Sky and Telescope - May 2012 - (Page 40)

Sungrazer
ing the comet’s tail projecting up from the brightly lit dawn horizon. Until this point, Comet Lovejoy’s coma displayed a normal, condensed appearance. But this abruptly changed on December 20th when the comet’s head exhibited a long, bright tailward-pointing ray instead of a compact central condensation. This ray grew longer each day as the comet’s head became weaker and more diﬀuse. Comet expert Zdenek Sekanina at JPL proposed that Lovejoy’s nucleus completely disrupted about December 17.6 UT, and that the stream of resulting debris was moving rapidly outward into the tail, forming the ray-like feature. This debris ranged from boulder-sized pieces nearest the position of the comet’s former nucleus to micron-sized dust stretching well out into the tail. We may well have been witnessing the creation of innumerable future pygmy sungrazers. With the comet moving away from the Sun and the tailing growing longer each day, Comet Lovejoy looked like a searchlight beam projecting up from the horizon before it was washed out by increasing twilight. On the morning of December 22nd, observers were describing the comet’s dust and ion tails as each about 16° long and similar in brightness to the Large Magellanic Cloud. That same day the crew of the International Space Station watched the comet as it rose over Earth’s curving limb. While some observers thought that the fading of the comet’s head heralded a rapid demise of the tail, the performance of past sungrazers suggested that the show was just beginning and that Comet Lovejoy might unfurl a truly enormous tail in coming days. And it did just that. By Christmas morning the tail had grown to at least 28°! As the New Year opened, there were reports of the comet’s dust tail stretching across 40° of sky, but the head had faded so much that it could hardly be seen with the unaided eye. Like the Great Southern Comet of 1887, Comet Lovejoy had essentially become only a tail. The searchlight appearance of the tail was maintained
Dec. 16, 2011 Dec. 17, 2011

In the days following perihelion, the bright central condensation at the comet’s head evolved into a slender ray-like feature that is thought to have formed as debris from the disrupted nucleus spread outward into the tail. This view was obtained at 17:49 UT on December 24th with the Uppsala Schmidt Camera at Australia’s Siding Spring Observatory.

as it faded and grew ever more ghostly and transparent with the passing days. Although the end of the tail faded into the Milky Way in Crux and Centuarus, images showed that its length was an astounding 45° when it peaked in mid-January. This corresponded to a truly enormous 1¼ astronomical units! But because of its faintness, the tail was noticeably shorter to the unaided eye. By January 20th, the comet could only be seen visually from the darkest, pristine observing sites. Photographs in February captured only hints of the grand visitor as the comet disappeared from view, but Comet Lovejoy’s legacy will surely be remembered for a long time to come. ✦ Amateur astronomer John Bortle has a long history of observing and writing about comets, variable stars, and lunar eclipses from his home in Stormville, New York.

STEREO IMAGES COURTESY NASA / NAVAL RESEARCH LABORATORY

Dec. 18, 2011

Dec. 19, 2011

The changing appearances of Comet Lovejoy’s brilliant, sweeping dust tail and slender, straight gas tail are evident in this daily sequence obtained by the Heliospheric Imager on the STEREO A spacecraft between December 16th and 19th as the comet moved away from the Sun. Streaks emanating from bright objects are artifacts due to overexposure. The notably bright “stars” in the ﬁeld are the planets Mercury (fainter of the pair) and Jupiter, which appeared relatively close together from the spacecraft’s vantage about 45° ahead of Earth on our orbit around the Sun.

40 May 2012 sky & telescope

ANU / UNIVERSITY OF ARIZONA / NASA / ROBERT H. MCNAUGHT

Table of Contents for the Digital Edition of Sky and Telescope - May 2012

Sky and Telescope - May 2012
Table of Contents
Spectrum
Letters
75, 50 & 25 Years Ago
News Notes
Saturn’s Raging Superstorm
Stars Above, Earth Below: Astronomy in National Parks
The Remarkable Case of Comet Lovejoy
In This Section
May’s Sky at a Glance
Binocular Highlight
Planetary Almanac
Northern Hemisphere’s Sky
Sun, Moon, and Planets
Celestial Calendar
Exploring the Solar System
Deep-Sky Wonders
Going Deep
S&T Test Report
New Product Showcase
Telescope Workshop
Planetary Imaging with Your DSLR Camera
Gallery
Focal Point

Sky and Telescope - May 2012