Comet SWAN Brightens Up!
October 27, 2006by Alan MacRobert
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Comet SWAN is creeping across the stars in the western sky, where it's highest and best seen soon after dark. Its position is shown here for 0 hours Universal Time on each date, which falls on the evening of the previous date in the time zones of the Americas. The tails point in the direction away from the Sun. Stars are plotted to 8th magnitude.
S&TAfter spending the last couple weeks as a nice, sedate little binocular comet, Comet SWAN (a.k.a. C/2006 M4) has about quadrupled in brightness in the last few days. It has brightened from 6th magnitude to about 4½ as seen in binoculars, while telescopes at high power are showing streamers in the inner coma near the nucleus. Observers with dark skies are even spotting it (dimly) with their unaided eyes.
According to an alert from the Central Bureau for Astronomical Telegrams, binocular observers were reporting the comet glowing at only magnitude 5.9 on October 23.75 Universal Time, but about 24 hours later it was variously being called 4.4 and 4.5 in 7x50 binoculars and 4.0 and 4.3 to the naked eye. (It's well known that comets appear to have brighter magnitudes the smaller the instrument you look with.)
At high magnification there's more going on. Imagers F. Montanucci, G. Sostero and E. Guido in Italy wrote, "The inner coma of this comet is showing a remarkable evolution. The visibility of several jets is strengthened (one obvious feature points toward SE). The central condensation shows a very odd, 'peanut-like' shape. Several streamers point to the ion tail."
Last night (Thursday), I swept up the comet immediately with an 8x50 finderscope despite suburban light pollution. The comet was a round, very condensed (centrally concentrated) greenish fuzzball with hints of a long, straight, upward-pointing tail. The comet's head was much brighter than the globular cluster M13 nearby. In my 12.5-inch reflector at 75x, the nucleus was very bright, but I saw no streamers. The outer coma practically filled the ¾° field of view with a greenish glow. (I didn't switch to higher power because a crowd of visitors were waiting in line for a view!)
To find the comet, you'll need to know exactly where to look. Comet SWAN has crossed into Hercules, as shown on the chart above. Note that the dates on the chart are for 0:00 Universal time, which falls on the evening of the previous date in the Americas. Through the weekend it's passing about 3° south of M13, the great Hercules globular cluster, which is magnitude 5.9 — much dimmer.
If you don't know where Hercules is, use the big evening constellation maps in Sky & Telescope or Night Sky, or the smaller versions here online.
Light from the waxing Moon will soon begin to interfere a bit; on Sunday the 29th the Moon is at first quarter. Full Moon comes on November 5th. Then on November 8th we'll again start getting moonless periods after the end of twilight, but by then the comet should be fading.
According to the best predictions, that is. Who knows?
More information is in the November Sky & Telescope, page 60. Here are some images.
