Physics Photo of the Week

April 18, 2008

Globular Cluster Messier 3

There are two kinds of star clusters: globular clusters and open clusters.  The globular cluster featured here is the third object in Messier's 18th century catalog of nebulous objects.  Globular clusters such as this contain about 500,000 stars, they are gravitationally bound, and are believed to harbor a black hole in the center.  Open stellar clusters, on the other hand, feature items such as the Pleiades (PPOW December 1, 2006), and several other Messier objects featured on PPOW May 6, 2005.  Open clusters contain at most a few hundred stars - many fewer than a globular cluster.  (Watch for future PPOW's featuring open clusters).  This image was made with with the assistance of astronomy students Kat Coker, Suzanne Lutsky, and Baldwin Saer on April 16, 2008.  The students worked the controls to line-up the telescope.  On the previous night (April 15, 2008) several other students (Karel Teifer, Phil Waidner, and Sam Wells) performed similar alignment and observed the cluster, but had run out of time before permanent images
could be procured.

Globular clusters are fascinating astronomical objects in a number of ways: the sheer number of stars located within each globular is overwhelming;  the gravitational stability of these clusters leads to very long life such that the more massive stars have become highly evolved, hence the clusters are very old; globular clusters are very distant (over 30,000 light years for M3) located on the perimeter of the Milky Way galaxy; each star of the cluster is in orbit about the center of mass of the cluster in a random direction somewhat like the windings on a ball of yarn. 

Globular clusters also contain many variable stars - especially Messier 3.  In spring of 2007 I photographed M3 over an extended period of time in one night.  At right we see two images that were taken 95 minutes apart.  The two images have been aligned and adjusted such that all stars show about the same brightness, and played back in rapid succession.  This is called "blinking".  The arrow indicates a rapidly varying star.  See if you can see any more stars that change in brightness as the blinking image switches between the two.  The preliminary processing and alignment of the images was performed by Jessica Harris in May 2007.

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.

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

Observers are invited to submit digital photos to: