Physics Photo of the Week

May 2, 2008

Pinwheel Galaxy M101
Several galaxies can be found in Ursa Major (Big Dipper).  M101 (appropriately named the "Pinwheel Galaxy") is one of the best examples of a face-on spiral galaxy.  This image was made by astronomy students: Emma Berger-Singer, Forrest Brown, Sean Moffitt, and Phil Waidner with assistance from Donald Collins on April 24, 2008.  The telescope is an 8-inch aperture telescope fitted with an SBIG CCD camera.

Galaxies are very distant and very large objects in the universe.  A galaxy such as this contains about 100 billion stars and lies abot 11 million light years distant.  The Milky Way Galaxy (our home galaxy), is believed to resemble this galaxy - about 100 thousand light years across. 
In the Milky Way the Sun and the Solar System exist as a tiny spec about half-way between the core and the outer perimeter.  About 100 million solar system diameters will fit in the 100 thousand light years of the galaxy's diameter.  If we made a model of a galaxy where the Sun and all the planets occupied a 1 milimeter circle, the whole galaxy would consist of a model about 100 km in diameter - a distance from Asheville, NC to the Great Smoky Mountains National Park.

As the pinwheel indicates, the galaxy is rotating.  About 2 million years are required to complete one rotation.  However the stars rotate faster near the core than on the periphery due to the laws of gravity. 

Most of the 100 billion stars lie within the core of spiral galaxies.  The disk consists mainly of dust, which is invisible but absorbs starlight.  (See PPOW for Nov. 11, 2005)  The spiral arms indicate where the dust has accumulated and piled-up partly due to the fact that the inner regions revolve more quickly than the the outer regions (see the leaf-litter piles from rainflow last October 19 PPOW).  The mass of the galactic dust actually forms new stars in clusters along the spiral arms.  Several regions of new star clusters can be seen along the spiral arms in M101, especially in the spiral on the left edge of the galaxy photo.

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