Comet 17P Holmes


An unexpected opportunity to see a comet that is normally too small and faint to be seen except in large telescopes.

Comet 17P/Holmes exploded on 25 October 2007, sending a cloud of debris into space and increasing it's brightness by a million times, 15 stellar magnitudes. This transformed it into a second magnitude star, rivalling those in its host constellation - Perseus.

As the cloud expanded away from the comet's nucleus at around 2km per second so it came to look less like a star and more like a comet. However, owing to the great distance at around 200 milliom km, the expansion was only around 1 minute of arc (1/60th of a degree) a day - too slow to watch in real time but quite evident from night to night.

The cause of the explosion is unclear, though the most likely explanations are a collapsing void in the nucleus as it evaporated due to heating by the sun or a collision with companion piece of nucleus . This is the second recorded such event on Holmes, the first in 1892 being the one that lead to the comet's discovery.

29 - 30 October 2007

I first saw Holmes on the 29th, 4 days after the outburst, and looking like a star to the naked eye but in binoculars as a well defined grey disk with a bright core. I brought my 80mm telescope to bear on the following day and was able to photograph it for the first time on the 31st.

Comet Holmes, thr Pleiades & the Moon

Images 1 to 3 of the above montage show Holmes, the Pleiades star cluster and the Moon all photographed at the same scale on the same night. The 4th picture is a larger view of the comet on the same evening, 31 October 2007.

1 and 4 show Comet Holmes as a circular disk, in reality a sphere, of dust with a well defined explosion front. A bright inner coma expands preferentially to the right, away from the sun, which shines from bottom left in this view. No hint of a tail is visible on these short exposures.

All photos taken by afocal projection (25mm eyepiece) with Minolta Dimage 7 on Helios 80mm refractor. Shown here at 50% of original size.

1 and 2. Zoomed at 7mm. 4s at equivalent f/1.4, ISO400

3. Zoomed at 7mm. 1/125s at equivalent f/1.4, ISO100

4. Zoomed at 15mm. 4s at equivalent f/2.8, ISO800

13 November 2007

Holmes continued to expand, though appeared fainter as the same material became spread out over a larger area of sky. These images, both at the same scale as the ones above show the change over 2 weeks.

Holmes on 31 Oct & 13 Nov 2007

Both photos were taken with an exposure of 4 seconds, though the 13th November one was taken at ISO800 rather than the original ISO400. To deal with the increased the amount of noise in the picture, I combined 10 images into 1 and increased the contrast to emphasise the stars and comet and reduce the background. 4 seconds is unfortunately the longest exposure possible owing to my having lost the camera remote!

It still shows the comet to have a clearly defined front to the cloud of material thrown off in the original explosion .It is now 1.5 million km in diameter, larger than the Sun, and appears nearly as large as the Moon in the sky. However, as the coma has expanded so it has grown fainter so requires a longer exposure to capture, and similarly is becoming harded to see to the naked eye.

This exposure is long enough only to show the coma, which has still retained its roughly circular outline despite the expansion it has undergone. There is no hint of a tail, though other people's long exposures do show a faint blue gas tail - see Space Weather for a good collection of images.

The image below shows Holmes at the full scale at which I actually photographed it. Hover the mouse over the image to see a captioned version that points out the comet's nucleus, which is faintly visible just above a bright star which shines through the inner coma.

Holmes captioned

Full scale picture of Holmes on 13 Nov 2007. Hover the mouse over it to see a captions detailing the various elements of the comet and the names of some of the stars, or click the picture to open it in a new window.

It is hard to imagine that the very faintest star I have pointed out on the picture, at magnitude 11.2, is 6 magnitudes or 250 times brighter than the entire comet under normal circumstances. Magnitudes work in reverse so the fainter the object the higher the number - normally Holmes is around magnitude 17.

29 November 2007

There were few clear nights in the weeks that followed, and with the Moon coming around again, my next view of Holmes was not until 29th November - another 2 weeks later.

The comet had moved by only a few degrees and was still within Perseus, but by now it was not easy to see with the naked eye. However, it was obvious enough with slightly averted vision - staring just 1 or 2 degrees sideways the comet popped into vision!

Reports said that the dust cloud was now 8 times the volume of the sun, but with little new material being added to it, it meant that it would be more thinly spread, and hence fainter. It was still a fine sight in binoculars, the circular outline seen previously having expanded further and taken on a slighly oval shape, with the cloud being pushed to the right, away from the sun. Unfortunately no opportunity for a photograph, but hope to take another soon. Meanwhile the gallery at Spaceweather continues to grow...



The comet has now faded to invisbility but observations by NASA's Spitzer Space Telescope suggest that Holmes exploded like 24 kilotons of TNT and ejected 10 million metric tons of dust and gas into space. These numbers fit a model favored by Reach in which a cavern of ice some hundred meters beneath the comet's crust changed phase, from amorphous to crystalline, releasing in transition enough heat to cause Holmes to blow its top. Who knows when it may happen again?



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