it? Is it interaction with the very few molecules out in the solar system retarding them?