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CHICAGO (Reuters) - The normally sedate Comet Holmes made a bright splash in the sky about two weeks ago, unexpectedly becoming a million times brighter than normal overnight and causing a stir among astronomers.
The comet and its expanding ball of dust have become the biggest object in the solar system, with a diameter appearing even bigger than the sun, according to astronomers at the University of Hawaii Institute for Astronomy.
The fireworks began on October 23, when the comet brightened in a 24-hour period. It is now fading but can still be seen by the naked eye in the northeastern sky.
"Something that spectacular is almost like a supernova going off," said Hal Weaver, a comet expert at The Johns Hopkins University Applied Physics Laboratory.
"Now, we're trying to observe the event and figure out what the heck happened," Weaver said in a telephone interview on Thursday.
Weaver and fellow astronomers, using the powerful Hubble telescope, have some clues that may help explain the display. So far they have noticed that the comet's nucleus, the small solid body that is the main source of all the comet's activity, is still surrounded by dust.
He and colleagues believe some sort of explosion sheared off a pancake-shaped slab of the comet, which crumbled into tiny dust particles that are being lit up by the sun.
Hubble images of the comet from June 1999 showed no dusty halo. At that time, they measured the comet's nucleus at some 2.1 miles, about the length of New York City's Central Park.
They are now hoping to examine the comet's size to see how much of it was sheared away in the blast.
Weaver said comets typically get most active when they are nearest to the sun, but Comet Holmes, known officially as 17P/Holmes, made its closest approach to the sun six months ago.
One reason he offered for the delayed effect was the presence of volatile ice made up of carbon monoxide, dry ice, ethane, methane and acetylene and hydrogen cyanide. These, he said, may have gotten heated up but could not escape the comet's nucleus, causing a build-up of pressure that eventually exploded.
Weaver also thinks there might be a seasonal effect, in which different portions of the comet's nucleus are visible only at certain points of the comet's orbit around the sun, which happens every seven years.
Comet Holmes put on a similar display in 1892, which astronomers believed was caused by a collision with another comet, which sliced off a piece of Holmes' nucleus that slammed back into it again, sending up a showy spray of dust.
"Now, we step forward to 2007, and the same thing is happening again. It indicates that hypothesis is incorrect," Weaver said.
While the magnitude of the light show is spectacular, he said comets do tend to flare up every now and then.
Editing by Will Dunham and Xavier Briand