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
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
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