Physics Photo of the Week

February 15, 2008

Horsehead Nebula
The famous Horsehead Nebula is a popular deep sky object found in the constellation Orion.  The dark shape of a horse sillouette is a cloud of dust and gas in interstellar space.  The area of Orion, high in the southern sky in February, contains many such dust clouds - all part of the Milky Way.  A monochrome image of the Horsehead was featured a year ago (Feb. 2, 2007) where much of the physics of the nebula was discussed. 

This image was produced in the Contemporary Astronomy class at Warren Wilson College on Jan. 30, 2008 by students Kat Coker, Suzanne Lutsky, Daniel McKinny, Anne Liffiton, and Raya Cooper assisted by physics professor Donald Collins.
  The photograph involved many exposures for 30 seconds each through red, green, and blue filters.  The images through each filter were co-added or "stacked" to increase the image viability.  Finally the different colors are combined to give a final color result.  The telescope is an 8-inch diameter Schmidt-Cassegrain telescope and a SBIG CCD camera.

Notice that many more stars are visible on the right side of the photo than on the left.  The dark cloud blocks many of the background stars on the left side.  The visible stars on the left are probably foreground stars in front of the cloud.  The red color of the background is real.  There is a bright star (Sigma Orionis)
off the right side of the picture that is very hot and emitting invisible ultraviolet radiation.  The UV radiation impinges on clouds of hydrogen and cause the hydrogen to glow or "fluoresce" - emitting visible light of lower energy due to the partucular energy levels of atomic hydrogen.

The location of the Horsehead Nebula is indicated by the cross-hairs in the image of Orion at right.  Notice that one cannot see any of the horsehead formation in the image.  The star Sigma Orionis that causes the clouds to glow is located in the upper right quadrant of the cross hairs.

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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