Physics Photo of the Week

February 28, 2014

Messier 42 - The Great Orion Nebula
The Great Orion Nebula, also known as Messier 42, is the brightest deep sky object visible from the northern hemisphere.  It is easily observable with binoculars and even the naked eye.  This is the first photo of this popular nebula from the new College View Observatory using the 14-inch telescope generously donated by alumnus Gary Starkweather (WWC 1978).

The Orion Nebula consists of a giant cloud of hydrogen gas about 1600 light years distant in the constellation of Orion.  New stars are currently forming in the dark parts of this cloud.  If you look carefully in the central brightest region of the nebula are four close stars forming a trapezoid shape, appropriately called the "Trepezium" cluster.  The photo at right shows a "close-up" view of the trapezium.  The stars in the Trapezium are responsible for causing the very extensive Orion nebula to glow.  These Trapezium stars are extremely hot (about 30,000 K    or       5 1/2 times hotter than the Sun).  This is on account of the fact that they are extremely massive.  The brightest of the Trapezium stars is about 40 solar masses and very luminous (about 250,000 times the luminosity of the Sun), and very young (about 1 million years).  In contrast the Sun is about 6 billion years old.  These hot, massive stars do not last very long.  In only a few million years, Theta1 Orionis C (the designation for the largest of the Trapezium stars) will explode as a supernova.  A supernova this close to the Earth will easily be visible in the daytime sky.

The glow in the Orion Nebula is analogous to a fluorescent lamp.  The hot stars in the Trapezium emit most of their energy in the ultraviolet (UV) radiation rather than visible light.  UV is so energetic that it excites the electrons and ionizes the hydrogen of the cloud.  When the electrons recombine with the protons, the reforming atoms give off characteristic visible hydrogen spectral lines of a characteristic red, blue-green, and violet colors.  The spectrum of hydrogen emission was featured by PPOW on April 16, 2004.

The last time the Orion Nebula was featured in PPOW was March 11, 2011,  photographed with WWC's 8-inch telescope.

Due to travel, there will be no Physics Photo of the Week next week, March 7.  PPOW will return on March 14, 2014.

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

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