It's not easy being green, it seems you blend in with other things, and people tend to pass you over.
Or stars in the sky.
The frog.
I agree and disagree with my amphibious friend.
If you were a green star, you'd stand out, and astronomy would devote lifetimes to studying you, so finding one would be an amazing thing.
I disagree with him in that comets are usually green, but ones like that tend to be bright and beautiful, so you would stand out clearly.
It's not easy being green, even for a comet.
There are chunks of ice, rock, and dust on the Sun. Most are on long elliptical paths that reach far past Neptune's orbit. It's so cold that they stay frozen for a long time. If the comet drops close to the Sun, the ices turn into gas and become a spectacle.
The comet was stuck in the ice and this process releases a lot of dust. The dust tail is created by the pressure of sunlight and reflects sunlight. The gas gets zapped by the sun's rays and glows in the same way a neon sign does. The light hits the molecule and it loses an electron or two. The molecule emits light if an electron recombines with it. The ion tail is created by this.
The ion tail is made of a molecule. Blue comes from carbon monoxide. Many comets glow green because of the unusual molecule dicarbon, two carbon atoms bonding together.
There is a mystery here. The comet has a green glow from the gases surrounding its nucleus. The tail is not green. Why not?
Astronomers assumed that the molecule is abundant near the comet's head, but exposed to sunlight it will be destroyed. This has never been proven in a lab. If you try to make C2 in Earth's air it will immediately grab a bunch of oxygen atoms and combine with them. It has never been proven that photodissociation is the reason comet tails are not seen.
Until now. A group of researchers tried to see if the conclusions of the astronomer were correct. They are!
They created C2 by zapping the molecule of C2Cl4 with a laser. They hit the C2 with another laser to blast it apart, and then used a third laser to illuminate the molecule, which they used to measure the speed. How much energy it takes to photodissociate C2 could be figured out.
If a comet were at the same distance from the Sun as Earth, they could calculate how long it would take for a molecule of C2 to break apart. It's a very complex process that involves at least two photons of light from the Sun and a couple of more steps where the molecule changes configuration, so it takes time to proceed. It takes about 160,000 seconds, or two days. A lot of the C2 is gone by the time it gets to the comet.
It's not that far away from a comet's tail, which can be millions of kilometers long. The comet heads are green but the tails are not: The C2 breaks apart close to the comet's head and never makes it to the tail.
It's nice that these numbers match what astronomer observed almost a century ago.
We know how this works. There are a lot of complex organic molecules on the surface of comets. The UV light breaks those apart when they are close to the Sun. As ice turns to gas, it gets lofted off the surface. The green colored molecule gets rejuvenated and glows, but over the next day or two it gets torn apart.
Many comet heads have a green glow.
I'm always amused by nature. The same sunlight that creates C2 on the comet's surface is what destroys it in space as well. If there is a morality lesson there, you can explore it on your own.
Another lesson to take home from this? He was correct. At least, close enough. It's natural for comets to be green. It's complicated.
