☄ Comet Lulin

Those with sharp enough eyes, who know where to look, should be able to see Comet Lulin in the sky even as I type this. It should be somewhere in the constellation Virgo, with a magnitude of about +6 (you may need binoculars if you’re a city dweller). It reaches its closest approach to Earth on Tuesday 24th, at about 0.41 AU away.

Comets are fascinating things. Even more so when, like Lulin, it’s on its first visit to the inner solar system. Comets are, after all, full of ices. As it draws near to the Sun, all of those ices start to sublime into a huge extended atmosphere called a coma. This coma is huge, but not very dense. The end result is a tenuous glowing gas cloud that can easily be as big as Jupiter. The solar wind interacts with all of this, puffing it in the opposite direction to the Sun and causing the comet to sprout a (sometimes magnificent) tail.

Lulin is also a striking shade of green. That comes from a couple of particular molecules that it’s particularly rich in. C2 (dicarbon) and CN (cyanogen). Probably C3 too — a molecule which was actually first discovered by looking at comets. In spectroscopy, the brightest C3 band is still commonly known as the “comet head” band.

Other comets, like Halley, contain different ratios of these molecules because each pass near the Sun causes ices to ablate away or react together, chemically changing the comet. The result is that they glow in different colours.

Seemingly, objects that spend a lot of time in the outer solar system pick up a lot of cyanogen, so they often glow green. I’m just guessing, but it’s probably why the rarely seen Aurigid meteors a couple of years ago also glowed green. Meteor showers, after all, come from comets. All comets leave a trail of dust in their wake, leaving more and more on each orbit. Every time the Earth passes through one of this loops of cometary debris, we get a meteor shower — the more times the comet’s passed by, the bigger this dust cloud will be, and so the more meteors we’ll see from down here on Earth. The Aurigids are rare because the comet that leaves them is on a huge orbit, spending centuries deep in the Oort cloud before eventually falling back in towards the Sun. As a result, it doesn’t spend a lot of time in our neighbourhood…

This image, incidentally, must’ve been taken around February 6th, because that double star in the background would be Zubenelgenubi (also known as Alpha Librae).

Image beautifully photographed by Mike Broussard. Go and look at his other photographs. They’re rather lovely — especially all the comets!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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9 Responses to ☄ Comet Lulin

  1. invaderxan says:

    Bleh… The astronomer’s two worst foes!

  2. ryttu3k says:

    Not visible :( Not only is the light pollution out here bad, but it’s overcast!

  3. invaderxan says:

    Actually, with Uranus and Neptune, the colour comes from different chemicals. I forget which, but I’m sure it isn’t cyanogen.
    The ice objects in the outer solar system are rich in cyanogen because it can form from the ices (N2 and CH4) when they’re hit with UV light… There are probably other processes too, but I’m pretty sure that’s a major one.
    Zubenelgenubi. :)
    It is a cool name, isn’t it? The Arabic astronomers were truly awesome. All the brightest stars still have the names they gave them!

  4. invaderxan says:

    It’s definitely worth trying. I don’t think any of us will see Lulin again. It’ll be a while before it comes back again. Quite a while. :)

  5. helen99 says:

    > Seemingly, objects that spend a lot of time in the outer solar
    > system pick up a lot of cyanogen, so they often glow green.
    Interesting – Neptune and Uranus are also blue-greenish. I wonder why there’s such a high concentration of cyanogen in the outer solar system.
    Of course I had to look up “Zubenelgenubi” (Obiwan Kenobi, only different). It derives from an Arabic phrase meaning the “Scorpion’s southern claw”. The Arabs were way ahead of their time in astronomy during ancient times.

  6. ryttu3k says:

    *checks Heavens Above*
    Okay, it won’t be visible until abouuut 10 PM tonight, in the East. We have pretty bad light pollution, so I’m not sure if I’ll be able to see ANYTHING, but I’ll bring out the binoculars and give it a crack.

  7. invaderxan says:

    Hehe… cool!
    And thanks. There’s no great rush. Just whenever’s convenient. :)

  8. I was just reading the other day about different aldehydes in comets and space debris. It made me think of your research! BTW I still need to ask my old man about those orgo journals for you!

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