Physics Photo of the Week

February 2, 2007

Horsehead Nebula

One of the most amazing astronomical objects is the Horsehead Nebula in the constellation of Orion.  The silhouette of a horse's head is actually a dark cloud of dust.  The Horsehead is impossible to see without photographic time exposures.  The advent of inexpensive CCD cameras and digital cameras make astrophotography much easier than the photography with film or emulsions.  Astronomical image processing software is also needed to co-add or "stack" many short exposures in order to eliminate noise or random "speckle" in the images.  This image was photographed on December 28, 2006 using the 8-inch Celestron Ultima 2000 telescope donated to Warren Wilson College by Bernard Aghierre and a Meade Deep Sky Imager Pro (an "inexpensive" astronomical camera) and an infrared-blocking filter.

The Horsehead Nebula is part of a very extensive and massive dust cloud in the constellation of Orion.  The Orion Nebula is part of this same dust cloud.  (See PPOW for April 7, 2006 and PPOW for January 11, 2004).  The dust cloud is about 1600 light years distant and about 100 light years across.  The Horsehead itself is about 5-10 light years across.  The backdrop light that shows the silhouette is hydrogen light from less dense clouds beyond of the horse's head.  Ultraviolet light from a star, Sigma Orionis off the right side of the picture, causes hydrogen clouds to fluoresce.  The hydrogen atoms absorb the ultraviolet light and re-emit the energy as a longer wavelength at 656 nm characteristic of hydrogen.  Careful inspection of the top of the horse's head reveals a brighter glow just above the head.  This is believed to be fluorescence at the edge of cloud.  A photometric scan of the image brightness is shown in the panels below.

The image at right shows an enlargement of the top of the horse's head.  The "top" of the head is just left of the center of the image.   The box represents a strip for which the pixel brightness is plotted in the bottom panel.  As can be seen the brightness is enhanced just at the right edge of the cloud.  The fluorescence is enhanced by the presence of hydrogen in the cloud.  The density of the cloud (particulate matter) prevents the ultraviolet light from reaching the hydrogen within the cloud.

Dark clouds in interstellar space tend to be sculpted by the intense ionizing radiation from the nearby stars.  Notice how the right-hand edges of the dark cloud are slightly concave due to the sculpting process.

A color image of Horsehead Nebula has also been produced.  That process will be described in a later Physics Photo of the Week.

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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