Physics Photo of the Week

December 7, 2007

Comet Holmes is Growing

October 29, 2007
Image acquired by Jenna Blakley and Jake Lyerly
November 7, 2007
Image taken by Chloe Stuber, Pengye Su, and Emily Woodall.
Processed by Jenna Blakley
November 28, 2007
Image acquired by Taylor Sanford, Gordon Jones, and Valerie Moore

Astronomers as well as the astronomy students at Warren Wilson have been    ` recording Comet Holmes during the past month.  This comet is highly unusual in that it suddenly erupted in late October releasing a large cloud of gas and dust.  This cloud of gas and dust has been steadily expanding.  It is expanding into the vacuum of space.  The radiation pressure from the sun is also forcing the gas to move further away from the sun to form a tail.  The location of the sun relative to the Earth for the comet at this time forces the tail in the direction directly behind the comet, consequently we cannot see the general shape of the tail.  We are looking at the comet head on.  The comet is also travelling further away from the sun.  However, the cloud is growing in size faster than the comet is receding - thus appearing larger and larger.

The cloud of gas and dust has originated from one major eruption.  Since this eruption the gas has been expanding into the vacuum of space, becomming more diffuse.  As a result of the gas becoming less dense, it has become less bright - fewer molecules to backscatter the sunlight.  The comet is so bright in the first image that only a couple of stars are visible.  In the last image the comet is quite faint compared to the stars.  Consequently we can see many more stars through the cloud of the comet.  In each image, the brightness levels is adjusted so that the comet does not look overexposed.  The image contrast is maximized in the last image to make the comet look "normal", but the large increase in contrast results in many more stars made visible.

Most of the light from the comet is reflected or scattered sunlight - scattered by the gas and dust of the coma.  Visual observers using binoculars report a greenish appearance.  Careful spectral analysis by C. Buil reveal faint emission features caused by fluorescence.

Below are two images of the comet taken with a digital camera (Canon A630) with out a telescope.  A number of 15 sec exposures were taken and co-added.  We can see how much the comet has moved in the 18 day interim.  The bright star in the center of the photographs is Mirfak - the brightest star in the constellation of Perseus.   

November 10, 2007
Photo by Don Collins from Warren Wilson College
November 28, 2007
Photo by Emily Woodall and Don Collins from Bee Tree Gap on Blue Ridge Parkway

These two images are reproduced at the same scale.  The reason the picture on the left shows sharper star images is that the picture on the left was taken with a 4x optical zoom.  The image on the right was taken with normal focal length, but the picture had to be expanded digitally to match the scale at left.  The digital zoom loses resolution.

Astronomy Picture of the Day has featured Comet Holmes 11 times since the October 24 eruption.  This is the second PPOW feature on Comet Holmes

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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