The color green represents hope and life. It’s also a sure sign our three morning comets are heating up and becoming more active as they approach the sun. I don’t know anyone who’s noticed the green color with their eyes looking through a telescope – the comets are all still around magnitude 10-11 and too faint to fire up our color vision – but the camera records their limey appearance with ease.
In some of the pictures you can clearly see the color difference between the comet’s tail and its fluffy coma – that’s the bulbous glow surrounding the tiny and invisible comet nucleus at the center of all the activity. Comas form tenuous, temporary atmospheres tens of thousands of miles across around the icy comet nucleus.
Don’t expect to ever see a bare comet nucleus through a telescope. They’re not only very tiny, typically only a few miles across, but cloaked by dust and vapors boiled away by the sun. The closest you’ll get is the bright spot in the center of the comet called the false nucleus – a compact region where the dust and volatiles are densely concentrated around the true nucleus.
Tails glow pale yellow from sunlight reflecting off cigarette-smoke-sized dust particles released from vaporizing cometary ices. The Caribbean blue-green of the coma, pretty as it is, originates from toxic cyanogen (a compound related to cyanide) and diatomic carbon (two carbon atoms bonded to one another). When energized by ultraviolet light from the sun, the gases fluoresce an eye-appealing green.
In my experience, the eye can’t sense the green until the comets become bright enough to show in ordinary binoculars at around 7th magnitude. When viewed through 8-inch or larger telescopes the color is tantalizing, like the green and blue iridescence sometimes seen along the edges of high clouds.
Comets ISON, Encke and Lovejoy are all predicted to brighten into visible green territory come November. That’s only a few weeks today. We’ll take the good news that all three comets are on track and brightening steadily. Stay tuned for more updates. For the latest brightness predictions, check out Seiichi Yoshida’s Weekly Information About Bright Comets.