How to Estimate a Comet's Brightness
Judging the magnitude of a comet is not straightforward.
by John E. Bortle
Comets are notorious for not following predictions, but even judging the magnitude of a bright comet that's right in front of you is not straightforward.
Stellar magnitude estimates made by long-time variable-star observers often agree to within 0.1 or 0.2 magnitude. These observers are comparing stars with stars. But a comet's coma or head may be anywhere from a few arcminutes to a degree or more in size. Because comets appear radically different from the pointlike stars used for brightness comparisons, determining a comet's integrated (total) magnitude is far more difficult.
For centuries the reported magnitudes of naked-eye comets were very ambiguous. Often they seem to refer to the brightness of the intense nuclear condensation — the strong, sometimes starlike feature seen at the heart of the coma. As such, the total brightness of the comet's head was usually underestimated. Not until the turn of the century were satisfactory visual methods developed for determining the brightness of extended objects.
As with variable stars, ascertaining a comet's brightness requires two comparison stars of known magnitude — one slightly brighter than the comet and the other slightly fainter. It helps greatly if they are all in the same field of view and at a similar altitude above the horizon to avoid errors caused by atmospheric extinction.
Listed, on the next, are five widely recognized methods used by amateurs to estimate a comet's integrated brightness. Each has its faults, but all (except perhaps the last one) will give acceptably accurate results if carefully employed.
Secrets of High-Power Comet Observing
Here's what to look for before going to the eyepiece.
by Stephen James O'Meara
Before Comet Hyakutake (C/1996 B2) leapt high across the celestial stage in the spring of 1996, humankind had not seen a great comet for 20 years. Anyone who was under a dark sky will not soon forget how the comet gradually unfurled its tail — a long, ghostly banner of blue light — that ultimately traversed half the night sky. The unexpected splendor of Hyakutake's tail caused many of us to push aside the telescope and marvel at the phenomenon with the unaided eye. It seemed as if we had forgotten how beautiful a comet can be.
Yet the tail was only a part — albeit a big part — of the comet's spectacular close approach to Earth. Indeed, observers who found time to glimpse Hyakutake's bright head under high magnification were rewarded by dynamic displays of Sunward jets and a tailward spine. Then, all too soon, the comet and all it had offered faded into the twilight glow. Who really had time to fathom the dynamic action going on close to the comet's nucleus when so grand a sight was before us?
Fortunately, another bright comet, Hale-Bopp (C/1995 O1), arrived, and remained visible to the naked eye for months, not days. Amateurs with even small telescopes had an opportunity to probe this comet's nuclear regions and see some of its most secretive details.
A comet is a whimsical creature. Its behavior mimics that of a cat, being largely independent of our will and wishes. Yet Hale-Bopp was a splendid telescopic sight during the summer of 1996, some nine months before the peak of its display. By then a vast array of jets spewed dust and gas from a seemingly pulsating nucleus. Multiple parabolic hoods surrounded the inner coma, and a needlelike spine occasionally appeared in the antisolar direction. Such features are sometimes fleeting, sometimes dramatic. They go through cycles of brightness and periodically have odd appearances.
The clarity of the features in Hale-Bopp's head was outstanding by historic standards. Comet expert John Bortle notes that only a handful of bright comets in the last century have displayed such distinct, near-nuclear structure — particularly when more than 0.75 astronomical unit (110 million kilometers) from the Sun. Yet Hale-Bopp even displayed conspicuous structure when more than 4 a.u. out!