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Up in the Air


Keep eyes peeled for ISON comet

By Daniel B. Caton

Comet! It will be all over the news this month as attention turns to a dirty snowball that has swooped in from the outer reaches of the solar system to pass around the sun. Will you be able to spot it?

This comet is ISON, which is an odd sort of name.But comets are named after their discoverers.In this case, it was a telescope in the International Scientific Optical Network, a system that automatically searches for asteroids. Discovery by robot!

This comet was billed early as potentially the brightest comet ever seen, perhaps rivaling the full moon. But such predictions are difficult to make since a comet’s brightening depends on its structure and how it reacts to being heated by the sun.

A few weekends ago, I was observing all night at our Dark Sky Observatory at Appalachian State University with collaborators from South Carolina. After a night of measuring the changes in light of eclipsing binary stars – pairs of orbiting stars that eclipse each other – we decided to take a break and image ISON in the predawn sky. Trudging down to a smaller DSO telescope that has a wider field of view and lower eastern horizon, we found the comet and took a late-night portrait.

This was not for science, but for fun. We got a decent picture but stuck with black and white. Astronomical cameras are monochromatic to optimize their efficiency and sensitivity. To make those wonderful color images of celestial objects, we have to take separate images with blue, green and red filters and combine them with software. (The trouble with that? If the comet moves enough between exposures, combining them is difficult: Either the comet or the background stars end up being streaked.)

As the next few weeks go by, ISON will pass by the sun and brighten as it gets closer. That, of course, means it will only be visible low in the dawn sky, unlike notable recent comet views like last spring’s Pan-STARSS, 2007’s McNaught, Hale-Bopp (’97), Hyakutake (’96) or even the 1986 passage of Halley’s comet.

We ended our long night at DSO taking calibration images of the brightening dawn sky and then left as the just-rising, hybrid-eclipsed sun displayed a chunk bitten out of it. A fitting end to a night of studying eclipsing stars!

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email:
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