Physics Photo of the Week

April 25, 2014

M81 - A galaxy in Ursa Major

M81 is a prominent spiral galaxy near the Big Dipper - currently high in the evening sky and viewable with small telescopes.  It is perhaps the 2nd largest galaxy in terms of its area in the sky second to the Great Galaxy in Andromeda (PPOW for November 15, 2013 and Oct. 12, 2007). 

The angular size of this galaxy is so large that about 6 separate images had to be aligned and pieced together to make this composite image - it still didn't image the whole galaxy.  The telescope (a 14 inch aperture at the College View Observatory) has a rather large focal length, and the size of the CCD detector camera is such that the field of view only spans only about 1/5 degree.  The full Moon diameter is about 1/2 degree. 

M81 was discovered in 1774 by Johann E. Bode, a German astronomer.  Consequently, it is often referred to as Bode's Nebula (Bode's Galaxy in modern terminology).  Messier soon thereafter named it M81 in Messier's catalog of faint objects that were definitely not comets.  It wasn't until the early 20th century that Edwin Hubble discovered that these spiral nebulae were located at great distances from the Solar System by measuring variable stars using the 100 inch reflector on Mt. Wilson, California.  On account of the great distances of Galaxies (12 million light years for this galaxy; 2 million light years for the Andromeda Galaxy) these galaxies contain countless stars (about 100 billion) and they are about the largest structures in the Universe.

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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