Physics Photo of the Week

Nov. 3, 2006
Comet SWAN (C/2006 M4). 
Photos and discussion by Donald F. Collins

These nights (late October - early November 2006) a comet is visible - but optical aid such as binoculars or telescope - are needed.  This comet was discovered by the Solar Wind ANisotropy instrument on a spacecraft, hence the name "SWAN".  There will be a public viewing of this comet on Monday, November 6 at the Warren Wilson Physics Department at 7:30 pm. (weather permitting).

Comets originate in the outer parts of the Solar System from a large volume of space called the Oort Cloud.  Comets have been in the Oort Cloud for most of the 10 billion years of the existence of the Milky Way Galaxy.  Hence the composition of the comet is of intense interest among astronomers about the composition of the solar neighborhood before any planets were formed.

Comets consist of volatile materials (frozen methane and other ices) plus small pieces of rock or "dirt".  Hence the name "dirty snowball" coined by the late Fred Whipple.  When comets travel close to the sun in the inner Solar System the volatile materials vaporize from the heat of the sun.  This  produces a large coma or cloud of gas and dust surrounding the head of the comet - called the "coma".  One or two tails are also visible when the comet is close to the sun.  A long straight tail is visible in this photograph. 

The nucleus of a comet (the "dirty snowball") is very small compared to the size of the coma that we see.  The nucleus is only a few km in diameter, while the coma for comet SWAN is about 12 earth diameters (as measured by astronomy student Caitlin Harrison).  This number results in the coma cloud to be between 10,000 to 15,000 times larger in diameter than the nucleus of the comet.  On the scale of the picture above, the nucleus would be a tiny pinpoint - much smaller than a single pixel.

The photograph above also is deceptive regarding the density of the gas and dust in the head of the comet.  This digital photograph is produced with a CCD camera mounted on a telescope.  The CCD camera produces pixel values that are proportional to the amount of light.  Of course the comet does not produce its own light, but the dust scatters sunlight.  The scattered sunlight makes the comet visible.  With analytical software, we can profile the comet and plot the intensity of light as a function of position.  The photo at right is a negative image of the top photo in the region of the comet's head.  The horizontal line represents a line through the comet's head in which the intensity is plotted.  The intensity graph (pixel value vs position through the head) is plotted.  Notice that the intensity of light scattered from the comet head rapidly diminishes from a peak value in the center of the head to eventually blend in with the sky background.  The profile shows much more information concerning the density of gas and dust in the comet's head than the photograph alone.
These photos of Comet SWAN also show a second tail.  Comets often produce two or more tails.  A second tail is barely visible in these photos.  The long prominent straight tail is called an "ion tail".  The ultraviolet radiation and solar wind from the sun ionizes some of the comet gas molecules.  The charged ions are "blown" away from the comet by the pressure of sunlight and solar wind.  The ion tail (consisting of the ionized gas molecules) follows the interplanetary magnetic field, which is relatively straight in the neighborhood of the comet.  To the right of the long ion tail, a faint dust tail is barely visible.  On larger comets, the dust tail is curved and resembles a comma punctuation mark.  The dust is not confined to move along the magnetic fields, but is governed by the laws of momentum conservation as the dust is blown further from the comet.  The best way to exhibit the dust tail is the profile plot across the tail part of the comet as shown by the photograph at right and the plot at left.  Both tails are very faint, hence the photmetry of the tail structure is very "noisy" and jagged compared to the photometry through the head of the comet.
Finally the motion of the comet is shown in the animated photo at left.  Each of the four images shown in the animated sequence consists of a "stack" of 16 10second exposures.  The total span of time is about 10 minutes.  The comet is seen moving to the southwest in the sky.  The tail always points in the direction away from the sun.  Notice that the whole comet and tail structure is moving "sideways" relative to the direction of the tail.  The motion is the result of the comet's orbit around the sun as well as the Earth's motion.

Excellent high resolution photos of Comet SWAN can be found at Astronomy Picture of the Day for October 28, 2006, APOD for October 4, 2006, and Earth Science Picture of the Day for November 1, 2006.

Many thanks to Mr. Bernard Arghiere for the donation several years ago of the Celestron Ultima telescope to Warren Wilson College.

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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