November 9, 2007

Brighter Comet Now Visible to Naked Eye

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Correction appended. See below.

Since its discovery in 1892, Comet Holmes has not drawn much attention. However, the comet’s recent “explosion” on Oct. 23 has increased its brightness more than half a million times. The comet has now become the latest sensation among amateur and professional astronomers at Cornell and worldwide.
The comet, which was previously invisible to the unaided eye, can now be easily seen.
“It is visible in the constellation of Perseus, which rises in the Northeast in early evening. It’s very high in the North sky at around midnight, and it forms a nice triangle with two of the brightest stars in Perseus,” said Michael Roman ’06, radiophysics and space research.
“It is easily visible as a fuzzy disc with the naked eye. Binoculars, however, will reveal more details,” he added.
According to Prof. Joseph Veverka, astronomy, the comet’s drastic increase in brightness is due to the expansion of its outer dust layer. Prior to the outburst, Comet Holmes was a dim object. Due to unknown reasons, a large amount of dust, water ice and other materials was recently ejected from the comet’s surface. These materials form the coma, a large thin, cloud that surrounds the nucleus of the comet and reflects sunlight. Although the comet is currently between the orbits of Jupiter and Mars, its incredible diameter of about 600,000 miles, which is 2.5 times the distance of the earth from the sun, contributes to its current visibility from earth.
The reasons behind the comet’s sudden expansion of its coma remain largely speculative.
“We don’t know exactly why it happened,” said Veverka, who is also involved with NASA’s asteroid missions. Even so, he offered two possible explanations.
The first hypothesis suggests that the outburst may be due to an external collision. The comet may have run into a rock in space, and such a collision would have resulted in the comet’s expulsion of materials.
Another hypothesis suggests that the comet’s internal structure helped to bring about its outburst. This hypothesis assumes that the comet contains a large number of carbon-dioxide ‘pockets’ beneath its surface. When the comet orbited closer to the sun, its external water ice may have evaporated and exposed these ‘pockets’ to the sun’s heat. This situation would have resulted in a large outburst as carbon-dioxide quickly evaporated.
Although it has been more than two weeks since its surprising outburst, Comet Holmes’s coma continues to grow both in size and brightness. According to the journal New Scientist, the coma is now expanding at a rate of 0.5 kilometers per second. This growth in size can be easily observed through the telescope in Cornell’s Fuertes Observatory, as noted by Shianne Beer ’08, vice president of Cornell Astronomical Society.
“It’s not static … it gets bigger every time I see it,” said Beer.
“What you see in general is a big cloudy disc, and the core is a lot brighter in the centre. You can see it’s a bit dissymmetrical,” said Russell Wolf ’11, who also saw the comet at Fuertes’s special open house on Halloween night.
Although observers can easily identify a brighter spot in the midst of the comet’s surrounding fuzz, Veverka pointed out that the nucleus of the comet is too small to be seen as it is only about two miles across.
“You’re still not seeing the comet itself, but the dust around it,” he said.
The latest photographs also show that the comet may be forming a tail. It is unlikely, however, that observers can find the tail easily without a very powerful telescope.
“Any tail that is from the recent outburst is oriented towards us, [so] we’re seeing it from an oblique angle,” said Roman.
Veverka noted that due to its small size, the comet probably does not have much gravity. In the next few weeks, as the dust leaves the comet and spreads off into space, the comet will return to its previously dim state.
Members of the Cornell community are welcomed to visit the Fuertes Observatory to catch a clearer glimpse of the comet through its telescope. The observatory is open to the public every clear Friday night. The comet is also visible with the naked eye and a pair of simple binoculars. Now all we need are clear skies in Ithaca.

Brighter Comet Now Visible to Naked Eye incorrectly states that 600,000 miles is 2.5 times the distance of the earth from the sun. 600,000 is actually 2.5 times the distance of the earth from the moon. The Sun regrets this error.