Astro Bob: Winter encounters with Comet ZTF
Cold weather and clear skies means it's time to see the comet!
Maybe you've had a chance to look for Comet ZTF . In the Duluth, Minnesota region it's been cloudy for so many nights I almost despaired of seeing it before the waxing moon would wash it out. Finally, on Thursday, Jan. 26, my vigil paid off. I got up at 4 a.m., looked out the window and saw stars. Glorious stars!
Wary of clouds returning at any moment, I immediately tried to spot the comet without optical aid. It was conveniently positioned a few degrees above the Little Dipper's bowl at the time. Playing my gaze around the spot I soon found it — a soft, faint glow about half the size of the full moon. While hardly obvious, if you knew exactly where to look, there was no doubt even through barely awake eyes.
Next, I brought my 10x50 binoculars to the task. Right away, I could make out a big, fuzzy, wedge-shaped glow. Examining the comet carefully, the main tail (pointing west) stood out clearly. The other two tails — gas and anti-tail — took more sussing, but I saw both faintly with averted vision, a technique where you gaze around the object instead of staring at it directly.
I eagerly set up the telescope while the wind bore down from the spring stars overhead. Yes, spring. When you wake up to stargaze in the morning hours you get to "cheat the season." The Earth rotates while you're asleep. Go to bed at 10, and the winter constellations of Orion, Gemini and Taurus still dominate the sky. But 6 hours later, they've shoved off to the west to be replaced by the spring groups. I looked up to see Arcturus high in the southern sky, while the Summer Triangle and Milky Way rose in the east.
Despite having looked at photographs showing Comet ZTF's green coloration, caused by carbon molecules fluorescing in sunlight, I wasn't prepared for the sight in the telescope. Wow! It was the first thing I noticed. We're not talking golf course green but something much more subtle. Imagine seeing a green traffic light from a distance through heavy fog.
Through the telescope the comet was so big it was hard to make sense of its hazy appendages at first. But by moving the scope this way and that I was better able to distinguish one tail from another. Off to one side of the comet's head a bright spot of light called the false nucleus got my attention. It looked just like a fuzzy star. Photographs often overexpose this feature; it really stands out in person.
This woolly nugget is where all the action begins. Within the false nucleus is the actual comet nucleus, an icy object several kilometers across that partially vaporizes when heated by the sun.
Embedded in the ice are organic molecules, dust and frozen gases such as water, methane and ammonia. When released they create a temporary, fuzzy atmosphere around the nucleus called the head or coma. Tails form when sunlight either fluoresces (in the case of gas) or physically sweeps dust particles back behind the coma.
Comet ZTF's false nucleus was very obvious. I used high magnification to see if any dust jets were visible, but being low contrast features I wasn't sure of seeing them. Jets form when ice inside the comet turns to vapor and blasts out through a crack or hole in the surface. They look like small geysers close to the nucleus.
After photographing the comet I took a few minutes to look around and enjoy the early summer sky. I also dropped in on several seasonal treasures — the Ring Nebula, Hercules Cluster and one of my favorite double stars, Acrab (Beta Scorpii), in the head of the Scorpion. With the wind determined as ever and the temperature at 2 below, it was a joy to return to a warm house.
The very next night was clear again. This time a whole group of us looked at the comet from a snowy road overlooking Duluth's Lakeside neighborhood. Despite moonlight and light pollution everyone loved seeing the comet. Most described it as a smudge, but there were no complaints. Sometimes you don't need a lot of fanfare and rah-rah. The simple "is-ness" of nature is enough.