Physics Photo of the Week

September 19, 2014

Galaxy M101
Galaxy M101, visible in small telescopes near the "handle" of the Big Dipper, is stunning by its spiral appearance.  It is thus called the "Pinwheel Galaxy".  At about 21 million light years distant this galaxy contains about 100 billion stars and is about 1 1/2 times larger in diameter than the Milky Way galaxy.  We see no individual stars of the galaxy in this image.  All the bright stars are "foresky" stars relatively close in our own Milky Way galaxy.  The distant galaxy stars are so numerous that they blend together in "clouds" of stars.

Spiral galaxies are obviously dynamic systems and rotate slowly, but the inner stars and clouds rotate faster than the outer parts.  As a result of the differential rotation, clouds build up along spiral paths, these clouds have enough self gravity that they eventually condense gravitationally and form stars making the spiral arms visible.  The spiral arms are thus young stars, much younger than the galaxy itself. 

Galaxies are also huge systems.  The width of this photograph spans a distance of 50,000 light years based on the galaxy's distance of 21 million light years.  At the typical rotation rate of about 80 km/sec about 1/3 the distance from the center to the fringe, a star would require about 1 billion years to orbit the galaxy.  The Solar System in the Milky Way galaxy rotates considerably faster requiring only about 100 million years.

Most of the mass and luminous stars of a spiral galaxy are in the central bulge of the galaxy called the nucleus.  The stars of the nucleus are much older than the stars in the spiral arms.  The nucleus of a spiral galaxy resembles an ellipsoidal galaxy. 

Three years ago there was a very bright supernova in this galaxy featured in PPOW for September 16, 2011.  The 2011 image of the galaxy was taken with a smaller focal length telescope resulting in a larger field of view.  Unfortunately, the location of the supernova is not available in this week's image.   However, three years later, the bright supernova of 2011 would have faded to invisibility for all but very large telescopes. 

This is the third galaxy photograph featured in 2014 in Physics Photo of the Week - all images made at the College View Observatory.

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: