by David Jewitt

Spanish version

Last updated 09 Nov 2007
from DavidJewitt Website

 

Summary


Formerly, the Sun was the largest object in the Solar System. Now, comet 17P/Holmes holds that distinction.

Spectacular out-bursting comet 17P/Holmes exploded in size and brightness on October 24. It continues to expand and is now the largest single object in the Solar system, being bigger than the Sun (see below figure). The diameter of the tenuous dust atmosphere of the comet was measured at 1.4 million kilometers (0.9 million miles) on 2007 November 9 by Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii Institute for Astronomy.

 

They used observations from a wide-field camera on the Canada-France-Hawaii Telescope (CFHT), one of the few professional instruments still capable of capturing the whole comet in one image. Other astronomers involved in the UH program to study the comet include Bin Yang, Nuno Peixinho and David Jewitt.

 

The present eruption of comet Holmes was first reported on October 24 and has continued at a steady 0.5 km/sec (1100 mph) ever since. The comet is an unprecedented half a million times brighter than before the eruption began.

 

This amazing eruption of the comet is produced by dust ejected from a tiny solid nucleus made of ice and rock, only 3.6 km (roughly 2.2 miles) in diameter.

Caption:

(Left) Image of comet Holmes from the 3.6-meter Canada-France-Hawaii telescope on Mauna Kea showing the 1.4 million km diameter coma. The white ''star'' near the center of the coma is in fact the dust-shrouded nucleus.

(Right) the Sun and planet Saturn shown at the same scale for comparison. (Sun and Saturn images courtesy of ESA/NASA's SOHO and Voyager projects).

The new image also shows the growth of a tail on comet Holmes (the fuzzy region to the lower right in the comet picture), caused by the pressure of sunlight acting on dust grains in the coma. Over the next few weeks and months, the coma and tail are expected to expand even more while the comet will fade as the dust disperses. Comet Holmes showed a double outburst in November 1892 and January 1893.

 

It is not known if the present activity in the comet will follow the pattern from 1892, but continued observations from Mauna Kea are planned to watch for a second outburst. Most comets show small fluctuations in brightness and some have distinct outbursts. The huge event on-going in comet Holmes is unprecedented, however.

The orbit period of comet Holmes is about 6 years, putting it in the class of Jupiter Family Comets whose orbits are strongly influenced by Jupiter. These objects are thought to have spent most of the last 4.5 billion years orbiting the Sun beyond Neptune, in a region known as the Kuiper Belt. Holmes probably was deflected into its present orbit within the last few thousand years and is losing mass as it evaporates in the heat of the Sun.

 

In another few thousand years it is likely either to hit the Sun or a planet, be ejected from the Solar system, or simply die by running out of gas.
 

 


Contacts

 

 

 

 

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A comet that has delighted backyard astronomers in recent weeks after an unexpected eruption has now grown larger than the sun. The sun remains by far the most massive object in the solar system, with an extended influence of particles that reaches all the planets. But the comparatively tiny Comet Holmes has released so much gas and dust that its extended atmosphere, or coma, is larger than the diameter of the sun.

The comparison is clear in a new image.

"It continues to expand and is now the largest single object in the solar system," according to astronomers at the University of Hawaii.

The coma's diameter on Nov. 9 was 869,900 miles (1.4 million kilometers), based on measurements by Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii Institute for Astronomy.

 

They used observations from the Canada-France-Hawaii Telescope.

 

University of Hawaii/CFHT (comet); NASA/Voyager (Saturn); NASA/ESA/SOHO (sun)Comet Holmes, the Sun and Saturn at all about the same scale.

 

 

NASA,ESA, and H. Weaver (The Johns Hopkins University Applied Physics Laboratory); Alan DyerAmateur astronomer photo, left, shows Holmes with a faint tail; Hubble image, right, reveals bow-tie pattern created by dust.

NASA/ESA/T. Dickinson, Ontario, CanadaThe location of Comet Holmes on various dates is superimposed on this amateur astronomer's image.

 

Percy MuiOct. 27: Comet Holmes, captured in a 2-minute exposure taken by Percy Mui from the Karl G. Henize Observatory at Harper College in Palatine, Ill.

 

Arkansas Sky Observatories/Clay SherrodOct. 25: The usually dim Comet 17P Holmes blazes away in an image from the Harvard MPC H45 observatory at Petit Jean Mountain in Arkansas.

 

The sun's diameter, stated differently by various sources and usually rounded to the nearest 100, is about 864,900 miles (1.392 million kilometers). Separately, a new Hubble Space Telescope photo of the comet reveals an intriguing bow-tie structure around its nucleus.

The comet's coma — mostly microscopic particles — shines by reflecting sunlight.
 


See for yourself

Holmes is still visible to the naked eye as a fuzzy star any time after dark, high in the northeastern sky. It is faintly visible from cities, and from dark country locations is truly remarkable.

"Right now, in a dark sky it appears as a very noticeable circular cloud," said Joe Rao, SPACE.com's Skywatching Columnist.

Rao advises looking for the comet this weekend, before the moon becomes more of a factor. The comet will likely diminish in brightness yet remain visible for the next two to three weeks, he said.

"Over the next few weeks and months, the coma and tail are expected to expand even more while the comet will fade as the dust disperses," Stevenson and her colleagues write.

On Monday, Nov. 19, the comet will create a unique skywatching event with its see-through coma, according to the Web site Spaceweather.com:

"The comet will glide by the star Mirfak [also called Alpha Persei] and appear to swallow it — a sight not to be missed."

A small telescope will reveal the fuzzy coma. Lacking a long tail characteristic of some great comets, however, Holmes is not the most dramatic object in the sky for casual observers.
 


Mystery outburst

Nobody knows why Holmes erupted, but it underwent a similar explosive brightening in 1892. The recent display, which began Oct. 24, brought the comet from visual obscurity to being one of the brighter objects in the night sky. It has since dimmed somewhat as the material races outward from the nucleus at roughly 1,100 mph (0.5 km/sec).

The Hawaiian astronomy team writes in a press statement:

"This amazing eruption of the comet is produced by dust ejected from a tiny solid nucleus made of ice and rock, only 3.6 kilometers (roughly 2.2 miles) in diameter."

The new image from the Hawaiian observatory also shows a modest tail forming to one side, now just a fuzzy region to the lower-right. That's caused by the pressure of sunlight pushing on the gas and dust of the coma. But the comet is so far away - 149 million miles (240 million kilometers), or about 1.6 times the distance from Earth to the Sun - that even Hubble can't resolve its nucleus.

The offset nature of the coma, seen in ground-based images, suggests,

"a large fragment broke off and subsequently disintegrated into tiny dust particles after moving away from the main nucleus," Hubble astronomers said in a statement Thursday.

The comet's distance, plus all the dust, prevent Hubble from seeing any fragments, however.