The Extraordinary Outburst of Comet 17P/Holmes


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A collage of Comet Holmes images taken between October 27 and December 14, 2007, showing the comet's changing
appearance from 3 days to 6 weeks after its outburst, its rapid expansion even as its surface brightness  decreased.
Photographs by Tony Hoffman, taken with a Canon Digital Rebel XTi and a 200mm lens. Most are composites
of multiple RAW images stacked in DeepSkyStacker. The collage was produced in Microsoft Paint.

If comets are like cats, as David Levy has quipped, then Comet 17P/Holmes is surely a Manx. Nearly tailless, it's spent most of its history in obscurity, and was even lost to astronomers for 58 years, fullly half of the time that's elapsed since its discovery in 1892. When visible at all, it's as a very faint object, only to be seen in large telescopes. Thus, it came as a big surprise when, in little more than a day in late October 2007, it brightened more than 600,000-fold to become an easy naked-eye object, even from large cities. (I saw it with my unaided eye on more than a dozen nights from New York City, and many more nights in binoculars. In appearance, shortly after the outburst, the comet resembled a planetary nebula--a star that has ejected a shell of gas, which surrounds the star, appearing like a disc--and the resemblance was more than coincidental. The comet had expelled a spherical cloud of gas, followed by a similar dust cloud, both expanding rapidly.  (Comet expert Zdenek Sekanina has proposed that the outburst occurred when a pancake-shaped companion object, which had peeled off Comet Holmes, completely disintegrated.)



Comet 17P/Holmes, photographed on October 27, 2007,  
from Pughtown, Pennsylvania, 3 days after the comet's outburst
Photographs by Tony Hoffman, taken with a Canon
Digital Rebel XTi and a 200mm lens.


I got my first look at Comet Holmes on October 27 from rural Pennsylvania. Despite the nearly full Moon it was easily visible to the unaided eye. I was struck how beautifully luminous it looked, a bright, yellow center surrounded by a fainter outer circle. We were blessed by a stretch of excellent weather, and I was able to observe the comet from New York City on about 10 of the next 12 nights, and many more nights to come. In binoculars, the comet's expansion from night to night was easy to see. As awesome as it was to have a naked-eye comet visible from the city (it remained so for 3 weeks, looking pretty much starlike for the first week, then increasingly fuzzy), the sight was even more spectacular under dark skies. On November 10, I observed Holmes from West Hurley, NY, near Woodstock. The comet was an obvious, fuzzy glow among the stars of Perseus to the naked eye. In 12x60 binoculars, it looked like a dull cotton ball about the diameter of the Full Moon. In large (20x100) binoculars and a small telescope, it looked more like an eye, the oblong or spindle-shaped inner coma surrounding the ghostly outer coma, which was raggedly symmetrical, as if there should have been a tail although none was visible.By the end of November, the comet looked enormous on November in large binoculars, giving the illusion that it was very close to Earth, though it was really more than two Astronomical Units (Earth-Sun distances) away. Since then, it has continued to expand and fade as it recedes from both Earth and Sun.



Comet 17P/Holmes, photographed on November 27, 2007,  
from West Hurley, New York. Image by Tony Hoffman, a composite
of multiple RAW images
taken with a Canon Digital Rebel XTi
and a 200mm lens, and stacked in DeepSkyStacker.



Comet 17P/Holmes was discovered by Edwin Holmes, a London amateur, in 1892. Since a supernova had burst into view in the Great Nebula in Andromeda in 1885 (it would still be several decades before it was recognized as a galaxy), Holmes would regularly observe that vast, spiral "city of stars". On the rather murky night of on November 7, 1892, he was turning his telescope towards his favorite object when to his surprise he came across a comet, at fourth magnitude nearly as bright as the Andromeda "nebula" itself. The comet slowly faded, then briefly brightened in January 1893, then faded again. Comet Holmes, a periodic comet that orbits the Sun every 6.9 years, then remained inconspicuous for over 100 years--so much so that it was lost to astronomers for 58 years, from 1906 to 1964. It seldom got any brighter than 17th magnitude or so--until its great outburst of October 2007, two months ago today as I write this on Christmas Eve. As it fades back into obscurity (tonight, in bright moonlight, I can hardly see it in 9x63 binoculars from West Hurley as an enormous smudge, barely visible against the background of sky), I can't help but wonder if the comet's next outburst will be in a month, a year from now, or sometime in the 22nd Century.

E-mail to tonyhoffman [at] earthlink [dot] net