Physics Photo of the Week

April 11, 2014

M13 - Globular Cluster in Hercules
Star clusters, very photogenic deep sky objects, come in two basic types: globular clusters and open clusters.  Globular clusters are very dense clusters (about 1/2 million stars), are very distant (more than 10,000 light years from the Solar System), and contain very old stars (about 10 billion years old - comparable to the age of the Milky Way galaxy, and much older than the Sun).  The globular cluster in Hercules (M13) is one of the closest (hence brightest) in the northern celestial hemisphere, and "easy" to photograph with a digital camera on a clock drive telescope.

Open clusters such as the Pleiades (shown below) contain relatively few stars (less than several hundred), are relatively close (less than 10,000 light years), and contain relatively young stars (less then 1 billion years old, with many only a few hundred million years old).  The Pleiades is perhaps the best known open clusters (click on the image for a link to its description).  The relative ages of the two clusters is apparent in the general colors of the stars.  The old age of the globular cluster is indicated by the lack of bright blue stars because the large stars of the globular cluster have had time to evolve into helium-burning red giants.  All the hydrogen fuel has been used up.  The predominant young stars of the Pleiades (the Pleiades' age is less than 100 Million years) are giant, hydrogen-burning hot stars - hence their bluish color representing a high temperature approaching 10,000 K (about twice the surface temperature of the Sun).

On account of the vast numbers of stars in globular clusters, the clusters' self gravity maintain their integrity.  Each star in the globular exhibits an orbit about the cluster's center in a random direction.  The much smaller total mass of an open cluster, on the other hand, presents much less central gravity to hold the cluster together.  As a result the stars in an open cluster disperse throughout the Milky Way galaxy within a few billion years.  New open clusters are formed all the time in areas such as the Orion Nebula (PPOW February 28, 2014).  The Sun is believed to have been formed in such an open cluster.  However, the advanced age of the Sun (4-5 billion years) has provided enough time for all the Sun's "siblings" to disperse throughout the Milky Way galaxy.  Astronomers have found candidates for some of the Sun's long lost "siblings"!  (Alan MacRobert, Sky and Telescope, March 10, 2009).

The high concentrations of stars in globular clusters result in substantial "collisions" between stars.  Before colliding, they lose energy through tidal interactions when passing close to each other.  The result is a rapid formation of binary stars within the cluster.  As more and more of these stars lose energy, they eventually have catastrophic collisions with each other forming larger and larger stars, eventually forming at least one massive black hole in the midst of the cluster.

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.

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