Physics Photo of the Week

October 15, 2009

The Trifid Nebula
Astronomy students - Kesari Fleury, Dan Segal, Marlon Cohn, Christina Grubb, and Madalyn Baldwyn - all helped produce this vivid color photo of the Trifid Nebula, also known as Messier 20 in early September, 2009.  This object in Sagittarius is a luminous "ball" of mostly hydrogen gas plus some veins of dust that make the dark lanes against the pink background.  This object is in our own Milky Way Galaxy.  It's luminosity is caused by fluorescence of the hydrogen gas.  Some very hot young stars near the center emit strong ultraviolet radiation.  The hydrogen clouds absorb the ultraviolet then re-radiate it in the strong visible color characteristic of hydrogen - mostly red.  A more general classification for this object is to call it an "emission nebula" because the colors are discrete emission lines as opposed to broadband radiation characteristic of stars.  (See PPOW for April 16, 2004 for a discussion of emission lines). 

Such an emission nebula is also a place where stars are currently forming.  The hydrogen clouds collapse under their own force of gravity, become denser and denser until they become dense enough to trigger nuclear fusion of hydrogen into helium - thus forming new stars.  The stars in these emission nebulas are about 100 times more massive than the Sun.  Consequently they burn at much higher temperatures than the Sun (30,000 K instead of 6,000 K) and emit much ultraviolet radiation.  The most famous example of an emission nebula giving birth to young, hot stars is the Orion Nebula (PPOW for Jan. 23, 2009, and PPOW for Apr. 24, 2009).

The color image was obtained using a deep sky CCD camera on Warren Wilson College's 8-inch portable telescope.  The monochrome camera is equipped with a set of red, green, and blue filters.  The red, green, and blue images are then combined by the students with special software to produe the composite color image.

There will be no Physics Photo of the Week next week on October 23 due to WWC's fall break.  The next Physics Photo of the Week will be published on October 30, 2009.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature 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 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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